COMPARISON OF 6 PHOSPHATE ROCKS AND SINGLE SUPERPHOSPHATE AS PHOSPHATE FERTILIZERS FOR CLOVER-BASED PASTURE

Six phosphate rocks (PRs), Sechura (SPR), North Carolina (NCPR), Egypt (EPR), Arad (APR), Zin (ZPR), and Nauru (NPR), were compared with sin gle superphosphate (SSP) as annually applied P fertilisers for mown ry egrass/ white clover swards at two sites, in the central North Island (Te Kuiti) and southern South Island (Woodlands) of New Zealand over a period of three years. NPR was finely ground before application but t he other PRs were applied as received and so differed considerably in particle size distribution. Measurements were made of annual dry matte r (DM) production and % P in harvested herbage, and Olsen P in the soi l at the end of each year. Overall, responses were in the order SSP>SP R similar to NCPR>APR similar to EPR similar to ZPR similar to NPR. Th ere were important differences in responsiveness to PRs at the two sit es. The agronomic performance of PRs was better related to their extra ctability by formic acid than to extractability by citric acid. Replac ement of the standard citric acid test used in New Zealand by the form ic acid test is recommended.