Availability of phosphorus and nitrogen from modified mono-ammonium phosphate (MAP) fertiliser compounds

Moderating nutrient release to match plant requirements more closely can improve nutrient use efficiency. The formation of lower-solubility ammonium salts may be a simple and cost-effective way to slow the release of nitrogen (N) from fertiliser sources. Several modified mono-ammonium phosphate (MAP) fertiliser compounds were prepared by adding magnesium silicate to regular MAP fertiliser and reconstituting the granules. This process results in the formation of schertelite that may potentially slow the release of both N and phosphate to soil solution. These modified MAP fertiliser compounds include more citrate-soluble N and phosphorus (P) than standard MAP fertiliser. The fertiliser compounds were added to a P-responsive soil and barley plants were grown for 5 weeks to investigate shoot yield responses to nutrient availability of the modified MAP fertiliser compounds. Reverse dilution tracing techniques were also used to compare the fertiliser solubility and P availability of the modified MAP fertiliser compounds with regular MAP fertiliser. Barley (Hordeum vulgare) plants recovered P equally and efficiently over 5 weeks of growth in the P-responsive soil, suggesting that the fertiliser compounds were sufficiently soluble to meet plant requirements and that phosphate release was not slowed by the formation of schertelite. However, shoot yields were generally lower when the barley plants were grown with the modified MAP fertiliser compounds compared to standard MAP fertiliser. This reduced growth was likely due to decreased N availability through a slower release of N, thus further research is warranted to determine the potential for these products to improve N use efficiency.