Isotopic determination of sulfur (S) availability from S coated mono-ammonium phosphate

Abstract

An evaluation of three novel elemental sulfur (S°) coated mono-ammonium phosphate (MAP) products ((MAP with coarse (75-100μm) S° coating (MAP75), MAP with fine (17-24μm) S° coating (MAP17), and MAP with 1/3 coarse S°, 1/3 fine S°, and 1/3 sulfate coating (MAPM)) was undertaken in an intact core pot trial using red clover ('Trifolium pratense') and compared with single superphosphate (SSP). Sulfur (S) uptake from coated S sources was not significantly different (p<0.05) from SSP after 120 days of growth. Between 34% and 53% plant S was derived from the fertiliser sources. There was no significant difference in % fertiliser recovery between MAPM and SSP at either harvest and at both harvests fertiliser S recovery was in the order MAP < MAP75 < MAP17 < MAPM < SSP. The key finding was that a MAP coated with 1/3 sulfate and 2/3 S° was able to supply clover with the same amount of S, at the same rate, as SSP over 120 days. Under leaching conditions, this availability may result in slower, longer release of S as the coarser fraction slowly meets plant demands and protects added S from leaching.