Potassium (K) is one of the major macronutrients and plays an important role in plant growth promotion. Potassium deficiency causes chlorosis, leaf falling, slow growth rate, poor root development, reduced production of seeds, and reduced yield in plants. Therefore it is necessary to apply an alternate potassium source such as bioformulations containing potassium solubilizers for improved plant growth and sustainability in agricultural crops (Prasad et al., 2019). Potassium mainly exists in three forms including soil minerals, nonexchangeable, and available form. About 90%–98% of the total K exists as insoluble rock and silicate minerals, micas, or feldspars in the rooting zone, which is relatively unavailable for plant uptake (Scheffer, 2002). The nonexchangeable form of K constitutes 10% of total K and exists as reserve to manage loss from the soil. Only 1%–2% of total K is available and found either in the solution or as part of the exchangeable cation on clay mineral. Further constraints such as imbalanced fertilizers, intensive cropping, soil erosion, leaching, and introduction of hybrids and other high-yielding crop varieties also cause difficulties to the plants. Different species of bacteria Bacillus subtilis, Bacillus mucilaginosus, Bacillus edaphicus, Bacillus circulans, Burkholderia spp, Paenibacillus spp., and Pseudomonas spp. are able to solubilize potassium by producing enzymes and organic acids (Saha et al., 2016; Liu et al., 2012; Hafeez and Hassan, 2011; Basak and Biswas, 2009). Other microbes including fungi, arbuscular mycorrhiza, and yeast also solubilize complex K sources including illite, micas, and orthoclase into soluble forms with the production of organic acid (Zeng et al., 2012). Members of fungi including Aspergillus terreus, Aspergillus niger, Glomas mosseae, Glomas intraradices, and Penicillium sp. have been reported promising in solubilizing complex sources of K (Rajawat et al., 2016; Meena et al., 2015; Lian et al., 2002).
These microbes adopt different approaches including direct and indirect mechanism, polysaccharide secretion, and biofilm formation on mineral surfaces to solubilize complex forms of K into simple ions. A direct method of solubilization is accomplished by the production of organic acids like oxalic, tartaric and citric acids, acidolysis, and enhanced solubility of minerals in the rhizosphere and chemical weathering based on carbonic acid (Mendes et al., 2013; Gerke, 1992; Park et al., 2009; Gadd, 2007). An indirect method of K solubilization includes chelation of the cations, followed by exchange and solubilization on mineral surfaces, formation of metal-ligand complexes, and release of plant hormones (Uroz et al., 2009; Sattar et al., 2019).
In addition to direct and indirect methods, beneficial microbes also adopt K solubilization by releasing exopolysaccharides (EPS), which helps in adhering of microbes over the surface of minerals to enhance the production of organic acids (Liu et al., 2012). These EPS are biodegradable, high-molecular-weight polymers made up of monosaccharides, and play an important role in aggregating soil particles, maintaining water potential, ensuring strict contact with bacteria and roots of plant, and protecting against phytopathogens (Pawar et al., 2016). EPS also adsorb organic acids, maintain the equilibrium between soil and minerals, and enhance dissolution and release of K+ (Lian et al., 2002). Bacteria including Bacillus, Clostridium, and Thiobacillus secrete capsules made of polysaccharides for degrading feldspar and illite to release K+ (Sheng and He, 2006).
Another important approach to solubilize K is the formation of biofilms. Bacteria produce biofilms to adhere to the surfaces of minerals and release various metabolites and organic acids, which lower the pH and help in solubilization of complex minerals and facilitate uptake by plants. The biofilms also help to protect, to adapt, and to survive in extremities of the environment by extracting nutrients through the release of extracellular polymers, polysaccharides exudates, and enzymes and mobilization and weathering of complex minerals.
by Richa Salwan, Vivek Sharma, in Plant Nutrition and Food Security in the Era of Climate Change, 2022