Keywords

12.1 Introduction

The current world population is ~7.3 billion, and it will further increase to 8.5 and 9.7 billion during 2030 and 2050, respectively, and this is expected to be stabilized at ~11.2 billion by the end of the twenty-first century (UNDESA 2015). This additional three to four billion people will require extra food grain production, from shrinking land and water resources, to ensure food and nutritional security (Sulieman and Tran 2015). Increase in quality food grain production under intensive agricultural management practices would only be possible by doubling the use of energy and fertilizers consumption (Sulieman and Tran 2015). Food grain production increased manyfold at global level during the twentieth century, and a tremendous yield increase was observed due to the increase in the net cultivable area. Further, use of short-duration high-yielding varieties, synthetic fertilizers, and pesticides is inevitable (Sihag et al. 2015). This approach has resulted in gradual degradation of soil organic matter (SOM) because of the breakdown of stable soil aggregates and decomposition of organic matter (OM). Consequently, soil health is deteriorated in terms of reduction in water holding capacity (WHC) of soils, surface and groundwater pollution, and multiple nutrient deficiencies (Gill et al. 2008; Meena et al. 2013). Soluble nutrient is provided by synthetic fertilizers for crop production that are easily vulnerable to loss, if soils and irrigation water are not properly managed. Moreover, higher application of synthetic fertilizers has led to an imbalance of the nutrient cycle, particularly N, illustrated by the growing accumulation of several reduced (NH3) and oxidized (NOx, N2O, NO3 −) forms (Fagodiya et al. 2017), causing water pollution (NO3 −), air pollution (NOx), and climate change (N2O) (Galloway et al. 2003). There are severe concerns about sustainable soil productivity, and today most of the countries have moved into a post-green revolution phase and are facing the problem of stagnation or declining crop productivity. Hence, both farmers and researchers have opted for conservation agriculture practices, resources conservation, and use of green manuring into the farming system to enhance further food grain production while maintaining soil health (Meena and Majumdar 2016; Meena et al. 2016a, b, c).

It has been widely reported that leguminous green manure crops play an important role in soil health management (Whitbread et al. 2000) and recently received higher attention for improving soil fertility and agricultural sustainability (Ray and Gupta 2001; Fageria 2007). Green manuring is the practice of incorporation of undecomposed fresh/dry plant material into soils, both either in place or brought from a distance (Pieters 1927). In addition to this, green manure legume crops also fix atmospheric N biologically. Biological nitrogen fixation (BNF) is a microbiological process in which atmospheric N2 is converted into a plant-usable form, which offers an economically attractive and ecologically sound option of reducing external inputs and improving internal resources (Sulieman and Tran 2014).

Legume green manuring (LGM) can enhance agriculture sustainability by improving nutrient retention (Dinnes et al. 2002), enhancing soil fertility (Fageria and Baligar 2005), by decreasing soil erosion (Smith et al. 1987), and reducing global warming (Robertson et al. 2000). LGM also has a major role to improve the SOC pool, thereby improving soil physicochemical and biological properties (Fageria and Baligar 2005; Fageria 2007). The incorporation of legume green manure crops into soil releases organic substances like organic acid, amino acids, sugars, vitamins, and mucilage (Shukla et al. 2011) during crop growth as well as after decomposition. These substances are capable to bind soil particles together and form better soil aggregation (MacRae and Mehuys 1985), resulting in increased hydraulic conductivity (MacRae and Mehuys 1985; Boparai et al. 1992), water holding capacity (WHC), water infiltration (Raimbault and Vyn 1991), and total pore space (Anderson et al. 1997) of the soil. Further, the green manure incorporation provides carbon (C) and energy to soil biota required for OM decomposition and nutrient recycling (Griffin and Garren 1976; Hu et al. 2006). In the process of LGM, soil pH is changed by addition of OM (Singh et al. 1992). The addition of organic amendments into soils, particularly green manure, has potential to control weeds and soil-borne diseases and to disrupt the life cycle of agriculture pest (Kumar et al. 2014; Varma et al. 2017). Green manure is important to small-scale farmers, for whom it is difficult to buy expensive mineral fertilizers (Meena et al. 2014). Therefore, legume green manure crops have great potential for sustainable food grain production. Keeping above facts in mind, the objective of this chapter is to provide information on the LGM for sustainable soil management and crop production.

