Fixing Phosphorus: Considering Cation Complexing Co-Applicants to Maintain Phosphorus Lability in Calcareous Soils.

Phosphorus (P) fertilizer use efficiency is often very poor in acid and calcareous soils around the world. The “fixing” reactions that limit P solubility waste grower capital and provide a nutrient pool that does little more than serve as a nonpoint source for surface water pollution. The superiority of liquid to granular fertilizer formulations to improve P lability in calcareous soils is well established, but the use of fulvic acids to further increase plant availability is not. This study investigated the ability of cation complexing coapplicants to increase the use efficiency of liquid formulations of orthophosphate and polyphosphate fertilizers. Technical grade monoammonium phosphate (TGMAP), ammonium polyphosphate (APP), and an 80/20 TGMAP/APP mixture were applied with and without a commercial fulvic acid fertilizer enhancement product to a calcareous, silt loam from Finney County, KS. Soils were incubated for four weeks in petri dishes and sectioned in concentric rings from the point of application. Anion exchange resin extractability as a percent of total P assessed potential plant availability and synchrotron based X-ray absorption near-edge structure (XANES) spectroscopy was implemented to probe reaction products and pathways. As has been found with granular P fertilizers, fulvic acid applied at recommended rates does not appear to enhance the P lability of liquid treatments after four weeks. Interestingly, the 80/20 TGMAP/APP treatment demonstrated the greatest orthophosphate extractability after incubation followed by APP and then TGMAP in decreasing order. Contrary to common belief, a fair portion of polyphosphate remained extractable in the APP treatment suggesting temporary control of solution P concentrations by calcium pyrophosphate like minerals. Strategic use of polyphosphates in conjunction with orthophosphate fertilizers may allow for reduction in total P fertilizer application without risk of deficiency in calcium rich soils.