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10.1 Introduction

Potassium is a major and essential plant macronutrient and the most abundantly absorbed cation in higher plants. Potassium (K) plays an important role in the growth, metabolism, and development of plants. It activates plant enzymes, maintains cell turgor, enhances photosynthesis, reduces respiration, helps in transport of sugars and starches, helps in nitrogen uptake, and is also essential for protein synthesis. In addition to plant metabolism , potassium improves crop quality because it helps in grain filling and kernel weight, strengthens straw, increases disease resistance against pest and diseases, and also helps the plant to withstand stress. Without adequate potassium, the plants will have poorly developed roots, grow slowly, produce small seeds, and have lower yields. With the introduction of high-yielding crop varieties and hybrids during green revolution and with the progressive intensification of agriculture, the soils are getting depleted in potassium reserve at a faster rate and available soil K levels have also dropped due to leaching, runoff, and erosion (Sheng and Huang 2002a). As a consequence, potassium deficiency is becoming one of the major constraints in crop production, especially in coarse-textured soils and even in fine-textured soils. Therefore, crops do respond to K fertilization in soils .

Potassium deficiency is not as wide spread as that of nitrogen and phosphorus. However, many soils which were initially rich in K have become deficit due to heavy utilization by crops. Potassium deficiency symptoms usually occur first on the lower leaves of the plant and progress toward the top as the severity of the deficiency increases. One of the most common signs of potassium deficiency is the yellow searching or firing (chlorosis ) along the leaf margin. In severe cases of potassium deficiency, the fired margin of the leaf may fall out. Potassium deficient crops grow slowly, have poorly developed root systems, stalks are weak, and results in lodging of cereal crops. Long before the symptoms of K deficiency become visible, severe losses in terms of yield and quality could be caused to the crop (Khanwilkar and Ramteke 1993).

In recent years, there is a growing awareness regarding the importance of potassium in crop production in several parts of India. Potassium level has declined in different kind of soils over the years due to intensive cultivation and imbalanced fertilizer application. Therefore, application of potassium fertilizer to these soils gives positive response. India ranks 4th in consumption of potassium fertilizers after the USA, China, and Brazil as far as the total consumption of K fertilizers in the world is concerned (FAI 2007). Because, there is no reserve of K-bearing minerals in India for production of commercial K fertilizers, therefore, whole consumption of K fertilizers are imported in the form of muriate of potash (KCl) and sulfate of potash (K2SO4). On an average, 1.7 million tonnes of K is being imported annually in India (Anonymous 2003).

Other major essential macronutrients required for plant growth and development such as nitrogen (N) and phosphorus (P) are provided through application of nitrogenous and soluble phosphatic fertilizers. These chemical fertilizers are applied at high recommended doses, which cause environmental and economic problems (Brady 1990; Xie 1998). This necessitates the search to find an alternative indigenous source of P and K for plant uptake and to maintain K status in soils for sustaining crop production. Thus, direct application of rock phosphate and rock potassium materials may be agronomically more useful and environmentally safer than soluble P and K applied as chemical fertilizers (Rajan et al. 1996). However, P and K nutrients from rock materials are not readily available to the plant because these minerals are released slowly in the soil (Zapata and Roy 2004). Therefore, isolation and identification of microbial strains that are capable of solubilizing potassium minerals quickly in large quantity can conserve our existing resources and may avoid environmental pollution hazards caused by heavy application of chemical fertilizers.

10.2 Potassium Availability in the Soil

Among the major plant nutrients, potassium is most abundant in soils. It is also the seventh most common element in the earth crust and on an average, the surface layer (lithosphere) contains 2.5 % potassium. However, actual soil concentrations of this nutrient vary widely ranging from 0.04 to 3.0 % (Sparks and Huang 1985). Plants can take up potassium only from the soil solution and its availability is dependent upon the K dynamics as well as on total K content. There are three forms of potassium found in the soil, viz., soil minerals, nonexchangeable, and available. Soil minerals make up more than 90–98 % of soil potassium (Sparks and Huang 1985; Sparks 1987). It is tightly bound and most of it is unavailable for plant uptake. The second is nonexchangeable potassium which acts as a reserve to replenish potassium taken up or lost from the soil solution. It makes up approximately 1–10 % of soil potassium and consists predominantly of interlayer K of non expanded clay minerals such as illite and lattice K in K minerals such as K-feldspars. Nonexchangeable K can also contribute significantly to the plant uptake (Memon et al. 1988; Sharpley 1989). Release of nonexchangeable K to the exchangeable form occurs when level of exchangeable and solution K is decreased by crop removal and/or by leaching and perhaps by large increase in microbial activity (Sparks 1987).

The third type of potassium found in the soil is the available potassium which constitutes 1–2 %. It is found either in the solution or as part of the exchangeable cation held by negative charge of clay minerals and organic matter in soils. The rate and direction of reactions between solution and exchangeable forms of K determine whether applied K will be leached into lower horizons, taken up by plants, converted into unavailable forms or released into available forms (Sparks 2000). Among three different forms of potassium in soils, the concentrations of soluble K in soils are usually very low, but the highest proportion of potassium in soils is in insoluble rocks and minerals (Goldstein 1994). A significant share of soil potassium occurs in unavailable form in soil minerals such as orthoclase and microcline (K-feldspars).

