Mobility and Lability of Phosphorus from Granular and Fluid Monoammonium Phosphate Differs in a Calcareous Soil

Abstract

Phosphorus availability is a major factor limiting crop production in highly calcareous soils. Recent field trials on calcareous soils in southern Australia have shown that fluid fertilizers may provide a useful alternative to granular fertilizer products. Fluid sources of P enhance P uptake and yield when compared with granular fertilizers applied at the same rate. This work aimed to compare the behavior of one fluid (technical grade monoammonium phosphate, TG-MAP) and one granular (monoammonium phosphate, MAP) form of P fertilizer in a highly calcareous soil. Changes in soil pH, P diffusion, solubility, and lability (using isotopic dilution techniques) were determined at different distances from the point of application over 5 wk. Furthermore, reaction products in MAP granules were investigated using spectroscopic techniques. The results indicated that P from fluid TG-MAP diffused more and was more available than P supplied as granular MAP. Also, X-ray diffraction (XRD) and energy dispersive X-ray microanalyses (EDXMA) of the MAP granules indicated that a significant percentage (12%) of the initial P remained in the granules even after 5 wk of incubation in the soil. The enhanced P availability of fluid fertilizers observed in field trials compared with granular forms is discussed in relation to differences in the dissolution, diffusion, and reaction processes in soils.