Comparison of ground and mini-granulated phospate rocks with superphospate as phospate fertilisers

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Abstract
Perennial ryegrass was grown in pots over a period of 10 months to compare the effectiveness of ground Chatham Rise nodules (CR),ground and mini–granulated North Carolina phosphate rock (NCP and NCG respectively), and ground, uncalcined Christmas ‘C’ grade ore (CC), with superphosphate (super) using a highly phosphate retentive soil at pH 5.2. The fertilisers were applied at six rates, 3 mm below the surface. A no–P control was also included. At less than 77% of the calculated yield maximum of dry matter from super (total of 8 cuts), reactive phosphate rocks (PRs—CR, NCP and NCG) were superior to super, but above this level they were inferior. At near maximum production (90%), the yields from the reactive rocks were 88% that of super, that from CC was 75% that of super and the control plot produced 44% of the yield of the super treatment. The effectiveness of PRs were generally in the order of NCP < CR < NCG< CC. The relative performance of reactive rocks (CR and NC), at almost all of the rates ofP, was better than super 6 months after addition. Converting NCP into even mini-granules (0.08 to 0.33 mm) slightly reduced its effectiveness. From Olsen bicarbonate values of soil it was concluded that at high rates not all of the PRs were dissolved after 10 months time. Downward movement of phosphate in all treatments and at all rates was mostly confined to the top 20 mm soil layer.

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