The potassium content of Indian soils varies from less than 0.5 to 3.0 %. Ghosh and Hasan (1980) have documented the state-wise available potassium status in India and categorized that 21 % districts are in low, 51 % are medium, and 28 % are having high available potassium. The average total potassium content of these soils is 1.52 % (Mengel and Kirkby 1987). However, total K is poorly correlated with available K and is rarely used to describe K fertility status of a soil. The immediate source of K for plants is the small amount which is in the soil solution and it ranges from 1 to 2 %. As K is removed, the equilibrium is disturbed; K in the nonexchangeable and soil mineral fraction will be drawn upon. The supply of K to the plants depends directly on the concentration of K in soil solution and indirectly on soil, which maintains this equilibrium (Sparks and Huang 1985). In mineral soils, K occurs in the form of silicate minerals , viz., muscovite, orthoclase, biotite, feldspar, illite, mica, vermiculite, smectite, etc. Total pool of soil K is extremely complex and this can be solubilized by microbes for uptake by plants and microorganisms through production of acids or exopolysaccharides (Ullaman et al. 1996; Rogers et al. 1998).

10.3 Solubilization/Mobilization of Potassium from Rocks or Mica

Significant areas of cultivated soils in India, Korea, and China are deficient in available potassium and have low crop productivity (Xie 1998). The use of plant growth promoting rhizobacteria including potassium-solubilizing bacteria as a biofertilizer could work as a sustainable solution to improve plant nutrient uptake and production (Vessey 2003). Furthermore, direct application of potassium containing rock materials may be agronomically more useful and environmentally more feasible than soluble K (Rajan et al. 1996). Rock K materials are cheaper sources and most of them are readily available to a plant because the minerals are released slowly and their use as fertilizer causes significant yield increases of the various crops. The potassium present in the rock materials could be made available by the following mechanisms.

10.3.1 Bioweathering of Rocks

Bioweathering is a common geochemical process involved in erosion, decay, and decomposition of rocks and minerals mediated by living organisms (Barker et al. 1997; Burford et al. 2003). The bioweathering process plays a fundamental role in the release of nutrients from rocks and is associated with global environmental changes (Li et al. 2006). Microorganisms are the main bioweathering agents leading to rock degradation and soil formation. Microbes also provide nutrients, such as P, K, and silicon to support plant growth by changing environmental pH and oxidation reduction potential to solubilize rock minerals (Buss et al. 2007; Lian et al. 2008). Prokaryotic and eukaryotic algae also play an important role in colonizing barren or eroded land surfaces and contribute to soil development in different ecosystems. The algae cause corroding and weathering of rocks as they can grow on barren rocks on which other plants fail to grow. The weathering of rocks may be the result of carbonic acid formation from the respiratory CO2 release of the algae and its subsequent reaction with water (Barker et al. 1998) or it may be associated with the metabolic products, i.e., production of organic acids. The organic acids produced by various microorganisms have been found to facilitate the weathering of minerals by directly dissolving K from rocks or through the formation of metalorganic complexes with silicon ions to bring the K into solution (Friedrich et al. 1991; Bennett et al. 1998).

Argelis et al. (1993) found that weathering of unaltered sand stone, granite, and lime stone was carried out by Penicillium frequentans and Cladosporium cladosporoides. They reported that both fungal species have the capacity to produce large amounts of oxalic, citric, and gluconic acids in broth culture that caused extensive deterioration of clay silicates, mica and feldspar from both sand stone and granite and also of calcite and dolomite from lime stone. Thus, filamentous fungi were also found to cause an extensive weathering of stone due to organic acid excretion.

10.3.2 Composting of Mica

Low-grade waste mica (8–10 % K2O) is an alternative source of potassium, which is generated during the processing of mica sheets. Large amounts of mica waste are generated and dumped near the mica mines. This mica waste cannot be utilized by plants because most of the K is present as nonexchangeable form. However, this mica can be effectively used as a source of K fertilizer by chemical and/or biological modifications. One of the possible alternative viable technologies is the management of waste mica through composting technology (Nishanth and Biswas 2008). Rice straw, rock phosphate , and waste mica are inoculated with Aspergillus awamori and the acidic environment prevailing during composting has been reported to convert unavailable K into plant available form.

10.3.3 Solubilization of K-Bearing Minerals Using Microorganisms

Potassium-solubilizing microorganisms present in the soil could provide another alternative technology and these microbes play a key role in the natural K cycle. There are considerable population of K-solubilizing bacteria (KSB) in soil and rhizosphere (Sperberg 1958). A wide range of bacteria, such as members of the genus Pseudomonas, Burkholderia, Acidithiobacillus ferrooxidans, Bacillus mucilaginosus, Bacillus edaphicus, and B. circulans were found to release potassium from potassium-bearing minerals but only a few bacteria such as Bacillus mucilaginosus and Bacillus edaphicus have high activity in mobilizing potassium in accessible form in soils (Lian et al. 2002; Sheng 2005; Li et al. 2006). Potassium- and phosphate-solubilizing bacteria (KSB and PSB) are extensively used as biofertilizers in Korea and China as significant areas of cultivated soils in these countries are deficient in soil-available K and P (Xie 1998). Their use in agriculture can reduce the use of agrochemicals and support ecofriendly crop production (Glick 1995; Requena et al. 1997; Sindhu et al. 2010). Therefore, the use of K-solubilizing bacteria as biofertilizer for agriculture improvement and environmental protection has been a focus of recent research (Sheng et al. 2003).

The first evidence of microbial involvement in solubilization of rock potassium was reported by Muntz (1890). Several microorganisms like Aspergillus niger, Bacillus extroquens, and Clostridium pasteurianum were found to grow on muscovite, biotite, orthoclase, microclase, and micas under in vitro conditions (Reitmeir 1951). Subsequently, a variety of soil microorganisms have been reported to solubilize silicate minerals (Bunt and Rovira 1955). The microorganisms like bacteria, fungi, and actinomycetes were found to colonize even on the surface of mountain rocks (Gromov 1957). Norkina and Pumpyanskaya (1956) reported that the silicate-solubilizing bacteria (SSB) B. mucilaginous subsp. siliceous, liberated potassium from feldspar and alumino-silicates. Duff and Webley (1959) reported silicate-dissolving action of Gram-negative bacteria Erwinia herbicola or with Pseudomonas strains. Heinen (1960) reported the ability of Bacillus caldolyticus and Proteus sp. to grow and solubilize quartz. Webley et al. (1960) found that a Pseudomonas strain isolated from soil showed clearing zone in silicate medium. Webley et al. (1963) demonstrated that the silicaceous materials in rocks can be attacked through the metabolic products of microorganisms.

