Application of Zeolites for Sustainable Agriculture: a Review on Water and Nutrient Retention

Abstract

Developing urbanization, water shortage, watercourse pollution, and demands for more food due to population growth require a more efficient water irrigation and fertilizer application. Retaining nutrients and water in agricultural soils brings about higher crop yields and prevents pollution of water courses. Among different solutions, zeolites, which are environmental friendly, ubiquitous, and inexpensive, have been extensively employed in agricultural activities. These minerals are considered as soil conditioners to improve soil physical and chemical properties including infiltration rate, saturated hydraulic conductivity (K _s), water holding capacity (WHC), and cation exchange capacity (CEC). Natural and surface-modified zeolites can efficiently hold water and nutrients including ammonium (NH₄ ⁺), nitrate (NO₃ ⁻) and phosphate (PO₄ ³⁻), potassium (K⁺), and sulfate (SO₄ ²⁻) in their unique porous structures. Their application as slow-release fertilizers (SRFs) are reported as well. Therefore, zeolite application can improve both water use efficiency (WUE) and nutrient use efficiency (NUE) in agricultural activities and consequently can reduce the potential of surface and groundwater pollution. This review paper summarizes findings in the literature about the impact of zeolite applications on water and nutrient retention in the agriculture. Furthermore, it explores benefits and drawbacks of zeolite applications in this regard.

1 Introduction

Water is one of the key and fundamental substances, essential for development especially in arid and semi-arid regions of the world, where renewable water resources are scarce (Akhtar et al. 2016; El Kenawy et al. 2016; Jlassi et al. 2016). Inadequate water resources coupled with increasing demand for freshwater, due to increasing population growth rate and improved living standards, has been a challenging issue for countries located in arid and semi-arid regions (Al-Busaidi et al. 2008; Omar et al. 2010; Roghani et al. 2016). Among the various sectors consuming freshwater, agricultural industries should be considered as the chief consumer of freshwater, because more than two-thirds of the renewable water resources are used in agricultural activities (Xiubin and Zhanbin 2001). This issue has resulted in unbalanced water allocation to various users particularly in arid and semi-arid regions that can endanger sustainable development in these areas (Sepaskhah and Barzegar 2010).

On the other hand, increasing population takes higher efficiency in agricultural activities to yield more food production. Besides population growth that asks for greater amounts of food, change of agricultural land use to urban areas brings about less arable farms. Thus, an efficient cultivation is crucial to sustain the urban developments. While high-efficient agricultural activities are needed to satisfy the current food demands, several issues have appeared to achieve this goal. These issues regard irrigation and fertilization (Nagaraja et al. 2016; Nyssen et al. 2015).

Unfortunately, not all soil types are appropriate for agricultural activities. Soil in farms can either be clayey that hampers water infiltration or sandy, which discharges irrigation water from root zones in high rates. Modifying soil physical properties, such as infiltration and hydraulic conductivity, is a method that can influence water and nutrient movement in soil, especially in light-textured soils (Mamedov et al. 2016; Wang et al. 2016a), because these soils have high infiltration rate and low potential to hold water. Therefore, there has been an interest among researches to improve soil physical properties such as infiltration and hydraulic conductivity to increase irrigation efficiency (Anderson et al. 2009; Thierfelder and Wall 2009).

Another issue in increasing the efficiency of cultivation is increasing the ability of soil for holding nutrients, most importantly including nitrogen (N), phosphorous (P), sulfur (S), and potassium (K). Presence of more nutrients in the root zone can lead to higher crop yields. Therefore, to elevate the growing rates, the problematic soils need some curing to boost efficiencies of both irrigation and fertilization (Wang et al. 2016b). In agriculture, although N is known as kingpin and widely used in all crops and cropping systems, rarely its conventional use efficiency exceeds 50% (Ramesh et al. 2015a). For instance, agricultural activities are recognized as an important source of NO₃ ⁻ pollution. High NO₃ ⁻ concentrations (> 3 mg L⁻¹), which easily can be discharged from soils to the groundwater indicate anthropogenic reverse impacts on the groundwater quality (Hosono et al. 2013). Elevated NO₃ ⁻ concentrations endanger public health through (1) spreading different diseases, such as methemoglobinemia disease, which causes defects in the vision or even death of infants (Greer and Shannon 2005), and cancer in digestive organs (Aschebrook-Kilfoy et al. 2013); (2) eutrophication of waterbodies; and (3) production of nitrous oxide (N₂O), which is a greenhouse gas, through the denitrification process. Leaching of NO₃ ⁻ from fertilizers has been known as the primary source of groundwater pollution (Peña-Haro et al. 2010). According to a field survey in China, more than 28% of groundwater samples had exceeded the maximum contaminant level (MCL) of NO₃ ⁻ allowed by the World Health Organization (WHO)’s standard (Gu et al. 2013). Besides NO₃ ⁻, phosphate (PO₄ ³⁻) is another nutrient in the fertilizers, which can contaminate water resources (Delkash et al. 2014). Therefore, nutrients should be retained in the soil in order to provide the needed nutrients for plant growth, while leaching into the groundwater should be prevented (Bakhshayesh et al. 2014; Leggo 2015; Nakhli et al. 2014).

Applying additives to problematic soils is a popular approach, which has been pursued to improve physicochemical properties of soil (Al-Busaidi et al. 2008; Mahabadi et al. 2007; Sarkar and Naidu 2015). Natural zeolites are being considered as soil additives, in which because of their higher water holding capacity (WHC), they have the ability to improve water content of the treated soils. On the other hand, addition of excessive nutrients such as nitrogen to soils in order to enhance the level of nutrients might cause excessive foliage or soil contamination. Leaching of mineral fertilizers (nutrients) into water resources is a serious problem that causes water pollution (Abbasi et al. 2015). Therefore, one of the reasonable approaches to maintain clean water resources and meanwhile keep adequate level of nutrients in soil would be applying additives such as zeolite to soils.

Zeolites, which are hydrated aluminosilicates of alkaline and alkaline earth element, exist in more than 50 and 150 natural and synthetic forms, respectively (Jha and Singh 2016; Virta 2002). Natural and synthetic zeolites found a wide range of applications in a host of industries including soil construction and repair, soil amendment, animal nutrition and health, aquaculture, building materials, heat storage and solar refrigeration, absorption, adsorption, ion exchange, molecular sieves, ion exchange, and catalysts (Elliot and Zhang 2005; Ober 2017). Different zeolite markets are summarized and classified in Fig. 1 (Elliot and Zhang 2005). Among several zeolite markets, agricultural applications have the largest potential market volume for zeolites (Elliot and Zhang 2005). Zeolites are considered to be one of the widely used natural inorganic soil conditioners to improve physical and chemical properties of soil, such as water holding capacity, infiltration rate, saturated hydraulic conductivity, and cation exchange capacity (Chmielewska 2014b; Ebrazi and Banihabib 2015; Enamorado-Horrutiner et al. 2016; Inglezakis et al. 2012). Due to the unique characteristics of zeolites, their application can increase the water use efficiency (WUE) through increasing soil WHC (Xiubin and Zhanbin 2001). Zeolites improve different soil physical properties, in which infiltration and hydraulic conductivity are the most important ones (Gholizadeh-Sarabi and Sepaskhah 2013). It has been extensively acknowledged that soil amendment by means of zeolitic materials improves water holding and prevents it from deep percolation that can lower water usage in agricultural activities (Ming and Mumpton 1989; Mumpton 1999; Polat et al. 2004; Sharpley et al. 1994; Talebnezhad and Sepaskhah 2013). Their application has been tested in many studies in which the effects on soil hydraulic properties have been determined (Ibrahim-Saeedi and Sepaskhah 2013). It has been reported that adding zeolite to light-textured soils can improve their WHC (Bernardi et al. 2009; Bernardi et al. 2013).

Fig. 1

Classification of different zeolite markets (Elliot and Zhang 2005)

Natural zeolites show a cation exchange capacity (CEC) of between 100 and 200 cmol(+) kg⁻¹ (Inglezakis et al. 2015; Ming and Allen 2001). Zeolite is a silicate mineral whose characteristics differ from other silicate minerals such that it has spacious pores and channels within its structure (Chmielewska and Lesný 2012; Inglezakis and Grigoropoulou 2003). Natural zeolites are loaded with cations such as sodium (Na⁺), potassium (K⁺), and calcium (Ca²⁺). They have different significant and well-known properties. The first one is high CEC, which is greater than that of soils (Inglezakis et al. 2015; Stylianou et al. 2015), the second one is a large amount of free water within their structural channels, and the third property is great ability of adsorption with a high surface area. These properties have been used in environmental science, such as improving water quality and ameliorating soil (Cobzaru and Inglezakis 2012). Potential applications of zeolites and their modified forms for capturing cations, anions, and molecular species have been reported. The plant growth media where zeolite is mixed with soil is called zeoponic. This media is nutrient rich and has high capacity of cation exchange. Therefore, there is no drastic demand for providing nutrients with irrigation water (Allen and Ming 1995; Gruener et al. 2003; Rivero and Rodríguez-Fuentes 1989). This type of soil additive is called slow-release fertilizer. This concept will be extensively discussed later.

Zeolites can exchange or adsorb different cations such as cesium (Cs) and strontium (Sr) as well as heavy metals such as cadmium (Cd), lead (Pb), nickel (Ni), manganese (Mn), zinc (Zn), chrome (Cr), iron (Fe), and copper (Cu) (Faghihian et al. 1999a; Faghihian et al. 1999b; Ghasemi Mobtaker H, Kazemian H, 2006; Kazemian et al. 2001; Kazemian and Mallah 2006); anions such as chromate (CrO₄ ²⁻) and arsenate (AsO₄ ⁻³) (Kazemian and Mallah 2008; Menhaje-Bena et al. 2004); and organic pollutants such as volatile organic compounds (VOCs) including benzene, toluene, ethyl benzene, and xylene (BTEX) (Ghadiri et al. 2010; Seifi et al. 2011a; Seifi et al. 2011b; Youssefi and Waring 2012; Youssefi and Waring 2014; Youssefi and Waring 2015).

Worldwide zeolite mine production of large deposits of different natural zeolites in 2012–2016 is shown in Fig. 2 (Dolley 2014; Jewell and Kimball 2016; Ober 2017; Summaries 2015). The annual worldwide mine production of natural zeolites remains almost constant during the last 5 years and is about 2800 kilo metric ton. This high production rate reveals both their variety and interest in their potential application in agricultural and other industrial sectors.

Fig. 2

Zeolite mine production in different countries (2012–2016) (Dolley 2014; Jewell and Kimball 2016; Ober 2017; Summaries 2015)

Zeolites have been historically utilized to enhance agricultural efficiency. Mumpton (1980) mentioned that zeolites were used in Japan three centuries ago for several purposes including soil conditioning. Mumpton (1972) noted that zeolites were widely mined from the USA and Japan in 1950s and employed as soil conditioners. Several seminars held between the researchers of these two countries caused a noticeable progress in this field. Mumpton (1985) elaborately enumerated the application of natural zeolites in agricultural activities in that era. Acknowledging all the efforts that have been made for this research field, the applications of zeolites for improving agricultural activities have been widely paid attention to over the last two decades. The chronological trend of the publication rate in “Scopus” online database with keywords “zeolite” and “water retention” or “nutrient retention” is depicted in Fig. 3a. This trend reveals an annual ascending trend in number of publications of this topic that justifies a comprehensive review about it. The subject areas that researches about zeolites have been published are shown in Fig. 3b. According to this figure, about 23 and 11% of the total zeolite research papers deal with environmental and agricultural sciences, respectively. Different review papers have summarized the applications of zeolites in environmental sciences. The application of different zeolites in treating various polluted wastewaters such as urban runoff, landfill leachate, nuclear fallout, and acid mine drainage has been reviewed (Delkash et al. 2015). A book chapter covered some eco-friendly applications of zeolites for treating contaminated water and wastewaters (Kazemian et al. 2012). Latest scientific reports on the applications of zeolitic materials in aquaculture industry for selective capturing of ammonia and toxic heavy metals, for live transportation of fish, and for feed additives are discussed in a review article (Ghasemi et al. 2016). Some other review papers summarized roles of zeolites in cation exchange and treatments of some heavy metals from the aqueous phase. However, to the best of authors’ knowledge, there is not any comprehensive review article that covers recent achievements in the last two decades on zeolite applications in improving soil nutrients and water holding capacities in agricultural Industries. It is noteworthy that impacts of various zeolites on the behaviors of soil are very subjective. It means that each zeolite has its own effect on a specific soil and this conclusion might not be true for another soil type. This fact renders this review article necessary to individually summarize the recent findings in the literature to pave the ways for future investigations.

