A preliminary evaluation of Chatham Rise phosphorite as a direct‐use phosphatic fertiliser

In a glasshouse experiment using four soils the effectiveness of Chatham Rise phosphorite (CRP), in both powdered and pelletised form, as a source of P to ryegrass plants was compared to that of ‘granulated’ single superphosphate. The magnitude of the response to applied P varied between the soils. The type of fertiliser, its physical form, the method and rate of application, and time also influenced the response to applied P. Overall, powdered CRP was as effective as superphosphate in increasing ryegrass yields. When compared at 90% of the yield maxima of surface‐applied superphosphate (taken as 100), the relative values for surface‐applied powdered CRP were 96, 99, and 100 for Ramiha, Kumeroa, and Wainui soils respectively. Incorporation increased relative values to 100 and 106 on Ramiha and Wainui soils respectively. Pelletising reduced the overall performance of the CRP in supplying P to the plant to only a small extent. For surface‐applied pelletised CRP the relative values were 85, 93, 93, and 104 with Ramiha, Kumeroa, Wainui, and Tokomaru soils respectively. Total herbage P uptake for the six harvests combined showed similar trends in the performance of both powdered and pelletised CRP in comparison with superphosphate. Extrapolation of these results to the field situation is discussed.