Alkasandrov et al. (1967) isolated different bacterial species which were found to dissolve potassium, silica, and aluminum from insoluble minerals. Purushothaman et al. (1974) reported the distribution of SSB in marine environments and suggested that these bacteria play a role in cycling of silicon in sea water. Avakyan et al. (1981) reported that B. mucilaginosus solubilized insoluble silicates. Belkanova et al. (1985) reported the cleavage of siloxane bond in quartz by B. mucilaginosus. Among the K-bearing silicate minerals, mica was found to weather readily (Tandon and Sekhon 1988). Li (1994) isolated K-solubilizing bacteria from soil, rock, and mineral samples. Bacterial isolate MCRCp1 was later identified as B. mucilaginosus based on morphological and physiological characters. Muralikannan (1996) isolated SSB from rice rhizosphere and tentatively identified as Bacillus sp. Kannan and Raj (1998) carried out enumeration of silicate- and phosphate-solubilizing bacteria from soil’s tank sediments. Three out of 17 isolates were identified as Bacillus sp. based on biochemical characteristics. Lian (1998) also isolated silicate bacteria B. mucilaginosus from corn field.

Liu (2001) reported isolation of silicate bacteria B. mucilaginosus CS1 and CS2 from soil and these bacteria exhibited inhibitory activity on the growth of Gram-negative bacteria E. coli. They identified strain CS1 as B. mucilaginosus. It was reported that the ability of slime production by the B. mucilaginosus strain dissolved the silicates and also contributed in the colonization of rhizosphere as well as non-rhizosphere soil (Lin et al. 2002). Hutchen et al. (2003) isolated 27 strains of heterotrophic bacteria from feldspar-rich soil and studied dissolution of silicate mineral in liquid and solid minimal media. SSB were isolated from rice ecosystem in a medium containing 0.25 % insoluble magnesium trisilicate and reported that Bacillus sp. solubilized silicate minerals more efficiently under in vitro conditions (Raj 2004). Potassium-solubilizing rhizobacteria were isolated from the roots of cereal crops by the use of specific potassium-bearing minerals (Mikhailouskaya and Tcherhysh 2005).

Murali et al. (2005) isolated silicate solubilizers using modified Bunt and Rovira medium from soil samples collected from coconut palms. Majority of the silicate solubilizers were identified as Bacillus sp. and Pseudomonas sp. Hu et al. (2006) isolated two phosphate- and potassium-solubilizing strains KNP413 and KNP414 from the soil of Tianmu Mountain, Zhejiang Province (China). Both the isolates effectively dissolved mineral phosphate and potassium, while strain KNP414 showed higher dissolution capacity even than Bacillus mucilaginosus AS1.153, the inoculant of potassium fertilizer widely used in China. When grown on Aleksandrov medium, both strains were rod-shaped spore formers with a large capsule and they formed slimy translucent colonies. Based on G + C contents of DNA and 16S rRNA gene sequence similarity, strains KNP413 and KNP414 were classified to the genus of Paenibacillus, i.e., P. mucilaginosus. Zhou et al. (2006) characterized Bacillus mucilaginosus which solubilized silicon from illite at 30 °C. The bacterium was identified as Gram-positive, rod-shaped endospore former with thick capsule. Sugumaran and Janarthanam (2007) isolated K-solubilizing bacteria from soil, rocks, and minerals samples, viz., microcline, orthoclase, muscovite mica. Among the isolates, B. mucilaginosus MCRCp1 solubilized more potassium by producing slime in muscovite mica. Phosphorus and potassium nutritional status in the soil were markedly improved through inoculation of this bacterium. Zhao et al. (2008) isolated mineral potassium-solubilizing bacterial strains with multiple activities relevant for beneficial plant–microbe interactions, i.e., potassium solubilization, production of indole acetic acid (IAA), and siderophore production.

Parmar (2010) isolated 70 bacterial isolates from the rhizosphere of wheat by using modified Aleksandrov medium plates (consisting of glucose, 5.0 g; MgSO4·7H2O, 0.5 g; CaCO3, 0.1 g; FeCl3, 0.006 g; Ca3PO4, 2.0 g; mica powder, 2.0 g; and agar, 20.0 g). These bacterial isolates along with 67 standard reference strains were tested for potassium solubilization ability on Aleksandrov medium using mica as potassium source. Different rhizobacterial isolates were spotted on medium plates (Sindhu et al. 1999) and plates were incubated for 3 days at 28 ± 2 °C. Detection of potassium solubilization by rhizobacterial isolates was based upon the ability of solubilization zone formation. Twenty rhizobacterial isolates were found to solubilize potassium from mica powder.

Potassium-solubilizing fungi were isolated from ceramic industry soils and four fungal isolates gave the high ratio of clear zone on Aleksandrov agar supplemented with 0.5 % potassium aluminum silicate (Prajapati et al. 2012). Two fungal strains, i.e., KF1 and KF2 showed the highest available potassium in liquid medium containing potassium aluminum silicate. These isolates were characterized as Aspergillus niger and Aspergillus terreus. A. terreus showed more solubilization when grown in the presence of 1 % rock potassium (feldspar) than A. niger. Liu et al. (2012) isolated mineral-solubilizing Paenibacillus strain KT from a soil in Henan Province, China. After inoculation of this strain for 7 days, the concentrations of water-soluble Al, Ca and Fe released from the potassium-bearing rock in active bacterial culture were higher than from the control with autoclaved inoculum, but the concentration of water-soluble K in active bacterial culture was similar to that in the control. A potassium-solubilizing bacterium isolated from soil was characterized as B. circulans strain Z1–3, which dissolved potassium from feldspar (Xiaoxi et al. 2012). Potassium-bearing rock material served as the sole source of potassium to support the growth of this strain.