12.2 Legumes as Green Manure

The green manuring practices are of two types: (a) in situ green manuring crops and (b) ex situ green leaf manuring (GLM) (Singh et al. 1991). In in situ green manuring, short-duration (~45 to 60 days) crops are grown and incorporated into soil at the same site. In ex situ green manuring, foliage and tender parts of green manuring crops collected from nearby forests, shrubs, and trees are incorporated into the soil at 15–30 days prior to the sowing of main crops (SSSA 1997).

There are many crops which can be used for green manuring; however the selection of the green manure crop depends upon several factors like the prevailing climatic conditions, cropping system practiced, availability of seed, and other factors including local habits and prejudices. The legume crops have an edge over nonlegume crops due to ability of fixing atmospheric N (Rao 2014). LGM is categorized under following categories:

  1. (a)

    Grain legumes: pigeon pea (Cajanus cajan), green gram (Vigna radiata), soybean (Glycine max), or groundnut (Arachis hypogaea)

  2. (b)

    Multipurpose perennial legume trees: subabul (Leucaena leucocephala), gliricidia (Gliricidia sepium), and kassod tree (Cassia tora)

  3. (c)

    Non-grain legume crops: sunn hemp (Crotalaria juncea), dhaincha (Sesbania rostrata), centrosema (Centrosema acutifolium), stylo ( Stylosanthes guianensis ), and desmodium (Desmodium ovalifolium) (Palaniappan 1994)

The most commonly grown LGM crops are sunn hemp, dhaincha, berseem (Trifolium alexandrinum), and green gram. The leguminous crops, namely, cowpea (Vigna unguiculata), green gram, black gram (Vigna mungo), pigeon pea, chick pea (Cicer arietinum), black lentil (Lens culinaris), pea (Pisum sativum), lathyrus (Lathyrus latifolius), kidney beans ( Phaseolus vulgaris ), tephrosia (Tephrosia purpurea), groundnut, soybean, dhaincha, and sunn hemp; woody legumes, namely, subabul, gliricidia, karanj (Pongamia glabra), and delonix (Delonix regia), have the ability to fix the atmospheric N in their root nodules. Legume crops and tree species suitable for green manuring in different agroclimatic zones are mentioned in Table 12.1. Dry matter accumulation by these legume crops may vary from 1 to 10 Mg ha−1 year−1 under ideal soil and environmental conditions, and the total N accumulation in the aboveground biomass ranges from 0.02 to 0.3 Mg ha−1 year−1 (Lathwell 1990). Besides BNF, a green manure crop should have some desirable characteristics, viz., fast-growing habit, short duration, early onset of BNF, high N accumulation rate, high tolerance to biotic stresses (pest and disease), abiotic stresses (flood, drought, salinity, and adverse temperatures), wide range of ecological adaptability, timely release of nutrients, photoperiod insensitivity, high seed production, higher seed viability, and most importantly easiness in incorporation (Meena et al. 2015a).

Table 12.1 Leguminous green manure crops for different regions
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12.3 Biological Nitrogen Fixation in Legume Green Manuring

BNF is the process of conversion of atmospheric N2 into ammonia (NH3) or other molecules which are easily available to plants and other living organisms into the soil (Postgate 1998). The BNF and mineralization of leguminous green manure crop in soil are depicted in Fig. 12.1. Overall reaction of BNF is given below.

Fig. 12.1
figure 1

N fixation and mineralization of leguminous green manure crop in soil

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$$ {\mathrm{N}}_2+8\ {\mathrm{H}}^{+}+8\ {\mathrm{e}}^{-}\to 2\ {\mathrm{N}\mathrm{H}}_3+{\mathrm{H}}_2 $$

In many studies, it has been proven that the BNF is the most efficient process or way to supply the major amount of N required by legume crops or plant (Sulieman and Tran 2014) (Table 12.2). After sowing of the legume seed, soon after its germination, the inherent Rhizobium present in soil are externally added as seed inoculums and later on enters into the root hairs of legumes and moves through infection thread toward the main root. After invasion, these bacteria multiply rapidly in the root of legumes, which result into swelling of root cells to form nodules. The atmospheric N around the root hairs of legumes is “fixed” by binding it to other elements and converting it into a plant available ammonical form. The Rhizobium bacteria use carbohydrates as a source of hydrogen in the conversion of atmospheric N to ammonia (Fageria 2007), and this symbiotic association of legume with Rhizobium contributes 40% of world total N fixation (Ladha et al. 1992; Meena et al. 2015a). A detailed survey of the literature showed that on an average, grain legume crops could fulfill 50–80% of own N requirement through BNF. BNF through legume crop cultivation in agriculture adds 33 Tg N year−1, of which symbiotic association of Rhizobium with seed legume crops, forage leguminous cover crops, non-Rhizobium N-fixing bacteria, cyanobacteria in rice, and endophytic N-fixing organisms in sugarcane contribute 10, 12, 4, 6, and 3 Tg N year−1, respectively (Smil 1999).

