it could be answer by a fractionation experiment . theoritically, the expected findings is that, most of the P will be bonded to Ca forming CaPO4 bearing P. its speciation therefore will be a function of the concentration of Ca in your soils so added parameter to be determine is the Ca concentration of such soils. so that you can strongly support your argument later on.

Phosphorus Speciation in Calcareous Soils Following Annual Dairy Manure Amendments (SSSAJ , V ol. 80 No. 6, p. 1531-1542)

Abstract:Applying manure to crops may alter P speciation in the soil profile and thus affect its availability for plant uptake and transport to surface waters. The goal of this research was to determine how repeated manure amendments affect P speciation within calcareous soil. Soil samples were collected in 2013, 2014, and 2015 from two depths to analyze differences in P composition following annual applications of 17 Mg ha-1 manure, 52 Mg ha-1 manure, or NH4H2PO4 fertilizer, and control plots (no P). To speciate the soil P, sequential chemical extraction, P K-edge X-ray adsorption near-edge structure (XANES) spectroscopy, 31P nuclear magnetic resonance spectroscopy, and microprobe element mapping were used. Total P concentration in the manure-amended soils increased over 3 yr. The highest soil test P concentrations were in the 52 Mg ha-1 plots. Most extractable P in the sequential extraction procedure was removed with the most aggressive extractant, suggesting that the predominant form of P is associated with Ca-P minerals. The XANES results showed that P species were similar among all amendments and years: 54 to 74% Ca-P minerals (e.g., hydroxyapatite), 25 to 35% adsorbed P, and 0 to 19% organic P (predominantly phytic acid). Despite the poorly soluble Ca-P species predominating in all soils, soil test P increased in the manure-amended soils. The P speciation results provide a baseline to compare how long-term changes affect P availability and will be useful for designing long-term scenarios in manure-amended calcareous soils to limit excess soil P that could leach into water.