Developed and Evaluation of Slow Release Phosphorus Fertilizer Using Mono-ammonium Phosphate and Di-ammonium Phosphate

Phosphorus (P) availability is generally low in Calcareous soils due to its association with soil constituents forming other P compounds that are of low solubility and availability for plant uptake. In particular, phosphorus use efficiency (PUE) in Saudi Arabia agricultural soils is very low. Therefore, there is a growing interest in finding an effective approach to improve PUE. Fertilizer coating with polymers has become a recognized strategy for controlling the release of nutrients and regulating their availability in the soil. In this study, two fertilizers types of phosphate-bound alginate-graft polyacrylamide (P-Alg-g-PAM) were prepared using mono-ammonium phosphate (MAP) and di-ammonium phosphate (DAP). A matrix of sodium alginate (Alg) and poly (vinyl alcohol) (PVA) was phosphorylated using either MAP or DAP, followed by grafting with acrylamide. An incubation study was conducted by using three soils with different CaCO3 content to evaluated the performance of developed fertilizer (slow-release MAP and slow-release DAP) compared with the commercial MAP and DAP fertilizer. The developed fertilizer exhibited P release of 77% for the slow release MAP and 57% for the slow release DAP until the 45 days of the application at pH 7 and 25°C. The results of incubation study, indicated that the available soil-P significantly (P<0.05) varied between the different P fertilizers. In the beginning of the incubation period, the available soil- P into different soil from slow-release MAP and DAP was slightly lower than the commercial MAP and DAP, but with the time proceed, the slow-release MAP and slow-release DAP resultant in more available soil- P than the uncoated MAP and uncoated DAP. The results indicated that, the fertilizers could be classified into two groups: the first “fast release” group showed total P release in the 10 days after incorporation of uncoated MAP and uncoated DAP fertilizers into soil. The second “slow release” group included coated slow- release MAP and coated slow- release DAP fertilizer, released of about 70% during the period of 10-30 days after incubation. The results of P fractionation chemical analysis indicated that, the soluble, exchangeable-P, Ca-occluded-P fraction, was the most dominated P forms distributed among the soils.