THE AGRONOMIC VALUE OF CO-GRANULATED CHRISTMAS ISLAND GRADE-C PHOSPHATE ROCK AND ELEMENTAL SULFUR

Certain low grade phosphate rocks have low agronomic value as direct a pplied fertilizers and make poor quality superphosphates. With some ro cks there is potential to increase their solubility in soils and hence the fertilizer value by mixing with finely divided elemental sulphur (SO). The agronomic value of a prototype 'low cost', granular P and S fertilizer, Christmas Island Grade C phosphate rock/elemental sulphur (Xmas C/S degrees), was compared against single superphosphate (SSP) a nd reactive phosphate rock (RPR)/S degrees dry blends, North Carolina phosphate rock/S degrees (NCPR/S degrees) and NCPR/50% S-super (granul ar SSP + 44% S degrees). An eight- month glasshouse trial using ryegra ss (Lolium perenne L. cv. Nui), grown in a central yellow-grey earth ( Aeric Fragiaqualf), indicated that fertilization with Xmas C/S degrees produced dry matter yields, between 54 to 73% and 10 to 40% lower tha n SSP and NCPR/S degrees blends respectively, and lower plant P and S uptake. Fertilization with Xmas C/S degrees however produced significa ntly higher yield and P and S uptake than unfertilized pots and pots r eceiving Xmas C PR and SO alone. The two RPR/S degrees dry blends, nam ely NCPR/S degrees 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 an d S by plants ranged from 3 to 35% and 2 to 45% respectively with the lowest recoveries for Xmas C PR alone and S degrees alone and the high est for SSP and SSP + S degrees. During the period of plant growth the percentage of S degrees oxidised from the S degrees, Xmas C/S degrees co-granule, NCPR/S degrees physical blend and NCPR/S degrees granular treatments were 37, 32, 32 and 45% respectively. Field evaluations, o r use, of the co-granule should consider it's slow P and S release rat es.