Abstract
Six phosphate rocks (PRs), Sechura (SPR), North Carolina (NCPR), Egypt (EPR), Arad (APR), Zin (ZPR), and Nauru (NPR), were compared with single superphosphate (SSP) as annually applied P fertilisers for mown ryegrass/ 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 the other PRs were applied as received and so differed considerably in particle size distribution. Measurements were made of annual dry matter (DM) production and % P in harvested herbage, and Olsen P in the soil at the end of each year. Overall, responses were in the order SSP>SPR∼NCPR>APR∼EPR∼ZPR∼NPR. There were important differences in responsiveness to PRs at the two sites. The agronomic performance of PRs was better related to their extractability by formic acid than to extractability by citric acid. Replacement of the standard citric acid test used in New Zealand by the formic acid test is recommended.
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