Fig. 3

a Trend of annual publications on zeolite applications in soil modifications. b Subject areas published in zeolite field (Scopus.com, last accessed August 25, 2016)

This paper reviews applications of zeolites in holding water and modifying infiltration rate and hydraulic conductivity. Besides, zeolites have been applied to hold nutrients in the soil and reduce their leaching into water bodies. Regarding these issues, scientific progress and development will be addressed and compared to illuminate the current state of the art in this field. Effects of different zeolite, and with or without surface modification on the different soils, will be discussed. The mechanisms that cause alteration in water and nutrient dynamics in soils will be argued and summarized too.

2 Impact on Water Retentions

2.1 Effects of Zeolite on Infiltration Rate

Infiltration rate is a measure of the rate of water entering the soil surface under a vertical hydraulic gradient, which varies in space and over the time. It is notable that neither high nor low infiltration rate is desirable for agricultural activities. In light-textured soils such as sandy soils, infiltration rate is typically high, which might lead to a significant water lost. On the contrary, in soils with lower infiltration rates such as loam and clay, high intensity of rains can generate a great deal of surface runoff, leading to soil erosion and nutrient lost. Depending on the purpose, various methods can be used to increase or decrease soil infiltration rate, including increasing soils organic matter content and vegetation cover and managing the crop residue. The application of these methods leads to a change in soil structure because of which infiltration rate alters. However, the application of these methods only modifies the soil structure and is considered as a short-term practice, whose efficiency can decrease over time (Franzluebbers 2002). Hence, in order to improve soil infiltration rate, soil texture as an inherent characteristic should be modified. One of the long-term method by which soil infiltration can be improved is applying a suitable natural zeolite to soils. Given the unique characteristics of natural zeolites (e.g., clinoptilolite) such as high pore volume and low bulk densities, they can be used to modify soil texture and consequently their infiltration rate. Therefore, their application in improving soil infiltration rate has been the subject of numerous studies worldwide.

According to the results of the related studies, soils treated with natural zeolites alter the infiltration rate compared to unamended soils, depending on soil, zeolite, and water characteristics and also experimental conditions (Gholizadeh-Sarabi and Sepaskhah 2013; Ippolito et al. 2011; Razmi and Sepaskhah 2012; Xiubin and Zhanbin 2001). In a study, the effects of natural mordenite with a particle size of less than 0.25 mm on infiltration rates of fine-grained calcareous loess soil was determined and compared to untreated soil under laboratory and field conditions (Table 1) (Xiubin and Zhanbin 2001). The zeolite had a CEC of 136.35 cmol(+) kg⁻¹ and a WHC of 121.32%. The organic content of the soil was 0.76% by weight. Particles with a size range of 0.05–0.01 mm fraction were 46% and the clay fraction was about 24%. To determine the effects of the zeolite on soil infiltration, water was applied with sprinkler irrigation method with an intensity of 2 mm min⁻¹ for amended soils at three different slopes (5°, 10°, and 20°). According to the results, the application of zeolite resulted in an increase in infiltration rate such that the beginning time of runoff for slopes between 5° and 20° went up. The extent of increase was higher for steep slope compared to gentle slope. The infiltration rate on gentle slopes increased by 7–30%, whereas it went up by more than 50% on steep slope. The observed increase in infiltration rate can be attributed to zeolites porous properties that can generate more routes for water movement (Szerement et al. 2014; Xiubin and Zhanbin 2001). It is suggested that the open network of the zeolites structure can lead to formation of new routes for water movement, which can consequently improve infiltration rate.

Table 1 Effects of zeolite application on the physical properties of different soils

In another study (Table 1), the effects of natural zeolite application rate and saline water on sorptivity of three different soils (sandy loam, loam, and clay loam) in a column study under laboratory conditions was determined (Gholizadeh-Sarabi and Sepaskhah 2013). It should be noted that the Philip equation is a two-term infiltration relationship and can be expressed as follows:

$$ z=s\ {t}^{0.5}+{K}_{\mathrm{fs}}\ t $$

(1)

where z is cumulative infiltration, s is the sorptivity, K _fs is the field saturated hydraulic conductivity, and t is the time. Sorptivity, which is an important factor in Philip’s infiltration equation, controls the infiltration rate of saline water and depends on both initial and final moisture content of soil (Bagarello et al. 2006; Moosavi and Sepaskhah 2012; Suarez et al. 2006). In this study, the zeolite application rate and salinity levels were 0, 4, 8, and 16 g kg⁻¹ and 0.5, 1.5, 3.0, and 5.0 dS m⁻¹, respectively. Then, soil samples modified with the zeolite were put in a column. The results illustrated that sorptivity was not affected in non-saline water for all three soil textures studied at different zeolite application rates. However, other results of this investigation demonstrated that in addition to zeolite, other factors play a significant role in the effects of zeolite on infiltration rate. For instance, salinity can cause interference with the zeolite application. It has been shown that saline water application can have a negative effect on soil infiltration rate. According to the findings, in clay loam salinity levels of 0.5 and 3.0 dS m⁻¹ with application of 16 g kg⁻¹ zeolite and a salinity level of 5.0 dS m⁻¹ with application of 8 g kg⁻¹ zeolite resulted in the lowest values of sorptivity (s = 0.5711 and 0.5423 cm min^−0.5 for 16 g kg⁻¹ and s = 0.5191 cm min^−0.5 for 8 g kg⁻¹ application rates, respectively). In loam, all salinity levels with application of 16 g kg⁻¹ zeolite increased sorptivity compared to other zeolite application rates (s = 2.003 cm min^−0.5 was the highest sorptivity). For sandy loam, only a salinity level of 0.5 dS m⁻¹ with application of 4 g kg⁻¹ zeolite increased sorptivity (s = 1.530 cm min^−0.5). Other zeolite applications decreased sorptivity in sandy loam. It was concluded that zeolite application rate along with soil texture and water salinity level affect infiltration rate in soils. The authors determined a relationship between sorptivity, Z (zeolite application rate), EC_iw (electrical conductivity of irrigation water), SAR_iw (sodium adsorption ratio of irrigation water), and mean soil particle diameter standard deviation (σ _g) as follows (Gholizadeh-Sarabi and Sepaskhah 2013):

$$ {\displaystyle \begin{array}{l}\mathrm{Log}(s)=0.736-0.003(Z)+0.011\mathrm{Log}\ \left({\mathrm{EC}}_{\mathrm{iw}}.{\mathrm{SAR}}_{\mathrm{iw}}\right)-0.045\left({\sigma}_{\mathrm{g}}\right)\\ {}{R}^2=0.838,\mathrm{SE}=0.068,n=144;\mathrm{P}\ \hbox{\pounds}\ 0.001\end{array}} $$

(2)

In addition to natural zeolites, synthetic zeolites have been used as amendments for improving the infiltration rate in soils. In a study (Table 1), Ca²⁺-type zeolite was added to a sand dune soil with a bulk density of 1.47 g cm⁻³, at the rate of 0, 1, and 5% (equivalent to 0, 1, and 5 kg m⁻²). The experiments were conducted in plots and pots with a height and diameter of 30 and 16 cm, respectively. According to the finding of this study, the infiltration rate of the soil tested was affected negatively with zeolite application, such that the higher application led to the lower infiltration rate (Al-Busaidi et al. 2008). Over about 4.5 min, the infiltration rate of the control soil, zeolite (1%), and zeolite (5%) decreased 41, 57, and 59%, respectively. The authors attributed this finding to the micro-pores of the zeolite particles, which slowed the percolation of water within the soil structure.

In another column study (Table 1), the effect of a polymer of lignin and natural zeolite on rainwater infiltration rate was determined. The natural zeolite used was obtained from a mine in Jinyum in Zhejiang province, China, and had a water content of 12 g 100 g⁻¹, specific gravity of 2.16 and specific surface area of 230–320 m² g⁻¹. The lignin was powdered and mixed with the natural zeolite. According to the results, both tested materials delayed and decreased the amount of surface runoff, which is an indication of an increase in infiltration. The combination of lignin and zeolite reduced the quantity of runoff by 44.4 to 50.55% (Zheng et al. 2006).

In general, infiltration is directly related to soil pore structure and structural stability. Zeolite application might alter the soil bulk density, total porosity, and aggregate stability, thereby changing soil structure and the numerous factors affecting the infiltration rate (Azooz and Arshad 1996). In structured soils, macropores and larger mesopores play the key role in conducting water, although they form only a small portion of total porosity (Schwen et al. 2011). It is reported that increasing zeolite application rate can decrease crack area density and therefore reduce the macropores in the soil surface (Razmi and Sepaskhah 2012). The authors did not mention the reason for this alteration. For future studies, more laboratory and field experiments are needed to mechanistically understand the effect of zeolite on infiltration rate, with specific attention on important factors affecting soil structure. It is also recommended to investigate spatial and temporal variation of infiltration in field works, which are common characteristics of the soil infiltration (Khatri and Smith 2006).

2.2 Effect of Zeolite on Hydraulic Conductivity

Hydraulic conductivity is another physical property of soil showing the easiness of water movements within the soil and is used for designing irrigation and drainage systems (Gholizadeh-Sarabi and Sepaskhah 2013). Saturated hydraulic conductivity, K _s, is the ability of soil to conduct water when water fills all pores (Lal and Shukla 2004). It is a parameter that is used for modeling water and solute movements in soil. This parameter has a significant effect on water deep percolations, as a factor playing an important role in efficient water use in the agriculture. In addition, movement of solutes within soil is controlled by hydraulic conductivity (Sepaskhah and Yousefi 2007). Improvement of soils hydraulic properties can lead to efficient water use (increased WUE) in the agriculture and an increase in crop production (Gholizadeh-Sarabi and Sepaskhah 2013). This can be done via improving soil physical properties, which can be achieved by applying soil amendments like zeolite (Gholizadeh-Sarabi and Sepaskhah 2013).

Zeolites can modify the hydraulic conductivity due to the existence of channels within their structure. However, it should be noted that zeolites have different effects on different soils textures. In heavy-textured soils, they are able to increase the hydraulic conductivity, while in light-textured soils, they lower the hydraulic conductivity. In a study (Table 1) clinoptilolite was added to four different soil textures including clay, loam, loamy sand, and sand. Zeolite with particle size of 0.05–0.1 mm was added to soils at different application rates of 0–15 g 100 g⁻¹ soil. The results indicated that application of zeolite decreased the hydraulic conductivity of sandy and loamy soils. On the contrary, it increased the hydraulic conductivity of clay soil. Change in the hydraulic conductivity was highly attributed to change the average particle size of the soil (Mahabadi et al. 2007). Larger particles create larger pores, in which water transport can be conducted faster. For sandy and loamy soils, zeolite application can effectively reduce the average particle size, resulting in lower hydraulic conductivity compared to control soils. In contrast, the addition of zeolite into clay soils might increase the average particle size, therefore resulting in higher hydraulic conductivity. Similar results were also reported in another study, supporting these observations (Lin et al. 1998).