Table 12.2 N accumulation in major leguminous green manure crops
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Ladha et al. (1988) reported an average accumulation of 2.6 kg N ha−1 day−1 under different legume green manure crops. Incorporation of such high N-fixing legume crops at 45–65 days of growth stage into soils results in rice yield equal to the application of N fertilizers at 50–100 kg N ha−1. Similarly, Ladha et al. (1988) reported that 45–60 days old dhaincha species could fix N equal to 200 kg N ha−1. Dhaincha legume crop age of 55 days fixed about 303 kg N ha−1 (at 5.5 kg N day−l) and 383 kg N ha−1 (at 6.96 kg N day−l) without and with inoculation of Azorhizobium bacteria, respectively (Ladha et al. 1988). The N supplied by hairy vetch (Vicia villosa) and crimson clover (Trifolium incarnatum) in cover crop experiments ranged from 72 to 149 kg N ha−1 (Hargrove 1986; Ladha et al. 1988; Holderbaun et al. 1990). Thus, it is clear that the amount of N fixed by different legume species varies, and it depends on the legume species, its variety, the number of effective root nodules, type of soil, agronomical and water management practices, and prevailing climatic conditions and their interactions with other factors (Buresh and De Datta 1991; Fageria and Baligar 2005). Sharma and Ghosh (2000) evaluated dhaincha as an intercrop with direct-seeded rice as well as incorporated pure dhaincha before transplanting of rice under flood-prone lowland conditions and found that dhaincha accumulated 80–86 kg N ha−1 in pure stand and 58–79 kg N ha−1 when intercropped with direct-seeded rice in alternate rows at 50 days of growth.

12.4 Nutrient Composition of Legume Green Manuring

LGM crops incorporation into the soil improves essential plant nutrients for succeeding crop (Bhuiyan and Zaman 1996). Legume green manuring crops, having low C/N ratio (high N), may behave like the chemical nitrogenous fertilizer application, when compared to the other crops used for BNF. However, the mineral composition and N content of legume may vary considerably depending upon the species (Singh and Bhattacharyya 1989; Verma et al. 2015a), crop growth duration, and growth condition. With increased crop growth duration, carbonaceous content (carbohydrate and cellulose) increases and nitrogenous content (amino acid and protein) decreases leading to increase in C/N ratio of crops which results in slower decomposition of plant material (Ishikawa 1988). To overcome this problem, green manure crops should be incorporated at the flowering stage. The N and C content in roots, shoots, and leaves may also vary. In general, leaves contain lower C/N ratio as compared to stem and roots (Palm et al. 1988). In addition to N and C content, green manuring crops may also contain considerable amount of other nutrient and trace elements. A study conducted by Bhuiyan and Zaman (1996) under protected cultivation in greenhouse showed that cowpea has a higher mineral composition than dhaincha under wetland rice. The nutrient composition of dhaincha and cowpea is depicted in Fig. 12.2.

Fig. 12.2
figure 2

Nutrient composition and supply of nutrient by 8-week-old green manuring crops. (Data source: Bhuiyan and Zaman 1996; Singh et al. 1992)

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12.5 Decomposition and Mineralization

Incorporation of legume green manure into soil undergoes decomposition and mineralization process (Fageria 2007; Meena et al. 2018). Decomposition is a biological breakdown and transformation of complex organic compounds into simpler organic and inorganic molecules (Joffe 1955; Fox et al. 1990).

$$ \mathrm{Complex}\ \mathrm{organic}\ \mathrm{molecules}+{\mathrm{O}}_2=\mathrm{Nutrients}+{\mathrm{H}}_2\mathrm{O}+{\mathrm{CO}}_2+\mathrm{Energy} $$

There are mainly two processes, namely, mineralization and immobilization (Gil and Fick 2001; Dinnes et al. 2002) as shown in Fig. 12.3.

  • Mineralization is the main process of conversion of organic carbon (OC) and N into the mineral form, i.e., CO2 and NH4 + by microorganism. During this process, nutrients are released to soil solution and are made available to plants.

  • Immobilization is the reverse process of mineralization in which the mineral form of C and N fixed into the organic form.

Fig. 12.3
figure 3

Mobilization and immobilization of nutrient in soil

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Decomposition and mineralization of green manures are affected by the type of soil, crops, crop growth stage at which it is incorporated, and prevailing climatic conditions (Fageria and Baligar 2005; Reddy 2016). Among these, the dominant factors are the quantity and quality of green manure crops. Soil factors, which affected the decomposition and mineralization of green manure, are the soil texture, structure, soil reaction, microbial activity, and the status of soil nutrients (Thonnissen et al. 2000; Dinnes et al. 2002; Dhakal et al. 2015). Decomposition and mineralization of OM mainly depend upon the availability of N in soil (Joffe 1955; Gil and Fick 2001). Due to low C/N ratio, lower lignin content, and high quantity of easily decomposable material, LGM generally leads to rapid and fast mineralization as compared to cereal residues (Janzen and Kucey 1988; Gil and Fick 2001). Decomposition rate of green manure legume was higher in sandy soils than the fine-textured soils (Verbene et al. 1990). Soybean residues incorporation into soils lost 68% of biomass within 1 month of incorporation (Broder and Wagner 1988). The incorporation of hairy vetch legume green manure rapidly released N within 15 days after incorporation (Varco et al. 1989). Legume residue incorporation into soils under field conditions led to that of the <30% of legume N recovered by a subsequent nonlegume cereal crop, and a major amount of it is retained in soil as organic forms of N (Ladd et al. 1983; Harris et al. 1994). Fractional C and N release was greater in dried and rewetted soil for green manuring legumes compared to continuously moist soil for all parts except for nodules (Franzluebbers et al. 1994). The NH4-N release and its accumulation increased significantly with a decrease in soil water; however there was a decrease in the release of NO3-N (Brar and Sidhu 1995). Soil temperature also has an effect on the pattern of N release during decomposition of added green manure residue in the soil (Brar and Sidhu 1997). The suitable range of soil temperature and moisture are 20–30 οC and −0.01 to −0.05 MPa for faster decomposition of green manure into soil and subsequent release of nutrients (Cassman and Munns 1980; Sinha et al. 2009). Magid et al. (2001) reported higher N mineralization of both black medic and white sweet clover occurred at low temperature. Thonnissen et al. (2000) indicated that soybean and Indigofera decomposed rapidly and lost ~70% of the biomass in 1 month after incorporation.

12.6 Legume Green Manuring and Soil Properties

Modern agriculture, having intensive cropping system coupled with frequent tillage of the soil, higher doses of chemical fertilizers, and overlooking of the application of organic manures, led to the breakdown of soil aggregates and destruction of SOM. This further led to soil physicochemical and biological health degradation. Due to faulty management practices, the agricultural land is becoming less productive day by day. The adverse effects such as soil compaction, reduction in SOM, and reduced crop productivity have been recognized in many areas over several years (Unger and Kaspar 1994; Meena et al. 2016a). Green manuring with legumes also had a magnificent role of providing OM into the soil, thereby improving the physicochemical properties as well as biological properties of soil (Ebelhar et al. 1984; Fageria and Baligar 2005). The role of leguminous green manuring on soil physical, chemical, and biological properties is depicted in Fig. 12.4.

Fig. 12.4
figure 4

Soil properties influenced by leguminous green manuring

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Besides the improvement of soil health, LGM helps in reducing insect-pest, disease incidence, and weed management (Kumar et al. 2014; Verma et al. 2015), acts as binding material in soil, and helps in improvement of soil structure (Schutter and Dick 2001). LGM between successive crop growth increased SOM (Pung et al. 2004) which stimulates the soil microbial activity and mineralization of plant nutrients (Eriksen 2005). Therefore, it enhances the soil quality and its fertility (Doran et al. 1988).

