Potassium sources, microorganisms and plant nutrition: Challenges and future research directions
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INTRODUCTION
Potassium (K) is one of the 16 elements essential for the growth and development of animals, humans, and plants. For plants, K is the third most important nutrient after nitrogen (N) and phosphorus (P) and is the most abundant nutrient after N in photosynthetic tissues of land plants. For some plant species, such as cotton and banana, the demand for K is higher than that for N and P (Mora et al., 2012). These latter are often added to soils through fertilization, whereas the K requirement of
K IN SOIL
Soil minerals from silt, clay, and sand are natural reservoirs of K. Based on their availability to crops, K is often subdivided into four fractions: solution K, exchangeable K, non-exchangeable or fixed K, and mineral K (K in primary mineral structure). These forms are in dynamic equilibrium (Fig. 1), which is controlled by the exchange properties, mineral makeup, and weathering rate of the soil. The relative abundances of the different forms of K are in the following order of mineral K (>
Root exudates and K availability
Plant roots exudate different compounds, including carbohydrates, organic acids, amino acids, phenolic compounds, enzymes, gaseous molecules (such as CO2 and H2), and inorganic ions such as HCO-3, OH–, and H+ (Dakora and Phillips, 2002), which are directly or indirectly involved in K solubilization. For instance, Yang et al. (2019) showed that, under K-deficiency treatments, tobacco plants exudate a large amount of organic acids and activate a greater amount of K-bearing minerals. The organic
ISOLATION AND CHARACTERIZATION OF KSMS FROM RHIZOSPHERE SOILS
The Aleksandrov selective agar medium is generally used for the isolation and quantitative assessment of KSMs. The medium contains 5.0 g L–1 glucose, 0.5 g L–1 magnesium sulfate, 0.005 g L–1 ferric chloride, 0.1 g L–1 calcium carbonate, 2 g L–1 calcium phosphate, 2 g L–1 K-bearing minerals, and 3% agar (Aleksandrov et al., 1967). Inspired by the identification of P solubilizers using the Pikovskaya agar plate method, the plate assay has been developed for KSM isolation. The formation of a
MAJOR FUNCTIONS OF K IN CROP PRODUCTION AND QUALITY
Potash fertilizer ensures optimal plant growth, and the K+ requirement for plant growth changes with the developmental stage, crop species, and quantity of K+ available in soil. K is involved in many physiological processes that are vital to plant nutrient and water uptake. The optimal cytoplasmic concentration for enzyme activity is approximately 100–200 mmol L–1. Furthermore, K plays an important role in many fundamental physiological and metabolic processes, such as controlling ion
ADVANTAGES AND DISADVANTAGES OF K FERTILIZER APPLICATION
K in silicate structures can hardly be used by plants when added as fertilizer. Therefore, manufacturing K fertilizers is required for crop growth in soils depleted of available K. Currently, K fertilizers are obtained from sedimentary deposit rocks. The most frequently manufactured and used forms are KNO3, K2SO4, K2CO3, and KCl. Among these products, KCl (more than 95% of K fertilizer) is the most widely used form for agronomic crops. However, its application is subject to some precautions,
IMPROVING K USE EFFICIENCY
Annually, over 30 million tonnes of K fertilizers are applied to agricultural fields worldwide (Bahadur et al., 2016). The demand increased at a rate of 2.5% between 2014 and 2019 and was distributed as follows: 56% in Asia, 27% in America, 11% in Europe, and 6% in Africa (Basak et al., 2017). However, the excessive application of K from chemical fertilizers leads to environmental concerns without improving yield, whereas K deficiency results in reduced crop yields. In fact, K+ from fertilizers
APPLICATION OF KSMS TO CROP PRODUCTION
Several studies have indicated that using KSMs as bio-fertilizers can reduce agrochemical consumption while improving crop production. In this regard, Meena et al. (2016) reported that silicate-dissolving KSMs could free K from insoluble minerals and increase K availability from 84.8% in uninoculated soil to 127.9% in inoculated soil. Similarly, Singh et al. (2010) used waste mica as the sole source of K and showed that three KSMs (B. mucilaginosus, Azotobacter chroococcum, and Rhizobium spp.)
CHALLENGES FOR COMMERCIAL K BIOFERTILIZERS AT THE INDUSTRIAL LEVEL
To date, few biofertilizers based on KSMs are available on the market, and most of them are based on Frateuria species. For instance, Symbion-K, Green K, ABTEC Bio-Potash, UPTAKE, and K Soil B® are based on Frateuria aurantia and ADVEN is based on Frateuria sp., a K-mobilizing bacterium (Kore et al., 2020). Other products are based on Bacillus spp., such as POTAK, a liquid formulation of K-mobilizing Bacillus licheniformis. Overall, there are few biofertilizers based on K-solubilizing and/or
MULTIDISCIPLINARY RESEARCH CONTRIBUTIONS TO KSM BIOFORMULATION
Providing new formulations with the potential to fulfill their expectations requires exploring multidisciplinary research involving biotechnology, nanotechnology, agrobiotechnology, chemical engineering, and material sciences. At the crossroads of these diverse disciplines, several techniques have been developed, such as bioencapsulation, seed coating, and cell- or spore-lyophilized beads with cryoprotectants, such as mannitol, microcrystalline cellulose, granular inoculants, and dried
CONCLUSIONS
This review shows that the potential application of KSMs in agriculture is still under exploration, and studies are currently confined mostly to the laboratory scale. Most publications have shown that the main mechanisms of K solubilization are organic acid production and rhizosphere acidification. In addition, the literature analysis shows that the number of published articles related to K solubilization mechanisms is scattered and lower compared to that of K solubilization microbes. The
SUPPLEMENTARY MATERIAL
Supplementary material for this article can be found in the online version.
ACKNOWLEDGEMENT
We would like to thank Younes En-Nahli and Dr. Sitor Ndour for their support with the statistical analyses.
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