10.4 Mechanisms Involved in Potassium Solubilization

Solubilization of illite and feldspar by microorganisms was reported due to the production of organic acids like citric, oxalic acid, and tartaric acids and also due to the production of capsular polysaccharides which helped in dissolution of minerals to release potassium (Sheng and He 2006; Liu et al. 2006).

10.4.1 Acid Production

Production of carboxylic acids like citric, tartaric, and oxalic acids has been reported as predominant mechanism contributing to K solubilization (Hazen et al. 1991; Styriakova et al. 2003; Sheng and He 2006). Silicate bacteria were found to dissolve potassium, silicon, and aluminum from insoluble K-bearing minerals such as micas, illite, and orthoclases by excreting organic acids which either directly dissolved rock K or chelated silicon ions to bring K into the solution (Aleksandrov et al. 1967; Friedrich et al. 1991; Ullman et al. 1996; Bennet et al. 1998). Organic acids can directly enhance dissolution by either a proton- or ligand-mediated mechanism. They can also indirectly enhance dissolution by the formation of complexes in solution with reaction products and as a consequence increase the chemical affinity for the overall dissolution (Ullman and Welch 2002). The potassium is made available to plants when the minerals are slowly weathered or solubilized (Bertsch and Thomas 1985).

Silicate bacteria were found to dissolve potassium, silica, and aluminum from insoluble minerals by liberation of phosphoric acids that solubilized apatite and released available form of nutrients from apatite (Heinen 1960). Moira et al. (1963) isolated several fungal isolates having the potential to release metal ions and silicate ions from minerals, rocks, and soils. The minerals used in this study were saponite and vermiculite. These fungal isolates were found to produce citric acid and oxalic acid that are mainly known to decompose or solubilize natural silicates and help in removal of metal ions from the rocks and soils. Vainberg et al. (1980) proposed that dissolution of minerals was caused by the formation of organic acids in the culture media. Berthelin (1983) demonstrated that potassium is solubilized from precipitated forms through production of inorganic and organic acids by Thiobacillus, Clostridium, and Bacillus. Production of carboxylic acids like citric, tartaric, and oxalic acids was associated with feldspar solubilization by B. mucilaginosus and B. edaphicus (Malinovskaya et al. 1990; Sheng and Huang 2002b). Organic compounds produced by microorganisms such as acetate, citrate, and oxalate were found to increase mineral dissolution rates in laboratory experiments and in the soil (Palmer et al. 1991). The production of gluconate promoted dissolution of silicates like albite, quartz, and kaolinite by subsurface bacteria (Duff et al. 1963; Vandevivere et al. 1994). Thus, production of organic acids such as acetate, citrate, and oxalate by microorganisms was found to increase mineral dissolution rate (Hazen et al. 1991; Barker et al. 1998). Welch and Ullman (1993) found that the rate of plagioclase dissolution in solutions containing organic acids was more compared to inorganic acids and further showed that polysaccharides produced by the bacterium during the process of reproduction can combine with the minerals to form bacterial mineral complexes which leads to degradation of the minerals.

10.4.2 Production of Exopolysaccharides

Groudev (1987) reported that production of slime or acidic exopolysaccharides (EPS) contributed to the mechanism of releasing potassium from silicates. Liu et al. (2006) demonstrated that polysaccharides strongly adsorbed the organic acids and attached to the surface of the mineral, resulting in an area of high concentration of organic acids near the mineral. It was suggested that the extracellular polysaccharides adsorbed SiO2 and this affected the equilibrium between the mineral and fluid phases and led to the reaction toward SiO2 and K+ solubilization. Welch and Vandevivere (2009) tested several naturally occurring polymers for their effect on mineral dissolution. Solutions of fresh microbial EPS extracted from subsurface microbes increased the dissolution rate of feldspars, probably by forming complexes with framework ions in solution. However, EPS was found to inhibit dissolution in experiments with both high- and low-molecular weight microbial metabolites by irreversibly binding to mineral surfaces.

10.4.3 Cumulative Effect of Different Mechanisms Leading to Potassium Solubilization

Jones and Handrecht (1967) reported that bacteria solubilized the insoluble silicates by production of CO2, organic acids and exopolysaccharides. The solubilization of silicates was investigated using Kaolin quartz and sand as model substances. The chemical leaching of silicates was carried out using inorganic and organic acids as well as sodium hydroxide. The process was more effective in the alkaline than in the acid pH range. The transformation of crystalline biotite, mica, vermiculite, and certain rocks to amorphous state was found due to the action of some organic products of microbial metabolism (Weed et al. 1969). The slime-forming and K-solubilizing bacterium Bacillus mucilaginosus produced endoglucanase, cellobiase, protease, ribonuclease, deoxyribonuclease, and phosphomonoesterase (Tauson and Vinogrado 1988).

