Preparation,forms and properties of controlled-release phosphate fertilizers |
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Authors: | N S Bolan M J Hedley P Loganathan |
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Affiliation: | (1) Fertilizer and Lime Research Centre, Department of Soil Science, Massey University, Palmerston North, New Zealand |
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Abstract: | Controlled-release phosphate fertilizers include phosphate rocks (PRs) for direct application, partially acidulated phosphate
rocks (PAPRs) and thermal phosphates. Phosphate rocks contain apatite as the main P containing mineral, the composition and
the chemical nature of which vary between PRs. Based on the solubility in chemical extractants PRs are broadly grouped into
‘reactive’ and ