The agronomic value of co-granulated Christmas Island Grade C phosphate rock and elemental sulphur

Certain low grade phosphate rocks have low agronomic value as direct applied fertilizers and make poor quality superphosphates. With some rocks there is potential to increase their solubility in soils and hence the fertilizer value by mixing with finely divided elemental sulphur. The agronomic value of a prototype 'low cost', granular P and S fertilizer, Christmas Island Grade C phosphate rock/elemental sulphur (Xmas C/elemental sulphur), was compared against single superphosphate (SSP) and reactive phosphate rock (RPR)/elemental sulphur dry blends, North Carolina phosphate rock/elemental sulphur (NCPR/elemental sulphur) and NCPR/50% S-super (granular SSP + 44% elemental sulphur). An eight-month glasshouse trial using ryegrass (Lolium perenne L. cv. Nui), grown in a central yellow-gray earth (Aeric Fragiaqualf), indicated that fertilization with Xmas C/elemental sulphur produced dry matter yields, between 54 to 73% and 10 to 40% lower than SSP and NCPR/elemental sulphur blends respectively, and lower plant P and S uptake. Fertilization with Xmas C/elemental sulphur however produced significantly higher yield and P and S uptake than unfertilized pots and pots receiving Xmas C PR and elemental sulphur alone. The two RPR/elemental sulphur dry blends, namely NCPR/elemental sulphur and NCPR/50% S-super, produced significantly lower yield and P and S uptake than the same two fertilizers granulated with water. The difference in yields and P and S uptake between blends and granulated forms increased with time. The recovery of fertilizer P and S by plants ranged from 3 to 35% and 2 to 45% respectively with the lowest recoveries for Xmas C PR alone and elemental sulphur alone and the highest for SSP and SSP + elemental sulphur. During the period of plant growth the percentage of elemental sulphur oxidised from the elemental sulphur, Xmas C/elemental sulphur co-granule, NCPR/elemental sulphur physical blend and NCPR/elemental sulphur granular treatments were 37, 32, 32 and 45% respectively. Field evaluations, or use, of the co-granule should consider it's slow P and S release rates.