Diffusion, Fate and Reaction Products of Phosphate Fertilizers with Varying Solubility Applied to a Tropical Soil.

The low concentration of P in unfertilized soils limits land-use for continuous crop production worldwide. Gradual depletion of high-grade phosphate rocks (PR) sources because of P mining is taking place all over the world.  Levels of impurities (insoluble P compounds) will certainly increase in the final acidulated P fertilizers as the industry is forced to use lower-grade PR ores. Better understandings of these compounds formed during fertilizer processing, its interaction with soil system and their availability to crops are essential to manage these heterogeneous fertilizers most environmental- and cost-effective manner. Although these impurities do not seem to cause reduction in agronomic efficiency, because of our lack of understandings of mechanisms those still considered as fertilizers that do not meet the legislation requirements in Brazil. This study aimed to investigate and compare diffusion and mobility of P from the traditional P fertilizers and fertilizers produced with low-grade PR. Short term laboratory incubation study was conducted using a soil collected in Itiquira, Mato Grosso State, in a wide agricultural area under Brazilian Cerrado. Petri dishes were filled with soil (1.2 Mg m^-3 bulk-density) and moisture was added to get 60% of maximum water holding capacity. Fertilizer granules were placed at the center of petri dishes and little bellow of the soil surface. All treatments except control, received the same amounts of P (8.8 mg / petri dish). Soils were incubated in the dark for five weeks in a controlled environment. After incubation period the petri dishes were opened and concentric rings of soil surrounding the fertilizer placement point (0- to 7.5-, 7.5- to 13.5-, 13.5- to 25.5-, and 25.5- to 43-mm) were separated. Soil pH, total P distribution, water soluble P and resin extractable P were measured. SEM-EDXA for both original and exposed granules was also conducted to further understand fertilizer P reactions.