In another column study (Table 1), the effects of natural zeolite (with application rates of 0, 4, 8, and 16 g kg⁻¹) and saline water (salinity levels of 0.5, 1.5, 3.0, and 5.0 dS m⁻¹) on K _s of sandy loam, loam, and clay loam soils were examined (Gholizadeh-Sarabi and Sepaskhah 2013). According to the findings, soils responded differently to different zeolite application rates and salinity levels. For clay loam and loam at salinity levels of 0.5–1.5 dS m⁻¹, saturated hydraulic conductivity increased with 8 and 4 g kg⁻¹ zeolite, respectively. For clay loam, this increase was 180 and 166% for the mentioned application rates, while for loam the extent of the increase was 20 and 26% for those application rates, respectively. Moreover, in sandy loam soil for 8 g kg⁻¹ zeolite at low salinity levels and for 16 g kg⁻¹ zeolite at high salinity levels saturated hydraulic conductivity dropped compared to the control soil. The amount of decrease for 8 g kg⁻¹ zeolite was 67 and 54% at salinity levels of 0.5–1.5 dS m⁻¹. The extent of decrease was 58 and 50% for 16 g kg⁻¹ zeolite at 3.0–5.0 dS m⁻¹, respectively (Gholizadeh-Sarabi and Sepaskhah 2013). In terms of the effect of zeolite application, an increase in conductivity of clay loam and loam soil and a decrease in conductivity of sandy loam might be attributed to increase and decrease of the average particle size, respectively. In terms of the effect of salinity, the adverse effect of salinity on saturated conductivity of soil can be due to plugging of pores by dispersed clay particles in the high saline water (Frenkel et al. 1978). However, the property of solution alone is not enough to characterize the effect of salinity on soil hydraulic conductivity and soil chemical composition is also a key factor that should be considered (Levy et al. 2005; McNeal and Coleman 1966).

In a column study (Table 1), the effect of a natural zeolite application on saturated hydraulic conductivity of a silty clay soil was determined. The zeolite application rate was 0, 4, 8, and 12 g kg⁻¹. Results showed that with increasing zeolite application rate to 8 g kg⁻¹, saturated hydraulic conductivity of the soil tested increased 130% compared to the control soil. However, results showed that further increase in zeolite application could decrease saturated hydraulic conductivity (67% reduction in 12 g kg⁻¹ in comparison to 8 g kg⁻¹ application rate). The authors concluded that the application rate of 8 g kg⁻¹ zeolite improved soil porosity and pore size, while higher application rate (12 g kg⁻¹ zeolite) had a negative impact on pore size and porosity of the soil sample (Razmi and Sepaskhah 2012). However, soil porosity and soil and zeolite particle size were not reported in that paper and without these information, making strong conclusion is impossible. A similar result was reported in another study illustrating that with an increase in zeolite application rate pore water velocity increased (Sepaskhah and Yousefi 2007). They showed that 4 and 8 g kg⁻¹ zeolite application rate increased the pore water velocity by 35 and 74%.

In another investigation (Table 1), the effects of adding two different natural clinoptilolite minerals (i.e., Clinolite and Ecolite) on hydraulic properties of a sand at 15:85% volume fraction (v v ⁻¹) was determined (Githinji et al. 2011). Bulk density of Ecolite and Clinolite were 0.95 and 0.97 g cm⁻³, respectively. The bulk density of the Ecolite-sand and Clinolite-sand were 1.56 and 1.57 g cm⁻³, while the bulk density of the sand was 1.67 g cm⁻³. The experiments were conducted in a column. Results showed that zeolite application increased hydraulic conductivity (Githinji et al. 2011). The saturated hydraulic conductivity of Clinolite and Ecolite was 1.42 and 1.29 m h⁻¹, respectively, which was higher than the saturated hydraulic conductivity of the sand (0.41 m h⁻¹). These results were in agreement with those reported by Waltz et al. (2003). They had shown that an increase in hydraulic conductivity value for sand-amendment compared to pure sand. This can be attributed to the improvement of soil texture due to the zeolite application. According to the results, geometric mean diameter, particle density and the bulk density of Ecolite-sand were 0.35 mm, 2.65 g cm⁻³, and 1.56 g cm⁻³. These parameters were 0.55 mm, 2.40 g cm⁻³, and 1.49 g cm⁻³ for Clinolite-sand. While these parameters were 0.31 mm, 2.67 g cm⁻³, and 1.67 g cm⁻³ for 100% sand. As can be seen, the amended sand had larger particles than that of sand and lower bulk density, which are the reasons for higher saturated hydraulic conductivity.

In addition to natural zeolites, modified zeolites can be used in soils to improve their hydraulic conductivity. In a research, the effect of modified zeolite on soil hydraulic conductivity was determined. Results showed that adding Ca²⁺-zeolite to sodic soils increased the hydraulic conductivity compared to unamended soil (Pal et al. 2006). The hydraulic conductivity of the treated soil was more than 10 mm h⁻¹. Moreover, the infiltration rate of the amended soil was higher. The authors linked this finding to Ca²⁺ release that facilitates the infiltration rate.

In general, saturated hydraulic conductivity is a function of several factors including total and effective porosity, pore size distribution, pore connectivity, and tortuosity (Jabro 1992; Vervoort and Cattle 2003). Zeolite application can alter the soil bulk density, particle density and particle size distribution, resulting in change in the factors affecting saturated hydraulic conductivity. However, the number of publications investigating the effect of zeolite application on hydraulic conductivity is not sufficient to make a general conclusion. It is recommended to assess change in pore size and connectivity, tortuosity, and potential segregation of zeolite particles using advance methods such as image processing in the future works.

2.3 Effect of Zeolite on Soil Water Content

Soil moisture is defined as the water held between soil particles and is considered as the major component of the soil characteristic playing a critical role in plant growth. Soil water content is critical for plant growth, microbial activity, regulating soil temperature, and runoff (Bittelli et al. 2015).

Zeolites can modify soils water content by altering the bulk density and total and aeration porosity. Bulk density is a basic soil physical property that can have an effect on the total porosity and topsoil stability, such that the bulk density of light-textured soils can be lowered with the application of zeolites (Ramesh et al. 2011). The lower the particle size of the zeolite and higher the application rates, the lower the bulk density of a sandy soil. Moreover, soils’ total and aeration porosity can be modified by using zeolites. However, the extent of this modification is higher for aeration porosity than that of total porosity (Ramesh et al. 2011). Furthermore, in sandy soils, decreasing of particle sizes of zeolites can lead to higher water holding capacities, which can be attributed to zeolites high pore volumes that enable them to hold more water in their structures (Ramesh et al. 2011).

Soil amendments are able to hold water and prevent it from deep percolation that can improve WUE in agricultural activities. Their application has been tested in a number of studies in which their effects on soil hydraulic properties have been determined (Ibrahim-Saeedi and Sepaskhah 2013). It has been illustrated that their application to light-textured solids can improve the WHC (Bernardi et al. 2009; Bernardi et al. 2013). It is noteworthy that one of the drawbacks of light-textured soils in agriculture and horticulture is attributed to its lower irrigation efficiency. This is because sandy soils do not have a proper water retention capacity. Treating light-textured soils with soil amendments such as zeolites can improve their WHC.

The capability of zeolites in improving soil WHC has been the subject of a number of studies. In a study, the effects of treating soils with a natural zeolite on WHC was determined (Table 1) (Xiubin and Zhanbin 2001). The natural zeolite used in this study was mainly mordenite with a size of less than 0.25 mm. The zeolite had a CEC of 136.35 cmol(+) kg⁻¹ and saturation water content of a 121.32%. Organic content of the soil was 0.76% by weight. The extent of particles with a size range of 0.05–0.01 mm fraction was 46% and the clay fraction was about 24%. Zeolite was added to fine-grained calcareous loess that had low WHC. Typical effects of zeolite on soil WHC is illustrated in Fig. 4. According to this figure, zeolite application increased the soil water content: after 25 h of adding water to treated and normal soils, the WHC of soil and zeolite was higher than that of the normal soil. This study showed that the capacity of the treated soil with natural zeolite in holding water in drought and general conditions, increased 0.4–1.8 and 5–15%, respectively, compared to control soil (Chmielewska 2014a; Xiubin and Zhanbin 2001).

Fig. 4

Soil water releasing processes of the control soil and zeolite-treated soil (Xiubin and Zhanbin 2001)

In another study (Table 1), a Brazilian natural zeolite, that was composed of stilbite, mixed with a smectite clay mineral and quartz, was added to a sandy soil (890, 30, and 80 g kg⁻¹ of sand, silt, and clay) at the levels of 33.3, 66.7, and 100.0 g kg⁻¹. According to the results, the amount of available water content of the treated soil was 10, 38, and 67% higher than that of the control for the mentioned zeolite application rates. Moreover, the amount of the readily available water is another soil characteristic that can be improved with zeolite application. It was illustrated that an increase in available water for treated soil samples compared to the control, such that easily available water went up by 15, 51, and 111% for noted zeolite application rates compared to the control, respectively (Bernardi et al. 2013).

In another study (Table 1), it is reported that with zeolite application WHC of sandy soil increased (Nus and Brauen 1991). The bulk density and CEC of the soil tested were 1.68 g cm⁻³ and 2.6 cmol(+) kg⁻¹, respectively. The zeolite used in this study was a natural clinoptilolite with a bulk density and CEC of 1.11 g cm⁻³ and 61.3 cmol(+) kg⁻¹, respectively. Distribution of zeolite particle size showed that 54.7% of the particles had a diameter of higher than 54.7 mm. In this study, the effects of increasing natural zeolite application rate (5, 10, and 20 v v ⁻¹) on water content of a sandy soil was determined. It was revealed that with an increase in zeolite application rate, the volumetric water content of the sand soil improved at − 10 KPa matric potential at each application rate. For 10% v v ⁻¹ zeolite application rate, moisture content was 5.85%, while it was about 3.51% for sandy soil (Nus and Brauen 1991).

Influence of natural zeolite application on a sandy soil water content was determined in another study (Table 1). Sandy soil was amended with (10% v v ⁻¹) natural clinoptilolite zeolite in which turfgrass was grown. The bulk density, porosity, hydraulic conductivity, and CEC of the sand soil were 1.66 Mg m⁻³, 0.413, 317 cm h⁻¹, and 0.8 cmol(+) kg⁻¹, respectively. The same parameters for zeolite-treated soil were 1.60 Mg m⁻³, 0.437, 212 cm h⁻¹, and 1.6 cmol(+) kg⁻¹, respectively. Based on the findings, natural zeolite application resulted in an increase in the soil volumetric water content such that the volumetric water content of the amended sandy soil was 20% higher compared to untreated sand (Bigelow et al. 2001).

In a study (Table 1), the effects of four natural clinoptilolite zeolite products on transpiration of “Tifdwarf” Bermuda grass, which was planted in sandy soil was examined (Miller 2000). All zeolites were added to soil at a rate of 8.5% weight fraction (w w ⁻¹). The author noted that zeolites increased the amount of transpirational water by 1% to 16% in sand (Dwairi 1998; Miller 2000). This is an indication of an increase in soil water content as a result of zeolite application.

One of the factors that might affect the efficiency of zeolite is the method whereby it is applied to soil. Effect of zeolite application on the water content of mixed zeolite soil versus banded zeolite soil was determined in another study (Table 1). The zeolite studied was natural clinoptilolite with CEC, bulk density, pore size, pore volume and permeability of 155 cmol(+) kg⁻¹, 0.76 g cm⁻³, 0.5 nm, 51%, and 10⁻³ m s⁻¹, respectively. The zeolite application rate for both methods (mixed and banded) was 0, 6.7, 13.4, 20.2, and 44.8 Mg ha⁻¹. Results showed that soils mixed zeolite contained 1.3% more soil moisture as compared to band zeolite applications. Furthermore, authors reported that 44.8 Mg ha⁻¹ application rate showed the highest water content, such that it had 2.1% more water than the 0 Mg ha⁻¹. According to the results, the differences among the application rates were minimal at potentials of 0 and − 10 KPa. Water retention at matric potentials of − 100 and − 300 KPa were the greatest for a mixed zeolite rate of 44.8 Mg ha⁻¹, compared to a lower zeolite rate and the control. This is an indication of having more water retained in the pore spaces of the sandy soil in 44.8 Mg ha⁻¹ (Ippolito et al. 2011).

Modified zeolites can also be used to improve soil water content. In a study (Table 1), the effect of a modified Ca⁺²-type zeolite on sand dune soil irrigated with saline water was determined. Sand dune soil samples were treated with the zeolite at the rate of 0, 1, and 5% (equivalent to 0, 1, and 5 kg m⁻²) and irrigated with seawater diluted to electrical conductivity (EC_w) levels of 3 and 16 dS m⁻¹. According to the results, a 5% zeolite application rate increased both the content of soil salt and water (Al-Busaidi et al. 2008). Authors attributed this finding to the fact that zeolite can enhance CEC of soils, hold cations on the surface, and release them at the expense of salts in the saline water, and the amount of the increase was more significant at the soil surface (Inglezakis 2005). The extent of increase for water content and salinity were 20 and 1.4%, respectively.