Bacteria have also been shown to accelerate the dissolution of silicates by the production of excess proton and organic ligands, and in some cases by the production of hydroxyl anion, extracellular polysaccharides, and enzymes (Berthelin and Belgy 1979; Hieberk and Bennett 1992; Vandevivere et al. 1994; Barker et al. 1998). Hinsinger et al. (1992, 1993) reported that dissolution of trioctahedral mica structure-like phlogopite occurred in the rhizosphere of ryegrass (Lolium multiflorum) and rape (Brassica napus) probably due to proton excretion by roots. Other workers also found that production of protons, organic acids, siderophores , and organic ligands were involved in the weathering ability of the bacteria (Paul and Clark 1989; Grayston et al. 1996; Welch et al. 1999; Liermann et al. 2000). Sheng and He (2006) reported that solubilization of illite and feldspar by wild-type strain of Bacillus edaphicus and its four mutants was due to the production of organic acids like oxalic acid and tartaric acids and also due to production of capsular polysaccharides (CPS), which helped in dissolution of minerals to release potassium. In liquid cultures, five bacterial strains showed better growth on Suzhou illite than on Nanjing feldspar. Oxalic acid seemed to be more active agent for the solubilization of Nanjing feldspar. Oxalic and tartaric acids were likely involved in the solubilization of Suzhou illite. Similarly, decomposition of silicate minerals by B. mucilaginosus was found due to production of oxalate and citrate as well as to the polysaccharides which absorbed organic acids leading to decomposition of minerals (Liu et al. 2006).

Styriakova et al. (2003) reported that the activity of silicate-dissolving bacteria played a pronounced role in the release of Si, Fe, and K from feldspar and Fe-oxihydroxides. Increasing evidence also exists for a mechanism of direct silicate precipitation by bacteria via metal sorption at the cell membrane (Beveridge and Murray 1980; Beveridge and Fyfe 1985; Urruti and Beveridge 1994; Konhauser and Ferris 1996). In a study to assess the weathering of finely ground phlogopitite, trioctahedral mica with heterotrophic bacteria B. cereus and acidophilic Acidithiobacillus ferrooxidans, it was found that cultures enhanced the chemical dissolution of the mineral. The X-ray diffraction analysis of the phologopite samples before and after 24 weeks of contact with B. cereus cultures revealed a decrease in the characteristic peak intensities of phologopite, indicating destruction of individual structural planes of the mica. On the other hand, Acidothiobacillus ferroxidans cultures enhanced the chemical dissolution of the mineral and formed partially weathered interlayer from where K was expelled. This was coupled with the precipitation of k and jarosite (Styriakova et al. 2004).

Parmar (2010) studied the mechanism of K solubilization in 20 rhizobacterial isolates for potassium solubilization ability on Aleksandrov medium supplemented with mica as potassium source. Rhizobacterial isolate HWP47 caused solubilization of potassium in mica by acid production only and isolates HWP28 and HWP69 caused K solubilization by production of CPS and EPS. Another rhizobacterial isolate HWP38 solubilized potassium by production of acid and CPS, whereas six isolates caused solubilization by production of acid, CPS and EPS.

10.5 Environmental Factors Affecting Potassium Solubilization

Many indigenous soil microorganisms have the potential to absorb and mobilize the fixed form of nutrients from trace mineral sources. Different environmental factors such as pH, temperature, nutrients, oxygen, agitation, and nature of the rock material affected the rate of potassium solubilization. For example, the efficiency of potassium solubilization by different bacteria was found to vary with the nature of potassium-bearing minerals. Yakhontova et al. (1987) found that the intensity of degradation of silicate minerals by the bacterium was dependent on the structure and chemical composition of the mineral used. Potassium-dissolving ability of strain HM8841 was measured using Kietyote and Pegatolite in which 47 and 44.4 mg of soluble potassium was released after 38 h of incubation time. Sugumaran and Janarthanam (2007) studied the effect of K-solubilizing bacteria B. mucilaginosus, isolated from soil, rock, and mineral samples, on solubilization form microcline, orthoclase, and muscovite mica minerals . One of the slime-forming B. mucilaginosus strain MCRCp1 caused maximum potassium solubilization (4.29 mg/L) in media supplemented with muscovite mica, whereas the potassium released in microcline and orthoclase was only 1.26 mg/L and 0.85 mg/L, respectively. The K-solubilizing activity of the five slime-producing bacterial isolates varied from 1.90 to 2.26 mg/L from acid leached soil. MCRCp1 was found to have maximum activity (2.26 mg/L) to dissolve the silicate than other isolates. Zhou et al. (2007) reported that Paenibacillus polymyxa promoted dissolution of microperthite by direct and indirect mechanisms and enhanced the release of K, Al, and Si from the mineral. P. polymyxa and its metabolites were also found to promote dissolution of basalt (Zhou et al. 2008). Under the bacterial growth conditions, olivine was the most bioweathered mineral followed by augite but feldspar was found the most stable (Zhou et al. 2008).

Potassium release from minerals was affected by pH, dissolved oxygen, and bacterial strain used (Sheng and Huang 2002b). The content of potassium in solution was increased by 84.8–127.9 % by inoculation of bacteria as compared with the control. Welch et al. (1999) found that a variety of extracellular polysaccharides significantly enhanced the dissolution of plagioclase at pH 4 but had little effect at pH 7. Sheng et al. (2002) observed 35.2 mg/L potassium release from strains of potassium-solubilizing bacteria in 7 days at 28 °C at pH range from 6.5 to 8.0. Potassium-releasing characteristics of a bacterium from different minerals were studied by using soil column experiment (Badr 2006). Potassium and phosphorus solubilization capacity of SSB ranged from 490 to 758 mg/L at pH 6.5–8.0. More potassium was solubilized under aerobic condition than less aerobic conditions. The order of release of potassium was illite > feldspar > muscovite. The extent of potassium solubilization by B. edaphicus in the liquid media was more and better growth was observed on illite than feldspar (Sheng and He 2006).

