Potassium (K) is abundant in soil, however, only 1 to 2 % of Potassium is available to plants. Depending on soil type, 90 to 98% of soil K is in the structure of various minerals such as feldspar (orthoclase and microcline) and mica (biotite and muscovite). About 1 to 10 % of soil K, in the form of non-exchangeable K, is trapped between the layers of certain types of clay minerals. The concentration of soluble K, which is directly taken up by plants and microbes in the soil and is exposed to leaching, varies from 2 to 5 mg l-1 in agricultural soils. Imbalanced use of chemical fertilizers, a significant increase of crop yield (depletion of soil soluble K), and the removal of K in the soil system result in a large rate of K fixation in the soil. As a result, K deficiency has been reported in most plants. The annual increase in the price of K fertilizers and the destructive effects of them on the environment have made it necessary to find a solution for the use of indigenous K of soil. The use of biofertilizers containing beneficial microorganisms is one of these strategies. Although K solubilizing bacteria can be an alternative and reliable technology for dissolving insoluble forms of K, lack of awareness among farmers, the slow impact of K biofertilizers on yield, less willingness of researchers to develop K biofertilizers technology and deficiencies of technology in respect to carrier suitability and proper formulation, are the major reasons for why potassium solubilizing microorganisms and K biofertilizers draw low attention.
The purpose of this study was modeling and evaluating the effects of different vermicompost, phlogopite and sulfur ratios on the solubility and release of K by Pseudomonas fluorescens and indicating the optimized levels of these variables for efficient biofertilizer preparation. 20 experiments were carried out using the response surface methodology (RSM) based on the central composite design and the effect of different values of vermicompost, phlogopite and sulfur variables, in the four coded levels (+α, +1, 0, -1 and -α), was evaluated on K dissolution. The applied vermicompost, phlogopite and sulfur in the experiment were ground and filtered through a 140 mesh sieve and their water holding capacity were determined. According to experimental design, different amounts of mentioned materials were combined and samples were sterilized in autoclave. The required amount of water along with 1 ml of bacterial inoculant were added to the samples. The samples were kept in incubator for 2 months. At the end of experiment, amount of soluble K were measured by the flame photometer.
The analysis of variance (ANOVA) depicted the reliable performance of the central composite predictive model of K dissolution (R2= 0.949 and RMSE=0.8). Based on the results, the interaction of vermicompost with sulfur (p < 0.038) and the interaction of phlogopite with sulfur (p < 0.0083) were relatively high and significant. Sensitivity analysis of the central composite design revealed that the vermicompost (X1), phlogopite (X2) and sulfur (X3) had positive and negative impact on potassium dissolution, respectively. Therefore, when sulfur content increased to 91.70%, K dissolution decreased to around 31.61%. According to the prediction under optimized condition, maximum potassium dissolution was obtained at the presence of 41.78, 24.35 and 10.25% of vermicompost, phlogopite and sulfur, respectively.
The results indicated that the applied fertilizer composition (vermicompost + phlogopite + sulfur) had a desirable impact on Pseudomonas fluorescens solubilizing ability on a laboratory scale. Due to the fact that Iran soils are often calcareous, there are high amounts of insoluble and unavailable nutrients. Under these unsuitable conditions, the application of these nutrients chemical fertilizers cannot reduce deficiencies. Therefore, we must use the ability of efficient microorganisms to dissolve and mobilize soil native elements. A combination of 41.78% vermicompost, 24.35% phlogopite and 10.55% sulfur could create a proper potassium biofertilizer by providing favorable conditions for bacterial activity. Along with solubilizing activities of bacteria, the presence of sulfur reduces soil pH and thereby nutrients availability and stability increase in these soils. Because of its acidity, sulfur has a significant effect on nutrients dissolution such as phosphorus, nitrogen and potassium, and micronutrients. On the other hand, the presence of vermicompost in this fertilizer, while meeting the carbon and energy requirements of bacteria and acting as a suitable carrier, improves the physicochemical properties of the soil, increases the biodiversity of the microbial community and, as a result, promotes the soil quality and health. The evaluation of this fertilizer composition efficiency (using optimal amounts of materials) at the greenhouse and field scales is suggested.
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