AGRONOMIC EFFECTIVENESS OF PARTIALLY ACIDIULATED PHOSPHATE ROCK FERTILIZERS IN SELECTED NEW-ZEALAND SOILS

The agronomic performances of some longlife superphosphate (LSP) and partially acidulated phosphate rock (PAPR) fertilisers were compared with single superphosphate in glasshouse pot trials over 9-10 months using a range of New Zealand soils. The fertilisers used differed in the type (origin) of phosphate rock (PR), the acid to rock ratio used in the production of LSP, percentage acidulation of PAPR, and incorporation of elemental sulfur into LSP. The agronomic effectiveness of the fertilisers studied was primarily influenced by the type of PR and percentage acidulation of PAPR, but not by the acid to rock ratio. Variations in the agronomic effectiveness of LSP and PAPR fertilisers made from different PRs did not reflect the relative solubilities of the original PRs. This was mainly attributed to alterations in the chemistry and consequent solubility of PR which occurred during fertiliser manufacture. Plant phosphorus uptake was highly correlated with fertiliser solubility in 2% citric and 2% formic acids. The relative agronomic effectiveness of LSP and PAPR fertilisers was not significantly related to any single soil property.