Lian et al. (2007) studied a strain of thermophilic fungus Aspergillus fumigatus cultured with K-bearing minerals to determine if microbe–mineral interactions enhance the release of mineral K. It was observed that the K solubilization rate showed a positive dependence upon pH when fungi and minerals were mixed directly and exhibited no correlations with solution acidity if cell–rock contact was restrained. Bacterial inoculation on mica material improved the water-soluble, exchangeable, and nonexchangeable K pools in soils. It influenced the K dynamics of soils into those pools which are relatively more available to plant (Basak and Biswas 2008).

Lopes-Assad et al. (2010) reported that Aspergillus niger strains CCT4355 and CCT911 solubilized 62–70 % potassium from the rock powder after 35 days in 125 mL Erlenmeyer flasks; however, the percent solubilization decreased at higher volumetric scales. The injection of filter-sterilized air into the medium enhanced the potassium solubilization. The authors suggested that production of organic acids such as oxalic, citric, or gluconic, depending on the pH of the medium, by the fungus may be a mechanism of rock solubilization. Parmar and Sindhu (2013) found that the amount of K released by the rhizobacterial strains ranged from 15 to 48 mg/L. Maximum K solubilization occurred with glucose as carbon source at 25 °C incubation temperature and 7.0 pH of the medium.

10.6 Effect of Inoculation of Potassium-Solubilizing Bacteria on Growth and Yield of Different Crops

Plant growth-promoting bacteria associated with plant roots may exert their beneficial effects on nutrient uptake and plant growth through a number of mechanisms such as N2 fixation, production of phytohormones, siderophores, and transformation of nutrient elements such as phosphorus, potassium, and iron, when they are either applied to seeds or incorporated into the soil (Kloepper et al. 1989; Glick et al. 1999; Herridge et al. 2008; Sindhu et al. 2010). Moreover, rhizosphere bacteria have also been found to suppress various plant diseases (Weller 2007; Haas and Defago 2005; Sindhu et al. 2011).

10.6.1 Inoculation Effect of Potassium Solubilizers on Plant Growth and Yield

Beneficial effects of inoculation of K-solubilizing bacteria has been reported in sorghum (Zhang et al. 2004), cotton and rape (Sheng 2005), yam and tapioca (Clarson 2004), rice (Raj 2004), maize (Wu et al. 2005; Singh et al. 2010), egg plant (Ramarethinam and Chandra 2005), groundnut (Sugumaran and Janarthanam 2007), wheat (Sheng and He 2006; Singh et al. 2010; Parmar 2010), pepper (Supanjani et al. 2006), pepper and cucumber (Han et al. 2006), and forage crop sudan grass (Basak and Biswas 2008, 2010).

Increase in yield of maize and wheat by application of organo-minerals and inoculated with silicate bacteria was first reported by Aleksandrov (1958). Vintikova (1964) observed the beneficial effects of silicate bacteria on the yield of lucerne and maize. Khudsen et al. (1982) isolated potassium-solubilizing bacteria from rock and mineral samples which showed higher activity in potassium release from acid-leached soil and improved green gram’s seedling growth. Zahro et al. (1986) studied the effect of inoculation of the silicate bacteria Bacillus circulans on the release of K and Si from different minerals and in different soils. Bacteria persisted for a long time and high population densities were detected after 14 months particularly in soils containing higher levels of organic matter. An increased yield in rice crop was observed due to inoculation of SSB (Muralikannan 1996; Kalaiselvi 1999).

The inoculation effect of potash mobilizer on egg plant recorded an increased potash uptake and plant biomass as compared to the control plants (Nayak 2001). Lin et al. (2002) observed 125 % increase in biomass, whereas K and P uptake were more than 150 % in tomato plant due to inoculation of silicate dissolving bacteria B. mucilaginosus strain RCBC13 in comparison to uninoculated plants. Sheng et al. (2003) studied the effect of inoculation of SSB Bacillus edaphicus on chilly and cotton which resulted in increased available P and K contents in plant biomass. Park et al. (2003) found that bacterial inoculation could improve phosphorus and potassium availability in the soils by producing organic acid and other chemicals and thereby stimulated growth and mineral uptake of plants. Zhang et al. (2004) reported the beneficial effect of potassic bacteria on sorghum, which resulted in increased biomass and increased contents of P and K in plants than the control. The beneficial effect of silicate-solubilizing Bacillus sp. was observed on grain yield and silica content of rice and available silica in soil (Raj 2004). Ramarethinam and Chandra (2005) recorded significantly increased egg plant yield, plant height and K uptake compared to control in a field experiment due to inoculation of potash-solubilizing bacteria Frateuria aurantia.

Mikhailouskaya and Tchernysh (2005) reported the effect of inoculation of K-mobilizing bacteria on severally eroded soils which were comparable with yields on moderately eroded soil without bacterial inoculation that resulted in increased wheat yield upto 1.04 t/ha. Sheng (2005) studied plant growth-promoting effects of potassium releasing bacterial strain B. edaphicus NBT on cotton and rape in K-deficient soil pot experiments. The inoculation of B. edaphicus resulted in increased root and shoot growth, and potassium content was increased by 30 and 26 %, respectively. The bacterial isolate was found to colonize and develop in the rhizosphere of both the crops.

Christophe et al. (2006) reported that Burechulderia glathei in association with pine roots significantly increased weathering of biotite. B. glathei PMB (7) and PML1 (12) was found to affect pine growth and root morphology, which was attributed to release of K from the mineral. Sheng and He (2006) recorded an increased root and shoot growth and also showed significantly higher N, P, and K contents of wheat plants components due to inoculation of B. edaphicus and its mutants in a yellow brown soil that had low available K. In the field experiment, increased yield in tomato crop was recorded due to inoculation of silicate-dissolving bacteria B. cereus as a bioinoculant along with feldspar and rice straw (Badr 2006). Badr et al. (2006) studied the effect of bacterial inoculation combined with K and P bearing minerals on sorghum plants and reported increase in dry matter yield along with P and K uptake in three different soils, i.e., clay, sandy, and calcareous soils; 48, 65, and 58 % increase in dry matter, 71, 110, and 116 % uptake of P as well as 41, 93, and 79 % uptake of K, and improved fertility through inoculation of silicate dissolving bacteria. In a field experiment, increased rice grain yield was observed due to inoculation of SSB that recorded 5,218 kg/ha grain yield than the control yield of 4,419 kg/ha (Balasubramaniam and Subramanian 2006).