Generally, zeolite can decrease the bulk density and increase total porosity, which consequently increase soil water content. Its application changes the inter-particle porosity of soil. Zeolite is a porous medium with open pore network channels into its structure, which can also play an important role in water retention. For future researches, it is recommended to investigate the effect of zeolite on full range of water retention curve from saturated zone to dry zone, by distinguishing the effect of inter-particle and intra-particle of zeolite.

Effects of zeolite application on discussed physical properties of soils (i.e., infiltration rate, hydraulic conductivity, and water content) reported in the selected scientific papers are summarized in Table 1.

3 Impact on Nutrient Retention

3.1 NH₄ ⁺/NH₃ Retention and Release in Soil

Adding zeolites to soils can improve nutrient holding capacities by directly or indirectly affecting physical, chemical, and biological properties of soils, which in turn control the nutrient dynamics in soils. Because of high CEC of zeolitic minerals, these compounds have demonstrated high NH₄ ⁺ sorption selectivity and capacity as a result of electrostatic attractions between positively charged NH₄ ⁺ and negatively charged sites of zeolites structure (Aiyuk et al. 2004; Englert and Rubio 2005). The strong affinity of zeolites for NH₄ ⁺ cations can be extensively used to improve its retention and release in the soil media (McGilloway et al. 2003; Rabai et al. 2013; Sfechis et al. 2015). On the other hand, addition of zeolites to soils can increase the amount of NH₄ ⁺ adsorption into soils; that may be due to a high affinity of zeolites to entrap NH₄ ⁺ molecule by ion exchange processes (Demir et al. 2002). Nevertheless, the degree of retention of NH₄ ⁺ by zeolitic minerals can vary depending on silicon/aluminum (Si/Al) ratio, mobile and exchangeable cations of zeolites, pore size and structure of these materials, pH, contact time, temperature, and the concentration of other ions in the soil and water (Barros et al. 2003; Sarkar and Naidu 2015; Torma et al. 2014; Watanabe et al. 2005). Although the CEC of some natural zeolites are 2 to 3 times greater than other types of minerals existing in soils, there is a wide diversity among zeolites because of the difference in the nature of zeolite-cage structures, natural structural defects, adsorbed ions and their associated minerals (Kazemian 2002; Kazemian et al. 2012; Malekian et al. 2011b; Ramesh et al. 2015b). Therefore, understanding the adsorption and release kinetics and mechanisms in different sources of zeolites is important in order to increase NH₄ ⁺ retentions and decrease its leaching losses in soils.

3.1.1 Effect of Natural Zeolites on NH₄ ⁺ Retentions and Release in Soils

The peerless chemical and physical properties of natural zeolites among other aluminosilicate minerals, especially high CEC and strong affinity for NH₄ ⁺, have been exploited to minimize the N loss through leaching and to maximize the nitrogen use efficiency (NUE) in agricultural applications (Sarkar and Naidu 2015). NUE and N leaching are a function of each other; i.e., reducing NH₄ ⁺ leaching to groundwater and surface runoff by zeolite amendment will likely increase the NUE and vice versa (Ming and Allen 2001). Clinoptilolite is one of the naturally existing zeolites, with a theoretical CEC of 2.16 cmol(+) kg⁻¹ (Jha and Hayashi 2009). Although the ion exchange capacity of clinoptilolite is lower than some other zeolites, it generally exhibits a high selectivity for NH₄ ⁺ ion. This zeolite is an inexpensive mineral with huge sedimentary deposits, which is available in most regions of the world. Clinoptilolite has been utilized in several studies to decrease leaching of N, which leads to reducing the risk of groundwater contamination. For example, a column leaching experiment was conducted under a pulse application of urea and ammonium nitrate solution (UAN32) with a total N concentration of 443 mg-N L⁻¹ (NH₄ ⁺ concentration of 37 mg-N L⁻¹) (Piñón-Villarreal et al. 2013). From the total applied NH₄ ⁺-N pulse, only 3% was leached from the pure clinoptilolite zeolite column in comparison to 17% in the column of the loamy sand. Therefore, an average leaching reduction of 82% was observed (Table 2). It is also noted that the leaching and retention of NH₄ ⁺ in treatments of loamy sands with clinoptilolite using mass ratios of 60:40 and 80:20 insignificantly deviate from pure zeolites. Similar reductions in NH₄ ⁺-N leaching after application of the clinoptilolite zeolite have been reported. In another leaching experiment, after application of an NH₄NO₃ pulse at a rate of 350 kg-N ha⁻¹ under saturated conditions, the maximum relative concentrations of NH₄ ⁺-N breakthrough curves (BTCs) decreased by 43 and 50%, respectively, in the columns amended with 2 and 8 g kg⁻¹ of the clinoptilolite zeolite as compared to the control loam soil (Sepaskhah and Yousefi 2007). In another experiment, NH₄ ⁺ movement in a Rositas loamy sand amended with clinoptilolite was evaluated (MacKown and Tucker 1985). It was observed that in the column filled with a 50 g kg⁻¹ zeolite-soil-mix, the NH₄ ⁺ leaching was 83% less than the control column (Table 2). The leached NH₄ ⁺ ranged from 0.01 to 0.6% of the total applied N fertilizer. This result can be attributed to an increase in the CEC of soils. The sorption of NH₄ ⁺ in powder natural clinoptilolite zeolite was monitored in another study. The zeolite with an average particle size of 45 μm was applied to a soil at a rate of 600–1200 kg ha⁻¹ (Torma et al. 2014; Vilcek et al. 2013). In a sorption experiment, it was revealed that more than 90% of NH₄ ⁺ was adsorbed by zeolite in the first several minutes. Stabilization of the exchange equilibrium was reached in only a few hours, which can be attributed to a very small particle size and high surface area of zeolite materials. Furthermore, the results of mixing with a soil revealed a reduction of NH₄ ⁺ content in the soil (92.5 mg kg⁻¹ in the control soil and 77.2–81.0 mg kg⁻¹ of soil in the zeolite-soil-mix) 1 month after the zeolite application. This reduction might be attributed to entrapping of NH₄ ⁺ ions at some ion exchange sites within zeolite crystal lattice that are not easily accessible. Three months later, the opposite was true and the amount of NH₄ ⁺ contents in the soil with zeolites were found to be 24–59% more than in the soil without any zeolite.

Table 2 Selected studies in which natural and modified zeolites were used to reduce NH₄ ⁺ and NO₃ ⁻ leaching

Zeolite minerals also can protect NH₄ ⁺ in soils from biological conversions to NO₃ ⁻ through the nitrification process. The latter is more prone to leach out of the soil. This transformation can contribute to surface and groundwater NO₃ ⁻ contaminations (Sepaskhah and Yousefi 2007). In zeolites with small pore sizes of the crystal lattice structure (4–5 Å) such as clinoptilolite, cations, like NH₄ ⁺, can fit in. However, microorganisms, especially nitrifiers, which are a precursor in microbial conversions of NH₄ ⁺ to NO₃ ⁻, are not able to access the pores (Baerlocher et al. 2007). Therefore, when zeolites selectively adsorb NH₄ ⁺ in soils, the retained NH₄ ⁺ ions in the internal voids of zeolites are physically inaccessible to nitrifying bacteria and thus NH₄ ⁺ is protected against the nitrification process. Zeolites can effectively inhibit the nitrification process and as a result, reduce N leaching especially in well-aerated sandy soils, where nitrifying microorganisms are more active (Gholamhoseini et al. 2013). The effect of adding 10% (w w ⁻¹) clinoptilolite-rich tuff to sand-based putting greens on N leaching was evaluated. While (NH₄)₂SO₄ was applied as a source of N, regardless of the N application rates, zeolites reduced the both NH₄ ⁺-N and NO₃ ⁻-N leaching. At the highest N application rate (293 kg-N ha⁻¹), NO₃ ⁻ and NH₄ ⁺ leaching from the clinoptilolite zeolite-amended sand was 86 and 99% lower than those from the unamended sand, respectively (Table 2). The NO₃ ⁻ concentration in leachate from the amended sand did not exceed 10 mg NO₃ ⁻-N L⁻¹ (Huang and Petrovic 1994). In an incubation experiment, it is found that application of pre-adsorbed clinoptilolite with large particle sizes (2.0 to 1.0 mm) decreased nitrification in Rositas loamy sand and Gila silty clay loam by 24 and 31%, respectively (MacKown 1978). Another experimental result also revealed a less intense nitrification rate after application of clinoptilolite (600–1200 kg ha⁻¹) to a soil. NO₃ ⁻ content decreased by 66–78% in comparison with the unamended soil in the consecutive months (in an autumn period) (Torma et al. 2014; Vilcek et al. 2013).

Zeolites also have been widely applied along with other materials to increase the soil nutrient availability, plant growth as well as yield (Colombani et al. 2015; Lim et al. 2015). For example, it has been observed that an increment of the exchangeable soil N concentration (mostly NH₄ ⁺, and NO₃ ⁻ was not detected) after a co-application of the clinoptilolite zeolite and fly ash to a coarse loamy soil was more than the application of those alone. This substantial increase in the nutrient retention in soil is beneficial in terms of prevention of water contaminations by reducing the nutrient loss. However, it might not always improve the plant growth and yield productivity due to the nutrient limitation, especially in a low fertile soil (Lim et al. 2015). In another study, application of the Italian chabazite-rich tuff of Sorano (Grosseto) as a soil conditioner and slow nutrient fertilizer was evaluated using batch and column experiments. Zeolite was added to a silty clay soil and a sandy soil at the rate of 95:5 v v ⁻¹. A natural zeolite was mixed with a swine manure, with a solid fraction of 1% w w ⁻¹, in order to gain an NH₄ ⁺-pre-charged zeolite. Using an artificial rainwater (deionized water Milli-Q plus CaCl2 0.01 mM and NaCO3 0.01 mM, pH = 7.6) as eluent, NH₄ ⁺ was never detected in the leachate of a batch and in the effluent of column experiments (Colombani et al. 2015). It has been suggested that sewage sludge compost, combined with zeolites can be used as a potting soil for greenhouses and nurseries (Bugbee and Elliott 1998). Composting is one of the most popular biological technologies in the sludge treatment; in which sewage sludge nutrients can be recycled for plant growth and soil fertility improvements (Doublet et al. 2010; Jiang et al. 2014; Wang et al. 2014; Wei et al. 2000). However, a direct application of sewage sludge compost onto fields may lead to an increase in water pollutions by leaching of nutrients such as NH₄ ⁺. In a greenhouse study, the combination of the sewage sludge compost, sphagnum peat, and clinoptilolite-rich tuff, by volume ratio of 50, 30, and 20%, respectively, reduced the total N leached through the pot to 0.4 mg, compared with 1.4 g leached through the pot of the control mix consisting the sewage sludge compost, sphagnum peat, and sand.

Zeolite amendments may influence coarse-textured soils to a greater extent than fine-textured soils. Many of the beneficial effects are due to a significant change in CEC and an improvement in the physical characteristics of soils (Ippolito et al. 2011; Sarkar and Naidu 2015). Sandy soils have low water and nutrient retention capacities. Thus, zeolite amendments can demonstrate the remarkable influence on the reduction of NH₄ ⁺ leaching (Zwingmann et al. 2009). It is reported that the addition of 0.2% by weight of a clinoptilolite zeolite to a loamy soil is enough to absorb the applied NH₄ ⁺ and prevent its leaching by the inflow water (Sepaskhah and Yousefi 2007). Similar studied obtained a significant reduction of NH₄ ⁺ leaching rates after application of small amounts of the clinoptilolite zeolite in sandy and loamy soils (Huang and Petrovic 1994; MacKown and Tucker 1985; Pepper et al. 1982; Perrin et al. 1998b). In comparison, it is noted that a clinoptilolite zeolite reduced NH₄ ⁺-N leaching in a Nunn clay loam (Aridic Argiustoll) only at a high application rate (135 Mg ha⁻¹) compared to an unamended control, indicating that large quantities of the clinoptilolite is needed to diminish NH₄ ⁺ leaching in fine-textured soils (Weber et al. 1983).