12.6.1 Soil Physical Properties

The continuous use of LGM results in buildup of OM in soil, which improves the soil physical properties (Table 12.3) and quantities of organic acid, amino acids, sugars, vitamins, and mucilage (Shukla et al. 2011). These organic substances are capable of binding the soil particles and better soil aggregation (MacRae and Mehuys 1985) which led to better hydraulic conductivity (Boparai et al. 1992) and improved drainage by increasing infiltration and percolation (Raimbault and Vyn 1991). The infiltration rates are mainly controlled by bulk density (pb) and aggregate stability; higher infiltration rate reduces soil erosion (Martens and Frankenberger 1992). The LGM improves total pore space by decreasing the soil pb, enhanced root development, soil water content, and nutrient use efficiency (Anderson et al. 1997).

Table 12.3 Physical properties of soil affected by LGM under rice-wheat system
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The SOC is a more suitable index for crop production in arid and semiarid regions, as it has positive effects on the WHC (Dias-Zorita et al. 1999). The penetration of green manure roots, particularly dhaincha and sunn hemp in soil, holds it in place and prevents soil erosion (Schumann et al. 2000). Green gram and sesbania green manuring reduced the soil pb to the extent of 0.03–0.07 Mg m−3 and enhanced the mean weight diameter of soil aggregates in the upper layer of soil when compared to fallow soils during the growth of rice and wheat (Mandal et al. 2003). In sandy soils, green manure helps to hold more water by reducing drainage rate through improvement in soil physical properties (Selvi and Kalpana 2009; Yadav et al. 2017).

12.6.2 Soil Chemical Properties

Green manuring have effects on soil chemical properties, and particularly soil pH is reduced, (a) by addition of organic acids and generation of CO2 during OM decomposition, resulting in reduced soil pH of furnishing proton by organic manure; (b) simultaneously soil pH is increased by reduction of H+ by organic anions to H2O and CO2 during mineralization of organic manure (Singh et al. 1992; Buragohain et al. 2017). Reduction of organic substances may reduce Fe and Mn oxides causing soil pH to rise (Meena et al. 2017), because of oxidation of Fe and Mn consumed proton generated during decomposition of OM. The 12 year long-term experiments conducted by Yadav and Singh (1986) indicated that soil pH reduced with the time under green manuring. Soil pH after 12 years of uninterrupted sugarcane crop cultivation was 8.0 under green manuring and 8.5 under control treatments, respectively. Green manures also prevent nutrients from being washed out from the soil. The nutrients are drawn up by the green manure crop from deep soil layers and held inside the plant and recycled back to soil upon decomposition. Organic materials, acting as a slow-release source of N, are expected to more closely match N supply and N demand of crops, and this could reduce N losses (Becker et al. 1994). In flooded soils condition, the average N loss from applied green manure and split-applied urea were 14% and 35%, respectively (Becker et al. 1995). Thus, N from green manure crops contributes significantly lower to air and water pollution compared to urea application.

Green manure added around 50–60 kg N ha−1 to the soil for the succeeding crop of rice (Singh and Bhattacharyya 1989). The leaching losses of green manure N are normally expected to be smaller than inorganic fertilizers, because it must be mineralized before it can be lost via leaching (Singh et al. 1992), and make it available to succeeding main crop, thereby reducing needs of N fertilizer (Stute and Posner 1993). The NO3 − leaching from nonlegume manuring ranged 29–94%; while, from legume manuring, it was 6–48%, showing that green manuring with legume crop of hairy vetch had more potential of reduction of NO3 − leaching than nonlegume crops (rye, Secale cereale) (Sainju and Singh (1997). Green manuring enhanced the availability of phosphorus (P) from added rock phosphate in rice crop (Cavigelli and Thien 2003). Hundal et al. (1992)and Bah et al. (2006) evaluated the contributions of different green manures to P nutrition in rice crop in soils of arid tropical climate and found that the utilization of phosphorous fertilizers had markedly enhanced it from 3% to 39% in treatments having green manure.

The integrated effect of legume green manure crops with mineral fertilizers improved SOC, nutrient availability, intake of nutrients by crops, and yield of rice-wheat system (Kumar and Prasad 2008). The incorporation of dhaincha at flowering stage adds about 60–90 kg ha−1 N into soil (Pandey et al. 2008), and it also helps improve the soil chemical properties. Legume green manure crops prevent nutrient leaching, decrease weed growth, and reduce the harmful effect of agrochemicals and soil-borne phytopathogens (Kumar et al. 2010; Dhakal et al. 2016).