Sugumaran and Janarthanam (2007) reported that inoculation with slime-forming B. mucilaginosus strain MCRCp1 recorded increase in the plant dry matter by 125 % and oil content 35.4 % of groundnut plant. Available P and K increased from 6.24 to 9.28 mg/kg and 86.57 to 99.60 mg/kg, respectively, in soil due to inoculation of B. mucilaginosus MCRCp1 as compared to uninoculated control plants. Basak and Biswas (2008) found that inoculation of K-solubilizing B. mucilaginosus along with application of mica in sudan grass (Sorghum vulgare var. sudanensis) increased the biomass yield and uptake of K in both the soils. Significant correlation between biomass yield and K uptake by sudan grass and different pools of K in soils were observed. Parmar (2010) showed that inoculation of K-solubilizing isolate HWP47 in wheat (Triticum aestivum L.) variety WH711 caused 51.46 % increase in root dry weight (RDW) in soil at 60 days after sowing in pots. Similarly, 44.28 % increase in shoot dry weight (SDW) was found in HWP47-inoculated plants. Inoculation with isolate HWP47 showed 22.35 % increase in RDW and 73.68 % increase in SDW on addition of rock material. Isolates HWP15 and HWP47 also caused significant K uptake in the shoot tissues.

10.6.2 Coinoculation of Potassium Solubilizers with Other Beneficial Bacteria

Ciobanu (1961) showed that increase in the yield of cotton was by 50–94 % when Azotobacter (nitrogen-fixing bacteria ) and silicate bacteria were applied simultaneously. Similarly, phosphorus-solubilizing bacteria and silicate bacteria were reported to play an important role in plant nutrition through the increase in P and K uptake by plant (Datta et al. 1982; Nianikova et al. 2002). The increased K uptake coupled with increased yield in yam and tapioca was observed by treating the plants with potassium mobilizer in conjunction with biofertilizers and chemical fertilizers (Clarson 2004). Similarly, Chandra et al. (2005) reported an increased yield by 15–20 % in yam and tapioca due to the potash solubilizer application and in combination with other biofertilizers like Rhizobium, Azospirillum, Azotobacter, Acetobacter, and PSM. Wu et al. (2005) found that inoculation of K solubilizer B. mucilaginosus along with P solubilizer Bacillus megaterium and N2-fixer Azotobacter chroococcum increased the growth and nutrient uptake significantly in maize crop. Bacterial inoculation also improved soil properties such as organic matter content and total N in soil. Han and Lee (2005) found that coinoculation of PSB (B. megaterium) and KSB (B. mucilaginosus strain KCTC3870) in combination with direct application of rock P and K materials into the soil resulted in increased N, P, and K uptake, photosynthesis and the yield of eggplant grown in nutrient-limited soil. The combined treatment resulted in increase of N, P, and K uptake in the shoot (14, 22, and 14 %, respectively) and in the root (11, 14, and 21 %). The treatment which combined both bacteria and mineral rocks further increased shoot dry weight by 27 % and root dry weight by 30 % over the control 30 days following planting.

Han et al. (2006) evaluated the combined potential of PSB (B. megaterium var. phosphaticum) and KSB (B. mucilaginosus) inoculation on pepper and cucumber in nutrient-limited soil. It was found that coinoculation of both PSB and KSB, and fertilization with rock P and K, increased the N, P, and K uptake in shoot (21, 31, and 33 %, respectively, for pepper and 29, 41, and 29 % for cucumber) and in root (16, 33, and 26 % for pepper; 29, 34, and 50 % for cucumber). The treatment including bacteria and mineral rocks, further increased plant growth, i.e., 26 % in shoot and 29 % in root dry weight for pepper, whereas 22 % in shoot and 27 % in root dry weight for cucumber plant in comparison to controls during 30 days following planting. The increase in plant growth by combining together, rock materials and both bacterial strains, suggested their potential use as biofertilizer.

Supanjani et al. (2006) reported that integration of P and K rocks with inoculation of phosphorus- and potassium-solubilizing bacteria increased P availability from 12 to 21 % and K availability from 13 to 15 %, in the soil as compared with control and subsequently improved nutrient N, P, and K uptake in Capsicum annuum. The integration approach of rocks and bacteria also increased plant photosynthesis by 16 % and leaf area by 35 % as compared to control. On the other hand, the biomass harvest and fruit yield of the treated plants were increased by 23–30 %, respectively. Similarly, the combined potential of phosphate-solubilizing bacteria B. megaterium var. phosphaticum and potassium-solubilizing bacteria, B. mucilaginosus was evaluated using pepper and cucumber as test crops (Vassilev et al. 2006). The outcome of the experiment showed that rock phosphorus and potassium applied either singly or in combination do not significantly enhanced availability of soil phosphorus and potassium indicating their unsuitability for direct application. However, coinoculation of PSB and KSB resulted in consistently higher P and K availability than the control.