In addition to the soil texture, the zeolite particle size may also have a significant impact on the rate and kinetics of NH₄ ⁺ adsorption and desorption in soils. In a greenhouse study, N leaching from three different particle sizes of NH₄ ⁺-loaded clinoptilolite, including small (< 0.25 mm), medium (0.25–2 mm), and large (2–4 mm) was investigated. Zeolites were banned with rounded quartz sands at the rates of 112, 224, or 336 kg-N ha⁻¹ (Perrin et al. 1998a). It was found that the larger zeolite particles, the less N leached from amended soil. After 40 days of the simulated leaching experiment, the sand modified with large clinoptilolite particles leached 24.9–32.5% of the total N. In comparison the medium and small-sized clinoptilolite-amended sands had leaching rates of 53.4–68.4 and 44.4–72.1% of the total N, respectively. This inverse relationship between zeolite particle sizes and NH₄ ⁺ exchange rates can be attributed to the required time for intra-particle diffusion, which increases as the particle size increases. However, this result is not consistent with the result of another study, in which the NH₄ ⁺ concentration in leachate from a sandy soil amended with millimeter particle sizes of clinoptilolite zeolites was slightly greater than that amended with nanometer particle sizes (Malekian et al. 2011a).

The other factor that might affect nutrient retentions is the method of zeolite incorporations to soils. The effect of banding or fully mixing of clinoptilolite zeolites (up to 90 Mg ha⁻¹) and urea fertilizers (224 kg-N ha⁻¹) with a silty loam soil on N dynamics was investigated during 35 days of an incubation study. The results revealed a greater extent of urea mineralizations and more effective NH₄ ⁺ sorptions at a mixing application of zeolites in comparison to band applications (Ippolito et al. 2011). This result is consistent with another column leaching study conducted by the same authors; in which mixed versus band applications of zeolites and fertilizers to a silty loamy soil was evaluated. It is reported that when the N fertilizer (224 kg-N ha⁻¹) and clinoptilolite zeolite (up to 20.2 Mg ha⁻¹) were fully mixed into the silt loam soil, less NH₄ ⁺-N was leached as compared with a control, regardless of zeolite application rate (Tarkalson and Ippolito 2010). In contrast to the abovementioned observation, another researcher stated that the application of the natural zeolite to soils in a layered treatment is more efficient in pollution retentions in soils and consequently reduces the risk of groundwater contaminations (Taheri-Sodejani et al. 2015).

3.1.2 Effects of Manipulated Zeolite on Improving NH₄ ⁺ Retention in Soil

In addition to natural zeolites, modified and synthetic zeolites have been used to reduce the NH₄ ⁺ leaching and improve the NUE. MesoLite is an artificial zeolite, which is made by a caustic treatment of kaolin at 80–95 °C. It has a very high CEC (~ 500 cmol(+) kg⁻¹) and a moderately low surface area (9–12 m² g⁻¹) (Mackinnon et al. 2003; Thornton et al. 2007a; Thornton et al. 2007b). The effectiveness of the K-MesoLite and NH₄ ⁺-MesoLite compared to a natural clinoptilolite in retaining and releasing NH₄ ⁺ in columns of amended sandy soils was investigated (Zwingmann et al. 2009). The additions of 8 g kg⁻¹ natural zeolites and K-MesoLite both reduced leaching of the applied dissolved NH₄ ⁺ by bore waters. However, as it is shown in Fig. 5, a K-MesoLite-amended soil exhibited a much higher efficiency in NH₄ ⁺ retentions. The column of the K-MesoLite-amended soil retained more than 90% of the added NH₄ ⁺ compared to 30 and 4% retained by the clinoptilolite-amended soil and unamended soil, respectively (Table 2). Based on the amount of amendment needed to retain 25% of added NH₄ ⁺ after six pore volumes of leaching, the effectiveness of K-MesoLite was about 11.5 times higher than that of clinoptilolite zeolite. The higher efficiency of K-MesoLite might be explained as the CEC of K-MesoLite is about five times higher than that of clinoptilolite zeolite. Furthermore, the release of NH₄ ⁺ adsorbed on zeolites showed dependency on the existing exchangeable cations in the irrigation water. When Ca²⁺-rich bore water was used, 15% of the NH₄ ⁺ was released by six pore volumes. But only one-quarter of this amount was released when deionized water was used. This higher concentration of NH₄ ⁺ in leaching by bore water occurred in all treatments in this study. Thus, under field conditions, where irrigation or surface water contains abundant dissolved cations, both the K-MesoLite and NH₄ ⁺-MesoLite material will give the most benefits in N loss reduction from soils.

Fig. 5

The effect of natural zeolite and K-MesoLite on NH₄ ⁺ ion retention (Zwingmann et al. 2009)

In another research experiment, a natural clinoptilolite from Japan was modified chemically with NaOH treatment to increase its content of alkaline metal cations (Na⁺ ion which is more easily exchanged with NH₄ ⁺ ion); and also was modified mechanically with a wet ball milling to decrease particle sizes (increase the specific surface area) (Jha and Hayashi 2009). Either with a physical or chemical modification of the clinoptilolite, the NH₄ ⁺ retention capacity was sharply increased. The maximum NH₄ ⁺ retention capacities of natural clinoptilolite (NZeo), clinoptilolite treated with NaOH solution for 72 h (Zeo-72), and wet-milled clinoptilolite (WM-50) were 0.89, 1.15, and 1.39 mmol g⁻¹, respectively (Fig. 6). Figure 6 also illustrates the Langmuir and Freundlich sorption isotherms, indicated as solid and broken lines. In the case of natural and chemically modified clinoptilolite, the sorption isotherms were good fit to the Langmuir model, while in case of mechanically modified clinoptilolite the Freundlich model was favorable. This behavior is expected because during chemical modification the crystalline structure of zeolite was not altered, while the crystalline structure was crushed during mechanical modification. Because of a higher adsorption capacity of the physically treated zeolite, it was concluded that the enhancement of NH₄ ⁺ retention capacity of the natural clinoptilolite just by a decrease in the particle size is possible, without incorporating any further exchangeable cations within the framework of zeolites. Selected studies that have used natural and manipulated zeolites to reduce NH₄ ⁺ leaching through soil are summarized in Table 2.

Fig. 6

NH₄ ⁺ sorption of natural and modified clinoptilolite zeolites. Solid and broken lines illustrate predicted data obtained from the Langmuir and Freundlich sorption isotherm parameters (Jha and Hayashi 2009)

3.1.3 Using Zeolite as an NH₄ ⁺-Slow-Release Fertilizer (SRF)

The imbalance between the rate of nutrient releases from fertilizers and the rate of nutrient uptake by plants causes a lower NUE of the fertilizers. An effective way to mitigate the aforementioned problem is to develop SRFs, which can release nutrients slower than commonly used fertilizers (Behin and Sadeghi 2016; Bhardwaj et al. 2012). SRFs have better performance over commonly used fertilizers, they have lower fertilizer loss rate and sustainable nutrient supply, reduce the application frequency of fertilizers, and minimize the potential of negative effects associated with over-dosages (Guo et al. 2006; Ni et al. 2010a; Ni et al. 2010b; Zhang and Saito 2009; Zhang et al. 2010). The prominent large CEC and conspicuous selectivity of natural, synthetic, and modified zeolites including natural clinoptilolite and synthetic MesoLite for plant nutrients, especially NH₄ ⁺, can be exploited in the preparation of fertilizers to improve the capacity of the soil nutrient retention. This can be done by promoting a slower release of these elements synchronized with plant uptake (Sepaskhah and Yousefi 2007; Sfechis et al. 2015). Although the aluminosilicate framework of natural zeolites is relatively not affected when exposed to water, NH₄ ⁺ may be slowly exchanged and released from exchange sites of zeolites (Rehakova et al. 2004). A slow-release fertilizing not only increases the NUE and promotes crop yielding, but it also diminishes the potential migration of these elements into groundwater and runoff from plant root zone, and subsequently reduces environmental pollution. Zeolites have been extensively used as SRFs, either in the combination of another N fertilizer or in the form of a pre-loaded NH₄ ⁺ zeolite (Sepaskhah and Barzegar 2010; Zwingmann et al. 2009). In both ways, exchange sites within zeolite structures act as sinks for NH₄ ⁺. Over time, this retained exchangeable cation can be slowly replaced by another cation and thereby become available for plant uptake (Ming and Allen 2001). As an example in a greenhouse experiment, more N was assimilated when an NH₄ ⁺-loaded clinoptilolite was exploited as an amendment for a sandy soil at rates of 112, 224, or 336 kg-N ha⁻¹, compared to the (NH₄)₂SO₄. Less than 5% of the added N leached out from the amended sandy soil, regardless of the N rate and zeolite particle size, while 10% to 73% of the N leached when (NH₄)₂SO₄ was applied (Perrin et al. 1998a).

3.1.4 Effect of Zeolites on NH₃ Volatilization in Soils

In addition to N loss from soils through leaching, a significant portion of the loss could be due to NH₃ volatilization (Piñón-Villarreal et al. 2013). Natural zeolites such as clinoptilolite are renowned adsorbents for ammonia, in which they are selectively trap ammonia even at very high relative humidity (Asilian et al. 2004). In agricultural applications, this gaseous NH₃ loss will lead to a decrease in the N uptake efficiency and an increase in production costs (He et al. 2002; Omar et al. 2010). As part of a study, the effects of clinoptilolite zeolites, cellulose and combination of both on NH₃ volatilizations and N transformations in agricultural calcareous sandy soils were investigated. With the application of 15 g kg⁻¹ clinoptilolite zeolites, NH₃ volatilization from NH₄NO₃, (NH₄)₂SO₄, and urea (applied at the rate of 200 mg-N kg⁻¹ soil) decreased by 4.4-, 2.9-, and 3.0-fold, respectively, compared to that from the respective sources without treatments. It is also reported that a co-application of clinoptilolite and cellulose (at a rate of 15 g kg⁻¹ each) was the most effective way in decreasing NH₃ volatilizations. The effect of a clinoptilolite and cellulose amendment on the N holding capacity of soils may be attributed to increase in NH₄ ⁺ retentions in the ion exchange sites of zeolites and increase in the microbial biomass, which is responsible for the immobilization of N. However, it should be noted that NH₃ volatilization is a function of pH and temperature. It enhances in a higher pH and temperature (Meisinger and Jokela 2000). Other studies also reported similar reductions in NH₃ volatilization after amending sandy soils with zeolites. It is reported that by applying 1 g kg⁻¹ of clinoptilolite to a sandy clay loam and sandy loam, NH₃-N losses reduced by 49.23 and 51.09%, respectively. In this research, urea and an acidic agent (triple super PO₄ ³⁻) were utilized as fertilizers (Ahmed et al. 2010; Haruna Ahmed et al. 2008).

Besides, composting of animal manures is considered an effective treatment method for naturally recycling organic matters. It generates high NH₃ emissions because of the microbial decomposition of nitrogenous organic compounds in the manures. NH₃-N loss decreases N content of the product and also causes environmental pollutions (Baek et al. 2002; Kuroda et al. 2004; Tiquia and Tam 2000). Without an appropriate control, 33–62% of the total N content of the manure may be lost during composting. Therefore, it is required to utilize proper methods to diminish NH₃ emissions and N loss from the compost (Hong and Park 2005). Zeolite amendments, such as clinoptilolite, have demonstrated a great ability in reducing NH₃ emissions from animal manures, both in lab-scale and large-scale composting processes (Bautista et al. 2011; Li et al. 2008). Reduction of volatilization of gaseous N, such as NH₃ or N₂, can be attributed to the exchange of NH₄ ⁺ into zeolite exchange sites; so that it is unavailable for conversions to these gaseous phases via microbial processes (Ming and Allen 2001). The addition of alum and zeolite to swine manures has shown a great ability in reducing NH₃ emissions by 85–92%. The final compost retained three-folds more NH₄ ⁺-N than the unamended control (Bautista et al. 2011). Furthermore, the zeolite substantially improved the quality of the compost by sequestering 44% of retained NH₄ ⁺ at exchange sites, which can be exploited as a slow-release fertilizer for applications to agricultural soils and improve the crop productivity (Leggo 2015).