12.6.3 Soil Biological Properties

The decomposition of green manures serves major functions for microflora providing both C and energy for growth and formation of new cell material, which further multiplies its colony saprophytically on the decomposing OM (Ye et al. 2014). A large number of soil microorganisms exist in the soil as long as there is a C source for energy (Kumar et al. 2014). Soil-inhabiting microorganisms are very critical for decomposing organic residues and recycling soil nutrients (Akpor et al. 2006). The process of decomposition is of great significance because unless the energy and nutrients are released through microbial activity, the primary product cannot exist for a long time (Kumar et al. 2014).

The LGM has two main positive points from the microbiological point of view: (a) primarily it provides nutrient-rich OM for the microbial community which easily converts organically bound nutrients in plant residues to easily available nutrient form to the crops; (b) secondly it enhances the biodiversity of soil microorganisms. This microbial diversity can be increased by incorporating different legume green manure in crop rotation and cropping system programs (Schutter and Dick 2001; Eriksen 2005; Kumar et al. 2016). Since the begining of agriculture, it has been found that legume green manures and other organic amendments improve the soil tilth ability and fertility (MacRae and Mehuys 1985). The increase in beneficial microbial community and its activity is most often directly related to an overall increase in soil organic matter. Sikora and Stott (1996) and Griffin and Garren (1976) studied the colonization of Aspergillus flavus and Aspergillus niger fungi in different soil textures in deep-plowed, decomposing rye, a green manuring crop in the soils of groundnut field. The greater colonization of A. flavus was reported, in heavy textured soil adjacent to rye (Secale cereal) and groundnut crops i and A. flavus population was as high as 165 propagules g−1 soil adjacent to rye.

12.7 Effect of Legume Green Manuring on Crop Yield

The positive impact of legume green manuring on SOM and other soil properties associated with increase in nutrition to growing crop is well reflected on grain yield. It was found that the average yield of rice grain increased by 1.7 Mg ha−1 in green manuring treatments, over controlled plots. The increase in rice yield ranged from 0.5 to 3.3 Mg ha−l. The average application of N @ 80 kg N ha−l, green manure N shows an agronomic N use efficiency (NUE) of 20 kg rice grain increased kg−l N applied (IRRI 1990). Besides this, green manures supply N constantly, and due to slow release of N from the green manure incorporation in soils, this would match with the requirement of N by plants resulting in improved crop performance (Westcott and Mikkelson 1987). The yield response of high-yielding varieties of rice crop to different green manuring crops in India ranged from 0.65 to 3.1 Mg ha−1 (Singh et al. 1991). Ali and Narciso (1993) compared NUE in long-term fertilizer experiments conducted in India, Indonesia, and the Philippines by the International Network on Soil Fertility and Fertilizer Evaluation (INSURF). The NUE of lowland rice is higher in the case of green manuring when compared to mineral fertilizer N (Peoples et al. 1995). In general, the linear relationship of NUE and N application rate does not differ significantly among different N sources, but it tends to decrease more and more with a higher application rate of green manuring (Becker et al. 1995). The highest rice production was recorded in dhaincha green manuring and it was at on par with 20 Mg ha−1 of FYM (Misra et al. 1996). To find out the relationship between the rate of application of dhaincha green manure and rice yield, a fie1d experiment was conducted at Bangladesh Rice Research Institute, Regional Station, Barisal. The rice grain yield showed a quadratic relation to the added dhaincha green manure, and maximum rice yield was obtained at ~6 Mg ha−1.

The effect of dhaincha green manuring on crop growth and yield of direct seeded rice and transplanted rice under intermediate water stagnation condition was studied by Sharma and Das (1994) and Meena et al. (2016b). They found that the highest yield of the rice crop was observed when rice and dhaincha were grown at a 2:1 ratio as 20 cm row-to-row spacing. The increase in rice yield under dhaincha green manuring was attributed to greater panicle weight, which was probably due to a continued supply of N following decomposition of added dhaincha OM. The dhaincha green manuring intercropped with direct seeded rice and the conventional practice of before transplanting incorporation of dhaincha green manuring was compared under lowland flood-prone conditions. It was found that the grain yield of direct seeded rice was significantly higher when 20 kg N ha−1 was applied at sowing and dhaincha was incorporated at 50 days of growth (Sharma and Ghosh 2000).

