Optimization of Biofertilizer Formulation for Increasing Phosphorus Solubility by Aspergillus Fungus

Phosphorus is one of the macronutrient that its deficiency severely restricts plant growth. One of the simplest and least costly methods of providing phosphorus is direct application of rock phosphate but in calcareous soils it is not very effective due to its low solubility. The use of rock phosphate mixed with sulfur and organic matter along with phosphate solubilizing microorganisms is considered as a method for increasing rock phosphorus solubility.  This study aimed to model the effect of different ratios of vermicompost, rock phosphate and sulfur on dissolution and release of phosphorus by Aspergillus sp and to optimize the levels of these variables for efficient biofertilizer preparation. Accordingly, 20 experiments were designed using response surface methodology based on central composite design. The effects of different values ​​of vermicompost, rock phosphate and sulfur variables encoded in the constraint (+1, 0, -1) on the dissolution rate of phosphorus were modeled. The results showed a high efficiency (R2 = 0.8841) of the central composite design model in estimating P dissolution. The results also indicated that vermicompost interaction with sulfur (p <0.05) and interaction of rock phosphate with sulfur (p <0.05) were significant. The results of the statistical analysis of the central composite model coefficients indicated that the vermicompost, vermicompost*sulfur and rock phosphate*sulfur additives had a positive and incremental effect on the phosphorus solubility. According to prediction of optimum conditions for phosphorus solubilization, 58% vermicompost, 23.3% rock phosphate and 18.7% sulfur resulted in maximum phosphorus solubilization (773.04 mg / kg) by Aspergillus sp. in microbial fertilizer.