3.2 NO₃ ⁻ Retention and Release in Soil

NO₃ ⁻, because of its negative charge, typically does not have much affinity for soil particle surfaces and does not readily adsorb in soil (Feigin et al. 2012), although sorption of NO₃ ⁻ on a few acidic types of soils has been reported (Cahn et al. 1992; Eick et al. 1999). Because of anion repulsion between NO₃ ⁻ and soil surfaces, the application of a fertilizer containing anions, such as UAN 32 or KNO₃ to soil, can result in higher N loss and lower NUE (Li et al. 2006; Li 2003). Furthermore, it can be lost via denitrification, especially in moist soils. Denitrification losses also reduce the NUE and are an environmental concern for the potential production of N₂O; that may play in stratospheric ozone depletion (Qian et al. 1997; Ravishankara et al. 2009). Due to the low retention and high potential to loss in soil, an increasing amount of applied fertilizer is therefore required for appropriate plant growth. However, this increased dose will contribute more to contaminate water, due to elevated highly soluble NO₃ ⁻ concentrations in surface and groundwater. Therefore, it has been recommended to apply anionic fertilizers at lower rates but at a higher frequency, use other types of fertilizers that do not contain NO₃ ⁻, or utilize lower soluble fertilizers and SRFs. Another possible approach is incorporation inexpensive soil modifiers such as natural zeolites that can increase NUE and decrease NO₃ ⁻ leaching (Malekian et al. 2011a).

3.2.1 Effect of Natural Zeolite on NO₃ ⁻ Retention and Release in Soil

Zeolite minerals do not have an evident affinity for NO₃ ⁻ ions, due to negative charges on the surface of zeolitic framework. Nevertheless, contradictory results can be found among published literature. While various studies from the literature suggest that addition of natural zeolites promotes NO₃ ⁻ leaching from soils (Piñón-Villarreal et al. 2013; Sutherland et al. 2004), several others have shown opposite effects (Aghaalikhani et al. 2012; Rabai et al. 2013; Sepaskhah and Yousefi 2007). In most of the reported studies, leached NO₃ ⁻ cannot be differentiated among one of these two forms: direct application of inorganic fertilizer containing NO₃ ⁻ or zeolites pre-loaded with NO₃ ⁻, and NO₃ ⁻ produced from biological nitrification after incorporation of NH₄ ⁺/urea fertilizers or zeolites pre-loaded with NH₄ ⁺. To determine whether or not the addition of natural zeolite, especially clinoptilolite to soil has a positive effect on NO₃ ⁻ retention and leaching, N source in soil should be considered as the main factor.

In the case of direct application of NO₃ ⁻ to soil, due to negatively charged the surface of natural zeolite molecules, anion repulsion might occur. In this case, zeolites not only have no positive effect on NO₃ ⁻ retention in soil but rather increase its leaching rate. In a column leaching experiment with a pulse application of 443 mg-N L⁻¹ of UAN 32 fertilizer, 92% of the total applied NO₃ ⁻-N was leached from clinoptilolite zeolite, in comparison to 83% leached from loamy sand (Table 2) (Piñón-Villarreal et al. 2013). In another leaching study, the maximum value of relative concentration occurred in the pore volume of about 0.5 (Sepaskhah and Yousefi 2007). While theoretically it should occur at pore volume of 1.0 for non-absorbent ions, such as NO₃ ⁻. This was attributed to anion exclusion by negatively charged zeolite and soil surface. When solute molecules are repulsed from the surfaces of the porous media, they tend to migrate toward the center of the pore line, where the pore velocity reaches its maximum. It causes the solute to move faster than the average pore velocity (Leij and van Genuchten 2001). Nevertheless, it is reported that after pulse application of NH₄NO₃ (350 kg-N ha⁻¹) under saturated conditions, the maximum relative concentration for NO₃ ⁻ breakthrough curve was reduced by 15%, when clinoptilolite zeolite was added at a rate of 8 g kg⁻¹ soil. This indicated that some portion of the applied N was held in the soil by amending zeolite and was not leached by water (Sepaskhah and Yousefi 2007). Some studies have reported enhancement in NO₃ ⁻ retention by zeolite incorporation in soil. In a particular report, the reduction has attributed to the trapping of NO₃ ⁻ into the pores of zeolite framework (Sadeghi et al. 2010). Adsorption properties of the clinoptilolite zeolite were utilized to improve filtering properties of a silt loam soil, irrigated with a wastewater containing NO₃ ⁻ at the concentration of 14.2 mg-N L⁻¹ (Taheri-Sodejani et al. 2015). After passing through a 40-cm column, NO₃ ⁻ concentration of the leached wastewater in the control sand, mixed, and layered soil amended with clinoptilolite, decreased by 12.18, 32.19, and 54.90%, respectively (Table 2). The result from this experiment revealed that zeolite application in band method, smaller zeolite particle size (less than 63 μm) and higher dosage (4%), is more effective in reducing NO₃ ⁻ in wastewater leachate. Similarly, it has been reported that the mixed method of clinoptilolite zeolite application remarkably reduces NO₃ ⁻ leaching; when NH₄NO₃ was used as a fertilizer to a silt loam soil at a rate of 350 kg-N ha⁻¹ under saturated conditions. By adding 2, 4, and 8 g of zeolite to 1 kg of soil, NO₃ ⁻ leaching is reduced to 6, 28.7, and 47.6%, respectively (Sadeghi et al. 2010). Another study reported that odorless and cohesive zeo-sewage sludge can be produced, either by commixture of sewage sludge with the Hellenic Natural Zeolite (HENAZE) or as a precipitate from HENAZE-treatment of urban wastewaters, which is suitable for the reclamation of agricultural soils. Addition of HENAZE with an NH₄ ⁺ ion exchange capacity of 226 cmol(+) kg⁻¹ in agricultural soils, reduced seepage of NO₃ ⁻ to water environment by 55–92% and thus protected the quality of surface and groundwater (Filippidis 2010).

When other types of nitrogenous compounds, like urea and NH₄ ⁺, are incorporated to soils the presence of NO₃ ⁻ in the leachate can be attributed to nitrification process (Piñón-Villarreal et al. 2013). Zeolite addition to soil might reduce NO₃ ⁻ leaching because of its unique physiochemical properties. It renders NH₄ ⁺ unavailable to nitrobacteria. During an experimental study, NO₃ ⁻ leaching after application of urea fertilizer at three N levels (90, 180, and 270 kg-N ha⁻¹), and clinoptilolite zeolite at four rates (0, 3, 6, and 9 t zeolite ha⁻¹) to a sandy loam was monitored (Gholamhoseini et al. 2012). Minimum NO₃ ⁻ leaching was obtained at the higher zeolite application rate (9 t zeolite ha⁻¹). Compared with the control sand, this treatment caused a 36 and 37% decrease in NO₃ ⁻ leaching loss in the first and second year, respectively (Table 2). A linear relationship between N application rate and its leaching loss was also observed, similar to the result of the other study (Li et al. 2007). NO₃ ⁻ leaching reduction after application of a mixture of zeolite and urea fertilizer to a soil was explained by different mechanisms including placement of urea in pores of zeolite crystals, reduction in the transformation of urea to NO₃ ⁻ by nitrification, and a decrease in the nitrification process by NH₄ ⁺ adsorption on zeolite (Sepaskhah and Yousefi 2007). In another study, the effects of applying clinoptilolite zeolite on nutrient leaching under two irrigation regimes (full and limited) were determined (Gholamhoseini et al. 2013). Several combinations of clinoptilolite, cattle manure and urea were added to a sandy soil as chemical fertilizer. The integrated application of zeolite and cattle manure significantly decreased NO₃ ⁻ leaching as compared to the unamended soil, particularly under full irrigation regime. Maximum NO₃ ⁻ leaching (36 kg ha⁻¹) was obtained in the soil without zeolite and cattle manure at full irrigation regime. While minimum NO₃ ⁻ leaching (11 kg ha⁻¹) was observed in a soil amended with urea, cattle manure and highest zeolite application rate (21%) at limited irrigation regime. In some studies, NO₃ ⁻ leaching rate decreased by modifying soils with clinoptilolite zeolite, when N sources containing NH₄ ⁺ (such as (NH₄)₂SO₄) were applied as fertilizer (Huang and Petrovic 1994; Perrin et al. 1998a). Similarly, it is found that the NO₃ ⁻ leaching decreased by 30% when a loamy sand soil (6% clay) was amended with NH₄ ⁺-loaded clinoptilolite zeolite (Wang and Alva 1996). These results are not consistent with the result of other study, where clinoptilolite zeolite was added to golf course sand at two rates of 100 and 200 mL L⁻¹ and two particle sizes of fine and coarse. In this case, (NH₄)₂SO₄ in addition to two forms of urea was added to the soil at a rate of 97.6 kg ha⁻¹ per month. It is reported that significantly more NO₃ ⁻-N leached from the sand amended with coarse clinoptilolite zeolite particles than control sand, while no difference was observed between sand amended with fine clinoptilolite zeolite and control (Sutherland et al. 2004). However, in this case, the leached NO₃ ⁻-N might have formed faster in the sand by speciation paths other than nitrification causing NO₃ ⁻ to experience anion exclusion and to be leached out from the amended soil in larger amounts than with control soil (Piñón-Villarreal et al. 2013).

3.2.2 Effects of Manipulated Zeolite on Improving NO₃ ⁻ Retention in Soil

While almost all of naturally occurring zeolites and most of the artificial zeolites do not possess high affinity for trapping anionic species, by modifying their surface chemistries using cationic surfactant multifunctional adsorbents with capability to trap anions and non-polar organics can be manufactured (Seifi et al. 2011c). Schematic diagram of a typical surfactant-modified zeolite (SMZ) is illustrated in Fig. 7.

Fig. 7

Scheme diagram of surfactant-modified zeolite (SMZ), a multifunctional adsorbent for trapping organic molecules, anionic species, and cations (Kazemian et al. 2012), Copyright 2012, Bentham Science Publishers

It has been reported that SMZ, an inexpensive anion exchanger, can be exploited as a fertilizer carrier to comply this requirement and control NO₃ ⁻ release from soil (Malekian et al. 2011a). For instance, the quaternary amine hexadecyltrimethylammonium (HDTMA) is a long-chain surfactant with a permanent positive charge (Bowman 2003). The maximum loading of HDTMA is about 200% of the zeolites external CEC. At the HDTMA sorption maximum, the surfactant molecules form bilayers on zeolite surfaces with the lower layer held by electrostatic interactions between the negatively charged zeolite surface and the positively charged surfactant head groups, while the upper layer is bound to the lower layer by hydrophobic forces between the surfactant tail groups in both layers. The surfactant loading on the zeolite is a function of the external CEC of zeolite and the chain length of the cationic surfactant (Thirunavukkarasu and Subramanian 2014). Under the surfactant bilayer configuration, the positive surface charge of zeolites is reversed, making it as a mineral with greater potential of sorption and retention of negatively charged NO₃ ⁻ that is attributed to surface anion exchange (Cordoves et al. 2008; Zhang et al. 2007). By application of surface modified zeolite, therefore, the problem of low NO₃ ⁻ retention due to lack of significant positive charges in many soils can be effectively addressed.

In a study, the performance of SMZ in comparison to clinoptilolite zeolite in NO₃ ⁻ leaching reduction and crop growth enhancement was investigated (Malekian et al. 2011a). Both zeolite types were added to a sandy soil at different application rate (20 and 60 g kg⁻¹) and with a different particle size (millimeter and nanometer). The clinoptilolite zeolite was modified by HDTMA to 200% of the external cation exchange capacity (ECEC). NH₄NO₃ was applied as N fertilizer via irrigation water at a rate of 150 kg-N ha⁻¹. They found that both maximum and mean concentration of NO₃ ⁻ in the leachate of SMZ-amended soil were significantly lower than those of clinoptilolite-amended soil. For example, at the application rate of 60 g kg⁻¹ the leaching of NO₃ ⁻-N from SMZ- and clinoptilolite-amended soil was approximately 26 and 22% lower than that from the control system, respectively (Table 2). The decreased NO₃ ⁻ concentration in clinoptilolite-amended soil may be attributed to the increasing CEC of the soil by mixing with zeolite and the high affinity and selectivity of clinoptilolite zeolite for NH₄ ⁺. Therefore, it leads to decreasing NH₄ ⁺ availability to nitrifying bacteria. Although theoretically adsorbents with smaller particle size should exhibit greater capacity of adsorption due to the greater specific surface area (Huang et al. 2010), no significant difference in the amount of NO₃ ⁻ leaching from amendments with different particle size was observed. It might be attributed to the fact that particle size affects both adsorption and desorption of NH₄ ⁺ and NO₃ ⁻ from unmodified and modified clinoptilolite, respectively. While more N was adsorbed in smaller particle size zeolites, more N was also released from them. However, as it is depicted in Fig. 8, the results revealed that the application of both surfactants modified and unmodified zeolites to agricultural soil can reduce N leaching and increase the ratio of N uptake to the applied N fertilizer.

