(Lack of) Effect of Cation-Complexing Compounds On Phosphorus Fertilizer Efficiency.

Adsorption and precipitation reactions reduce the efficiency of phosphorus (P) fertilizers. In acidic soils, P strongly adsorbs to Al and Fe oxyhydroxides. In calcareous soils, large fractions of P may precipitate as Ca-phosphate minerals. It has been suggested that addition of cation-complexing compounds to P fertilizer may increase the solubility of added P and therefore increase the fertilizer efficiency. To obtain a better understanding of how metal-complexing ligands may affect P solubility in soils, batch experiments were carried out with 5 soils (varying in soil properties) and several cation-complexing ligands, with or without addition of P. Solubilization of cations, P and soil organic matter was determined. The strongest P mobilization upon addition of the ligands was observed in soils with low P status and slightly acidic pH. In soils with pH>7, the organic ligands had very limited effect. A Petri dish experiment was undertaken to test the effect of these ligands on P availability at commercially realistic rates. The products were coated on MAP fertilizer at 1% (by mass). A newly developed visualization technique was used to assess whether the ligand addition increased the diffusion of P from the granules. This technique showed that none of the ligands had any significant effect. This was also confirmed by chemical analyses after sampling of soil from concentric rings around the granules. In agreement with the Petri dish experiment, no significant effects of the ligand coatings were observed on yield, P uptake or shoot P concentrations in wheat in a pot experiment with three P-responsive soils. Theoretical calculations indicate that no effect is to be expected from the addition of cation-complexing compounds at commercially realistic rates on P availability.