Fixing Phosphorus: Considering Cation Complexing Co-Applicants to Maintain Phosphorus Lability in an Acid Soil.

Fixation reactions with iron and aluminum significantly reduce the use efficiency of phosphorus fertilizers in acid soils such as those found in the southeastern United States and Brazil. Finding methods to keep the nutrient in a more readily plant available form behooves growers by mitigating chances of deficiency and lowering production costs, while simultaneously reducing risks of non-point source pollution of nearby surface water. One proposed solution is to coapply humic substances with fertilizers to complex solution cations that can adversely react with P before fixation can occur and/or compete with P for sorption sites on soil colloids. This study investigated the effects of several commercially distributed co-applicants on phosphorus applied as liquid fertilizers in an acid soil. Technical grade monoammonium phosphate (TGMAP), technical grade diammonium phosphate (TGDAP), ammonium polyphosphate (APP), and an 80/20 TGMAP/APP mixture were applied with and without commercial fertilizer enhancement products to an oxisol from Brazil. 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, while a suite of other wet chemical assessments were employed to investigate how P fate and transport were impacted. P lability and partitioning varied with the type of co-applicant and the fertilizer that the product was paired with. Further study will be necessary to determine how humic substances can best be utilized in future agricultural systems.