Fig. 8

The amount of NO₃ ⁻ leached from different soil amendment types (S, SMZ; Z, zeolite clinoptilolite; C, unamended soil control), size (M, millimeter; N, nanometer), and rate (a, 20 g kg⁻¹; b, 60 g kg⁻¹). Different letters above each column represent significant difference at p < 0.05 (Malekian et al. 2011a)

The synthetic clinoptilolite and its surfactant-modified type by using solutions of hexadecyltrimethylammonium bromide (HDTMAB) and dioctadecyldimethylammonium bromide (DODMAB) were introduced. In addition, its potential in adsorption of NO₃ ⁻ and using as SRF was evaluated (Bhardwaj et al. 2012). The anion exchange equilibrium between NO₃ ⁻ions and counter bromide ions can be schematically expressed by:

$$ \mathrm{Zeolite}-\mathrm{HDTMA}-\mathrm{HDTMA}-{\mathrm{Br}}^{-}\left(\mathrm{s}\right)+{\mathrm{NO}}_3^{-}\left(\mathrm{l}\right)\iff \mathrm{Zeolite}-\mathrm{HDTMA}-\mathrm{HDTMA}-{\mathrm{NO}}_3^{-}\left(\mathrm{s}\right)+{\mathrm{Br}}^{-}\left(\mathrm{l}\right) $$

(3)

$$ \mathrm{Zeolite}-\mathrm{DODMA}-\mathrm{DODMA}-{\mathrm{Br}}^{-}\left(\mathrm{s}\right)+{\mathrm{NO}}_3^{-}\left(\mathrm{l}\right)\iff \mathrm{Zeolite}-\mathrm{HDODMA}-\mathrm{DODMA}-{\mathrm{NO}}_3^{-}\left(\mathrm{s}\right)+{\mathrm{Br}}^{-}\left(\mathrm{l}\right) $$

(4)

The powder X-ray diffraction (XRD) patterns of clinoptilolite and its modified forms are shown in Fig. 9. According to XRD patterns, a close resemblance observed on comparing the diffraction patterns of clinoptilolite with its modified surfactant forms indicated the structural integrity of material before and after surfactant treatment. Therefore, surface modification of zeolite does not alter zeolite structure, which is desirable. In aqueous solution with NO₃ ⁻ concentration of 160–280 mg L⁻¹, the surfactant-modified material exhibited much higher adsorption capacity than the unmodified clinoptilolite. The slow NO₃ ⁻ release studies have been performed using thin layer-funnel analytical test and soil column percolating system. In the funnel analytical test, unmodified zeolite released 25–35% of NO₃ ⁻ after first irrigation, while SMZs demonstrated slow release tendency with only 15–20%. Furthermore, at the first day of the column study, NO₃ ⁻ desorption was 61%, 30–35%, and less than 18% in the soil, clinoptilolite, and surfactant-modified clinoptilolite, respectively. Even after 15–20 days of leaching, the SMZs still released NO₃, clearly indicating that a slow release of N is achievable by application of these type of modified zeolite to soil. It is also noted that the presence of humic acid reduced NO₃ ⁻ adsorption. It can be attributed to the high affinity of surface modified zeolite for humic acid adsorption, which cause low availability of the adsorption site on zeolites for NO₃ ⁻ ion to be adsorbed (Klučáková 2010; Zhan et al. 2010). In another greenhouse study, NO₃ ⁻ and NH₄ ⁺ ions retention in a soil amended with 2 g kg⁻¹ of clinoptilolite zeolite (Z), iron oxide in the form of goethite (G), and SMZ with goethite at two forms (Z-G I and Z-G II) was examined (Molla et al. 2014). NH₄NO₃ was used as N source at two rates of 75 mg-N kg⁻¹ (300 kg-N ha⁻¹) or 150 mg-N kg⁻¹ (600 kg-N ha⁻¹). The results revealed that the modification of zeolite with goethite led to a positively charged modified zeolite surface, which can be used to promote NO₃ ⁻ retention in the soil. Consequently, the use of binary systems (Z-G) as an amendment to soil with high concentration of NO₃ ⁻ ion will reduce the amounts of nitric compounds to significant levels. A summary of selected studies are listed in Table 2.

Fig. 9

XRD patterns of clinoptilolite and its modified forms by HDTMAB and DODMAB (Bhardwaj et al. 2012)

3.2.3 Using Zeolite as NO₃ ⁻-Slow-Release Fertilizer

Presumably if cations, such as NH₄ ⁺, can be released slowly from zeolite due to cation exchange, a similar result can be obtained for NO₃ ⁻ release from SMZ due to an anion exchange reaction, even though the location of the ion exchange sites may differ. The feasibility of using SMZ as a slow-release NO₃ ⁻ fertilizer was evaluated (Li 2003). Clinoptilolite was modified using HDMTA at different loading rate (25–200% ECEC). Batch sorption experiment revealed that NO₃ ⁻ sorption onto SMZ increased by increasing the surfactant loading. In batch desorption study, 30–40% of sorbed NO₃ ⁻ still retained by modified zeolite after 100 pore volumes of water; indicating that this type of zeolite can be utilized as SRF. Furthermore, column leaching tests showed that a 95% decrease in the effluent NO₃ ⁻ concentration could be achieved, compared to that when soluble NO₃ ⁻ was mixed with zeolite at the same NO₃ ⁻ loading (Table 2). In the treatment with a mixer of soluble NO₃ ⁻ and clinoptilolite after two pore volume of water the effluent NO₃ ⁻ concentration was rapidly decreased to < 0.2 mM. While the effluent NO₃ ⁻ concentration remained > 0.3 mM after more than 50 pore volumes in the treatment with NO₃ ⁻-loaded SMZ. The result from both batch desorption and column study demonstrate the high ability of a NO₃ ⁻ loaded SMZ to control NO₃ ⁻ retention and release in agricultural soil (Li 2003).

It is noteworthy that most of the surfactants that are being used to modify zeolites are toxic chemicals. Furthermore, there are not enough scientific studies on potential leaching of the loosely attached surfactant from the zeolite surface. Therefore, further studies are needed prior to any suggestion for commercializing this type of modified products. Nevertheless, using more eco-friendly and less toxic compounds as potential surface modifiers can be considered as an alternative path.

3.3 Phosphorous (P) Retention and Release in Soil

P is one of the most critical major plant nutrients. Among its different compounds, PO₄ ³⁻ is the main constituent in the soil matrix. Although PO₄ ³⁻ exists as negatively charged ion in the soil solution, it has a large affinity to the soil matrix. It reacts quickly in soils and becomes immobile and insoluble (Gholamhoseini et al. 2013). PO₄ ³⁻ fixation in soils occurs via adsorption and precipitation reactions. Adsorption is the dominant reaction in a short period of time, and generally it is attributed to hydrous oxides of iron (Fe) and aluminum (Al) (Moharami and Jalali 2014). Nevertheless, several recent studies clearly have demonstrated that PO₄ ³⁻ can be leached out from soils under different conditions (Heckrath et al. 1995; Jalali and Kolahchi 2009; Jalali 2009). PO₄ ³⁻ accumulates when is added to soils in the amounts of more than those removed by the plant. Consecutive applications of the excess PO₄ ³⁻ in fertilizers will saturate adsorption sites of soils. In this condition, PO₄ ³⁻ can be washed off and leads to high concentration of this element in the surface runoff. Furthermore, it can be leached to deeper soil layers and eventually leads to increasing PO₄ ³⁻ concentrations in the groundwater (Bansiwal et al. 2006; Coblentz et al. 2004; Sharpley et al. 2007).

3.3.1 Effect of Zeolites on P Retention and Release in Soils

Zeolites are aluminosilicate minerals with poor amounts of Fe and Al hydrous oxides. They have a large negatively charged surface and no anion exchange capacity (Elliot and Zhang 2005). Therefore, negligible or no effect on PO₄ ³⁻ leaching is expected when it is added to the soil. For example, in a leaching experiment with (NH₄)₂HPO₄ solution, the addition of clinoptilolite zeolite to A (0 to 10 cm) and B (30 to 40 cm) horizons of a coarse sandy soil at a rate of 10% (w w ⁻¹) showed P sorption similar to unamended soils. This was attributed to the high pH and lack of sorption sites in the natural clinoptilolite zeolites (Phillips 1998). Nevertheless, the modification of zeolite could lead to an improvement in its ability to sorb more P. In a leaching experiment, P leaching from a sandy soil was evaluated in the presence of unmodified and modified zeolites with Fe³⁺ ions, amending at rates 10 and 3%, respectively. P retentions in the zeolite-Fe and zeolite were 14.2 and 10.5%, respectively, in comparison to 8.1% in the control soil. Compared with the untreated soil, the soil amended with zeolite-Fe leached slightly less amount of P. This reduction in P leaching is possibly attributed to the P adsorption by Fe and Al oxides and precipitation of stable form of Fe, Al, and Ca compounds of P (Moharami and Jalali 2014). In another experiment, the feasibility of using SMZ as a carrier for P was investigated. The HDTMAB was used to modify the surface of a commercial zeolite-A type, to increase its capacity of PO₄ ³⁻ retention. The P loading on the SMZ increased by a factor of 4.9, compared with the unmodified zeolite-A. The results also revealed that P released from PO₄ ³⁻-loaded surface SMZ was available even after 1080 h of continuous percolation, while P from KH₂PO₄ was depleted within 264 h (Bansiwal et al. 2006).

3.3.2 Using Zeolite as PO₄ ³⁻-Slow-Release Fertilizer

An effective way to alleviate the imbalance between the rates of nutrient uptakes by plant roots and the rates of nutrient releases from fertilizers is to use SRFs (Ni et al. 2010a; Zhang and Saito 2009; Zhang et al. 2010). Using zeolites as SRFs is limited to cationic nutrients which can be loaded on the zeolite sites, such as NH₄ ⁺ and K⁺. Therefore, loading of anionic nutrients like PO₄ ³⁻ is negligible on unmodified zeolites. Nevertheless, several studies showed that a controlled and gradual release of PO₄ ³⁻ is achievable by a combination of zeolite ion exchange and mineral dissolution (like apatite-rich phosphate rock (PR)) (Chesworth et al. 1987; Lai and Eberl 1986). Ion exchange and dissolution reactions drive each other. Zeolites capture dissolved cations by an ion exchange mechanism, thereby increase dissolution and mineral dissolution provides cations for ion exchange. The reactions occurring in these systems can be described by the following reactions in which Ca₅(PO₄)₃F is fluoroapatite, a frequent source of P, and K⁺- and NH₄ ⁺-zeolite are zeolites enriched with potassium and ammonium ions, respectively (Lancellotti et al. 2014):

$$ 0.5\ {\mathrm{Ca}}_5{\left({\mathrm{PO}}_4\right)}_3\mathrm{F}+1.5\ {\mathrm{H}}_2\mathrm{O}\iff 2.5{\mathrm{Ca}}^{2+}+1.5{\mathrm{H}\mathrm{PO}}_4^{2-}+0.5\ {\mathrm{F}}^{-}+1.5\ {\mathrm{OH}}^{-} $$

(5)

$$ 0.5{\mathrm{Ca}}^{2+}+{\mathrm{NH}}_4^{+}-\mathrm{zeolite}\iff {\mathrm{NH}}_4^{+}+0.5{\mathrm{Ca}}^{2+}-\mathrm{zeolite} $$

(6)

$$ 0.5{\mathrm{Ca}}^{2+}+{\mathrm{K}}^{+}-\mathrm{zeolite}\iff {\mathrm{K}}^{+}+0.5{\mathrm{Ca}}^{2+}-\mathrm{zeolite} $$

(7)

The first reaction shows a weak dissolution of the PR. In the second and third reactions, because of high CEC of zeolites, released Ca is adsorbed on the zeolite surface. Therefore, more PR will be dissolved with lowering Ca²⁺ activity in the solution. This system releases PO₄ ³⁻, NH₄ ⁺, and K⁺ ions. As a result of uptaking the released nutrients by plant roots, dissolution and ion exchange reactions will continue (Lancellotti et al. 2014). Although the P release rate from induced dissolution-ion exchange system may not be sufficient for intensive cropping systems, it is suitable to use where a low concentration of P must be maintained in soils (Pickering et al. 2002).