Bokhtiar et al. (2003) found that dhaincha and sunn hemp green manuring and supplemented urea increased yield of subsequent sugarcane by up to 57%. Besides this, there was a significant increase in SOM, total N, available P, and S of the soil. Comparison of the relative efficiency of green manures with inorganic N sources by Selvi and Kalpana (2009) revealed that the cowpea green manuring application at 0.17% (3.4 Mg ha−1) and dhaincha green manuring at 0.36% (7.2 Mg ha−1) on a dry matter basis would be more than enough to produce a yield of rice crop equal to that obtained with the application of the recommended dose of fertilizers, i.e., at 80-25-35 kg/ha N-P-K. Further, demonstration of rice grain yield response showed that the cowpea was a better green manure crop than dhaincha mainly for wetland rice. In addition, substantial residual effects of cowpea and dhaincha green crops were also observed and in some cases even up to two to three successive rice crops (Bhuiyan and Zaman 1996; Selvi and Kalpana 2009; Meena 2013).

Pooniya et al. (2012) conducted a field experiments with summer green manuring crops, namely, green gram, cowpea, and dhaincha, it was found that that the highest crop residue was added by dhaincha, i.e., 38.56 Mg ha−1, which in turn led to the recycle of 180.5, 22.6, and 267.8 kg N, P, K ha−1, respectively. Further dhaincha incorporation also led to a significantly higher yield of succeeding basmati rice crop, and it was 2.38%, 4.14%, and 10.82% higher over cowpea, green gram, and summer fallow, respectively.

12.8 Limitations of Legume Green Manuring

In spite of the wide range of literature which reveals associated benefits of legume LGM, its applicability still remains in the research farms. Their adaptation by farmers is still not a common practice due to lack of awareness and some limitations at farm levels. There are few cases to cite. Haryana Government promotes LGM in rice-wheat areas of state. Becker et al. (1995) reviewed several available literatures (Ladha et al. 1992; Ali and Narciso 1993; Garrity and Flinn 1988; Ashoka et al. 2017) and identified the following limitations of green manuring and the possible reasons behind it. They are listed below:

  1. (a)

    Establishment and incorporation of green manuring crops are relatively costly.

  2. (b)

    Narrow window period between the two crops for growing and incorporating green manure crops during most of the cropping season.

  3. (c)

    Green manure crop, if not incorporated at proper growth stage and time, may lead to immobilization of N on a temporary basis.

  4. (d)

    Being a high water requiring crop, it may not be suitable for dryland agriculture.

  5. (e)

    Problems of decomposition of green manuring in the sowing of the following crop if proper moisture is not available, particularly in semiarid regions (Aase et al. 1996).

  6. (f)

    No visible economic benefits are seen during initial few seasons of green legume manuring.

  7. (g)

    Easy availability of fertilizers and their ease of application in comparison to green manuring.

  8. (h)

    Prices of mineral fertilizers are relatively low when compared to the high price of land and labor.

12.9 Conclusion

The ever increasing human population poses a burden on soils to produce more food. The intensive use of agrochemicals is threatening the soil sustainability. Use of chemical fertilizers in intensive cropping systems may lead to degradation of natural resources, particularly soils. These degraded soils will not be fit for profitable agriculture. Therefore, incorporation of legume green manure crops into the soil is emphasized for crop production. In ancient times, also legumes were recognized as suitable crops for green manuring to improve soil health and crop productivity. In addition to fixing of atmospheric N, it helps in conservation of soil water and reduction of soil erosion. LGM is to be considered superior over the nonlegume crops due to a higher N content and lower C/N ratio, helping in easy decomposition of OM and mineralization for nutrient release at a faster rate. Besides this, it also reduced the N immobilization risk for succeeding crops. Therefore, practices of LGM have a large scope for inclusion, to make the farming system more sustainable.

12.10 Future Perspectives

Future research for legume green manures is needed on following topics:

  1. (a)

    The studies across the globe have established the benefits of green manures on soil physicochemical and biological health. However, their application to the farm level is still at a limited stage because of lack of awareness and suitability under particular environments and cropping system.

  2. (b)

    There is need to develop a location-specific cropping system with compatible legume green manure crop either partly or in the window period between two crops.

  3. (c)

    Benefits of green legume manure need to be quantified regarding fertilizer saving, water saving, increase in crop productivity, and more importantly soil health improvement, so that the extra cost involved in raising the green manure crop may be justified to the ultimate adapter, the farmers.

  4. (d)

    Efforts are to be directed to find out alternative techniques like brown manuring of legume crops by growing them as intercropped, which can save time as well as need of incorporation. Such easily acceptable techniques are to be devised.