Results from a 3-year field evaluation of Chatham Rise phosphorite (CRP) at 4 contrasting sites under permanent pasture are presented. On an annual basis, both powdered and pelletised CRP were as effective as superphosphate as a source of phosphorus (P) to pasture from the time of application at all 4 sites. At 2 of the hill country sites (Ballantrae and Wanganui), pelletised CRP showed a marked residual effect in the third year. This was particularly pronounced in the clover component of the sward. At these 2 sites, a single initial autumn application of 70 kg P/ha as CRP was agronomically as effective in the third year as 3 annual applications of superphosphate at 35 kg P/ha. This finding has implications for the strategy of fertiliser use. The residual effect shown by CRP in the third year at Ballantrae and Wanganui appears to result from the effect of CaCO3on the rate of release of P from CRP. In marked contrast to this, no measurable differences were found in the residual effect of CRP and superphosphate in the 2.5 years of study on the high P-sorbing Ramiha silt loam at Pahiatua, the third hill country site. This suggests that the residual effect of CRP may be dependent on soil type. The results from a site on a high producing dairy farm (Tokomaru), indicate that CRP would be an effective maintenance phosphatic fertiliser on soils of relatively high P status.
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