Clinoptilolite is one of the most abundant natural zeolites that has been used with the PR in several studies. A glasshouse study was undertaken to assess the enhancement of P uptake by sunflower after using clinoptilolite in combination with PR. Different saturated forms of clinoptilolite zeolites (Ca²⁺, K⁺, and NH₄ ⁺-saturated) were combined with PR at the ratios of 3.5:1 and 7:1. The P uptake in these treatments was lower than the treatment with soluble KH₂PO₄. However, the result clearly showed a great enhancement in plant uptakes of P from PR, when it was applied in a combination with NH₄ ⁺-zeolite (Pickering et al. 2002). In another study, untreated and treated (with NH₄ ⁺, H⁺, Na⁺) clinoptilolite were mixed with carbonate apatite at a ratio of 5:1 to examine the possibility of achieving a slow-release fertilization. The addition of all treated forms of clinoptilolite considerably increased the solution P concentration, in comparison to apatite without zeolite addition (Lai and Eberl 1986). Phillipsite and chabazite have also been used in induced dissolution-ion exchange systems as an SRF. In a study, phillipsite zeolite was combined with reactive and unreactive PRs at the ratios of 1:1, 1:10, and 1:100 and added to a sandy clay loam soil to evaluate its effectiveness to enhance the dissolution of PRs. Addition of zeolite had an insignificant effect on the P uptake from the unreactive PR. However, it approximately doubled in the soil containing phillipsite and reactive PR at the ratio of 1:100 (Mnkeni et al. 1994). Another research study reported that chabazite induced insoluble PO₄ ³⁻ dissolution, like phillipsite and clinoptilolite, when a mixture of natural and NH₄ ⁺ exchanged chabazite zeolite and animal bone ashes (as a P source) was used in comparison to animal bone ash without chabazite (Lancellotti et al. 2014).

The concentration of P compounds and their ratio to other nutrients in solutions are controlled by several factors including the ratio of zeolite to PR, type of zeolite and PR, type and equivalent fraction of cations such as K⁺ and NH₄ ⁺ on zeolite exchange sites, and pH. Increasing the zeolite to PR ratio would promote P solubilization and hence enhance plant growth. In this condition, more exchange sites are available for Ca²⁺ in the solution, thereby driving the PR dissolution (Pickering et al. 2002). For example, in an experiment that used an NH₄ ⁺ saturated clinoptilolite zeolite to induce the dissolution of PR (apatite), the results clearly revealed that both the rate and the quantity of the released P are increased with increasing zeolite/PR ratio (Fig. 10). The curves shown in Fig. 10 demonstrate that the zeolite systems have a more gradual release compared to mono-Ca PO₄ ³⁻ system, in which most of the P was released in the first few extractions (Lai and Eberl 1986). A similar trend of the P release was observed in a system containing different ratios of chabazite zeolite to animal bone ash (Lancellotti et al. 2014).

Fig. 10

Cumulative P released from zeolite systems (open circles) with different NH⁴⁺-zeolite to PR ratio (numbers next to the curve), compared to compared to a mono-Ca PO₄ ³⁻ system (solid circles) having an equivalent amount of P (Lai and Eberl 1986)

PR dissolution is enhanced by saturating zeolites with monovalent cations, such as NH₄ ⁺ and Na⁺, compared with zeolites saturated with Ca²⁺ or untreated natural zeolites which initially contain some exchangeable Ca²⁺. The results of an experimental study clearly revealed that P release rate from animal bone ash increased when NH₄ ⁺ pre-loaded chabazite was used, in comparison to natural chabazite (Fig. 11). Furthermore, in Fig. 11, it is obvious that the presence of NH₄ ⁺ in chabazite is more effective in promoting P release than the increase of the zeolite/PR ratio (Lancellotti et al. 2014). Changes in the relative charge fractions of cations such as NH₄ ⁺ and K⁺ affect the P release rate due to the ion selectivity of natural zeolites. For example, it was observed that NH₄ ⁺-loaded clinoptilolite increased the availability of P from reactive PR, while Ca²⁺- and K⁺-loaded zeolites did not increase the P availability of the PR. This observation can be attributed to more selectivity of clinoptilolite for K⁺ than for NH₄ ⁺ (Allen et al. 1993). P release is also increased in acidic soil media with lower pH. In a treatment containing the combination of the H⁺-saturated clinoptilolite and PR, P release was much higher than the other treatments (Lai and Eberl 1986). Furthermore, proton production through the nitrification process or root H⁺ efflux can promote PR dissolution (Pickering et al. 2002). Finally, an induced dissolution-ion exchange system is much more effective when reactive PRs are used than the non-reactive materials (Pickering et al. 2002).

Fig. 11

P concentration in each sample (left) and cumulative P released (right) after leaching treatment of specific solid mixtures (Lancellotti et al. 2014)

3.4 Other Ions Retention and Release in Soil

3.4.1 Effect of Zeolite on Potassium Retention and Release in Soil

Zeolite minerals have a pronounced selectivity for K⁺ over other ions such as NH₄ ⁺, Na⁺, Ca²⁺, and Mg²⁺. Hence, it is difficult to remove K⁺ from zeolite exchange sites by these less selective cations. In addition, the reported CEC of zeolite obtained using 1 M KCl and 1 M NH₄Cl was considerably higher than the values obtained using 0.1 M BaCl₂ and 0.1 M BaCl₂/NH₄Cl solutions (Phillips 1998). It is because of the preferred exchange of the natural zeolites with lower field strengths for cations having a low charge density such as K⁺ (Barrer et al. 1968; Colella 1996). The hydration spheres of high field strength cations prevent their close approach to the seat of charge in the zeolite framework. Therefore, cations of low field strength such as K⁺ are generally more tightly held and selectively exchanged by the natural zeolite than other ions (Barrer and Klinowski 1972; Chelishehev et al. 1988; Mumpton 1999).

Because of abovementioned characteristics of zeolites, numerous researches have been conducted to load zeolite sites with K⁺ and use it as SRFs. The plants can take the K⁺ from the loaded zeolite by an ion exchange reaction between the plant root hairs and the zeolite (Gonzales and Fuentes 1988). Using zeolite as K⁺ SRF leads to reduce the potential of K⁺ leaching to surface runoff and groundwater (Ming and Allen 2001). For example, the effect of zeolite application on K⁺ release in sandy soils amended with municipal compost was investigated. The results from column tests revealed that zeolite application reduced total K⁺ leaching up to six times, due to high K⁺ affinity for the mixture of soil-compost-zeolite (Moraetis et al. 2015). In another study, it was revealed that the use of clinoptilolite zeolite increased K⁺ contents in plant tissues and reduced its leaching. This increase in K⁺ uptake might be attributed to the high zeolite CEC, which leads to holding K⁺ in zeolite structure for slow release to the rhizosphere (Gül et al. 2005). A similar result was observed in a study, in which a soilless potting medium was fertilized with either K⁺-loaded clinoptilolite or KNO₃. About 10% of total K⁺ on zeolite exchange sites was leached out of the potting medium after collecting 3 L of leachate, while over 90% of total K⁺ was leached from the KNO₃ source (Hershey et al. 1980). Similarly, the addition of K⁺-saturated clinoptilolite to a soilless medium resulted in 23% less leaching of K⁺, compared with control substrate (Williams and Nelson 1997).

3.4.2 Effect of Zeolite on Sulfur Retention and Release in Soil

S is considered as the fourth major nutrient after N, P, and K. S is known for its role in the synthesis of proteins, vitamins, chlorophyll, oils, amino acids and flavored compounds in plants. Although both organic and inorganic forms of S can be found in soils, the organic form constituting more than 95% of the total S (Thirunavukkarasu and Subramanian 2014). However, many plants assimilate S in SO₄ ²⁻ form (Hawkesford and De Kok 2006; Li and Zhang 2010). The SO₄ ²⁻ fertilizers are readily available for plant uptake, following their dissolution. However, as they are mobile relatively large concentration of SO₄ ²⁻ in soils over a short period are likely to be lost by leaching or surface runoff.

Because SO₄ ²⁻ is a negatively charged species, zeolite minerals do not have high affinity to this ion. Nevertheless, it is reported that surfactant-modified clinoptilolite zeolites, using HDTMAB, possess the required properties to be used as carriers for SRF. The results of a study revealed that after leaching with 50 pore volumes, 70 and 85% of the pre-loaded SO₄ ²⁻ still remained on the zeolites modified to 150 and 200% ECEC, respectively. In addition, the initial SO₄ ²⁻ concentration in the leachate of SO₄ ²⁻-loaded SMZ was decreased by a factor of three, in comparison with the mixtures of zeolite and same loading of soluble SO₄ ²⁻ (Li and Zhang 2010). In addition to clinoptilolite, Epistilbite zeolite was modified by HDTMAB to develop nano-zeolite based nano-S fertilizer. The results exhibited that SO₄ ²⁻ supply from SO₄ ²⁻-loaded surface modified nano-zeolite was available even after 912 h of continuous percolation, while SO₄ ²⁻ from (NH₄)₂SO₄ was depleted within 384 h. This slow release of SO₄ ²⁻ from surface modified nano-zeolite might be attributed to the huge number of pores, cages and channels in the zeolite structure, which hold SO₄ ²⁻ (Thirunavukkarasu and Subramanian 2014).

4 Conclusion

Rapid population growth and urbanization have restricted the agricultural area to have more crop yield and food production. On the other hand, more efficient agricultural activities need higher nutrient application rate and more efficient water irrigation systems. However, if a high rate of nutrients is applied to soils, a noticeable percentage of it might be washed out, which pollutes water resources as well as undermines product yields. Thus, an inexpensive, pervasive and green solution is essentially required to improve the crop yielding. Zeolites have been widely used in different investigations to enhance agricultural efficiencies.

Zeolites have some impacts on soil physical behaviors. Decreasing soil bulk density and increasing soil porosity are the outstanding impacts. These effects along with high internal pore volume within their structure can efficiently improve water holding capacity. Furthermore, the open network of the zeolites structure can lead to formation of new routes for water movement, which can consequently improve infiltration rate and saturated hydraulic conductivity.

The exceptional chemical and physical properties of natural zeolites, especially high cation exchange capacity and strong affinity for NH₄ ⁺ and K⁺, can be exploited to maximize the nitrogen use efficiency in agricultural applications. Although almost all of naturally occurring zeolites and most of the artificial zeolites do not have high affinity for anionic fertilizers such as NO₃ ⁻, PO₄ ³⁻, and SO₄ ²⁻, by modifying their surface chemistries using cationic surfactant, multifunctional adsorbents with capability to trap anions and non-polar organics can be obtained. However, further studies should be conducted prior to any commercial applications to evaluate the risk of leaching toxic surfactants that are loosely attached to the zeolite surface.

The ion exchange properties of natural, synthetic, and modified zeolites can be exploited in the preparation of slow-release fertilizers for nitrogen, potassium, and sulfur. In addition, controlled and gradual release of phosphorous are achievable by a combination of zeolite ion exchange and mineral dissolution (like phosphate rock).

Generally, the impacts of zeolite application on soil physical and chemical properties depend on different experimental conditions, most importantly including zeolite type and application rate, method of the application, soil texture and structure, zeolite particle size and density, as well as water salinity. Zeolite amendments may influence coarse-textured soils to a greater extent than fine-textured soils.