Coinoculation of waste mica with potassium-solubilizing B. mucilaginosus and N2-fixing A. chroococcum A-41 resulted in highest biomass production and nutrient acquisition by sudan grass (Basak and Biswas 2010). Coinoculation of bacterial strains maintained consistently higher amounts of available K and N in soils even at 150 days of crop growth. Bacillus mucilaginosus strain was found more effective K solubilizer than Azotobacter chroococcum strain A-41. Similarly, inoculation of maize and wheat plants with Bacillus mucilaginosus, Azotobacter chroococcum, and Rhizobium spp. significantly improved the biomass accumulation, potassium content, and uptake by plants (Singh et al. 2010). B. mucilaginosus resulted in significantly higher mobilization of potassium than A. chroococcum and Rhizobium inoculation. Results revealed that PGPR could be used to mobilize potassium from waste mica, which in turn served as a source of potassium for plant growth. These results suggested that the treatment with P- and K-containing rock materials and inoculation with P- and K-solubilizing bacterial strains could be applied as a sustainable alternative to the use of chemical fertilizers. Thus, inoculation with PGPR including phosphate and potassium-solubilizing bacteria (PSB and KSB) as biofertilizers could be a sustainable solution to improve plant nutrition and crop production (Vessey 2003).

10.7 Possible Approaches to Increase Potassium Solubilization Efficiency and Its Application as Biofertilizer

Soil microorganisms play pivotal role in various biogeochemical cycles and are responsible for the cycling of nutrients in the plant utilizable form (Wall and Virginia 1999). Phosphate- and potassium-solubilizing microorganisms and other beneficial rhizobacteria cause the release of nutrients in plant utilizable form and exert beneficial effects on plant growth (Glick 1995; Sindhu et al. 2002; Marques et al. 2010). Thus, microbes influence aboveground ecosystems by contributing to plant nutrition, plant health, soil structure, and soil fertility. Therefore, microorganisms offer an environment friendly sustainable system and play a vital role in maintaining soil nutrient status.

Many rhizosphere bacteria are well known for their capacity to confer plant growth promotion and also increase resistance toward various diseases as well as abiotic stresses. These rhizobacteria often fail to confer these beneficial effects when applied in the field, which is often due to insufficient rhizosphere colonization (Lugtenberg et al. 2001). Lin et al. (2002) showed that silicate-dissolving bacteria increased 70 % in the rhizosphere soil and 20 % in non-rhizosphere soil, respectively. Sugumaran and Janarthanam (2007) also reported that the number of K-solubilizing bacterium B. mucilaginosus strain MCRCp1 increased to about 106–107 cfu/g in soil after 90 days of inoculation, whereas the count of K-solubilizing bacteria was only 103 g−1 in the control soil. New bacterial traits conferring strain survival in the rhizosphere have been found and opened a way to better understand specific signaling and the regulatory processes governing the plant-beneficial bacterial association (Matilla et al. 2007). Use of molecular techniques in genetic modification of microbial and plant biological activities allows their better functioning in the rhizosphere (Ryan et al. 2009) leading to substantial improvement in the sustainability of agricultural systems.

Plants could be selected by breeders with favorable traits or microorganisms can be engineered that increase nutrient accessibility, minimize biotic and abiotic stresses, and suppress pathogenic microbes or that encourage the persistence of beneficial microorganisms (Weller 2007; Dey et al. 2009; Sindhu et al. 2009). The release of organic anions such as citrate and malate has been reported to improve availability of poorly soluble organic and inorganic phosphorus (Richardson et al. 2001; Ryan et al. 2001). Therefore, the release of organic anions from roots can have an important influence on plant growth and nutrition. Since many of the genes controlling these exudates have now been identified, it is possible to manipulate conditions in the rhizosphere by modifying their expression via genetic engineering. Moreover, different methods and techniques have been developed recently to characterize and conserve various agriculturally important microbial communities from different environments for their optimal utilization for agriculture (Kirk et al. 2004; Naik et al. 2008). The knowledge generated on biodiversity and genetic manipulation of phosphate- and potassium-solubilizing bacteria will be useful to design strategies for use of these strains as inoculants in organic agriculture.

Thus, complex interactions between the KSB, PSB, other PGPR, the plant, and the environment are responsible for the variability observed in solubilization of bound nutrients and their uptake leading to plant growth promotion. Future strategies are required to clone genes from KSB involved in solubilization of insoluble potassium and to transfer these genes into the bacterial strains having good colonization potential along with other beneficial characteristics such as nitrogen fixation. Further, the efficacy of potassium-solubilizing bacteria can be improved by developing the better cultural practices and delivery systems that favor their establishment in the rhizosphere. The applications of mixture of PGPRs with different beneficial activities including potassium and phosphate solubilization ability, may be a more ecologically sound approach because it may result in better colonization and better adaptation to the environmental changes occurring throughout the growing season. In near future, the biotechnological approaches used in manipulation of bacterial traits will lead to improved potassium solubilization and their inoculation as potassic biofertilizer will enhance plant growth and crop productivity for sustainable agriculture .

10.8 Conclusion

The use of low grade, locally available soil minerals such as mica, feldspar, and rock phosphate in combination with selected efficient strains of potassium-mobilizing bacteria as biofertilizers are urgently required to replace chemical fertilizers and for reducing the cost of crop cultivation. Although, many bacterial strains have been found to improve the growth of plants under pot house conditions, the extent of growth stimulation by bacterial strains under field conditions usually remains unexplored. Therefore, effective potassium-solubilizing and plant growth-promoting bacterium–plant systems must be tested under field conditions with specific crop experimental designs keeping in consideration of the soil type, plant types grown, and the environmental factors. In addition, plant type/variety has also been found to influence the root colonization ability of the inoculated strains (Sheng 2005). Thus, competitive and effective bacterial strains must be selected from the pool of indigenous beneficial soil bacteria which could be adopted to the particular conditions of the inoculation site (Paau 1989; Sindhu and Dadarwal 2000). The application of efficient strains of potassium-solubilizing bacteria may find their use in the amelioration of potassium-deficient soils and further research could lead to an alternative mean of potassium nutrition for sustainable agriculture.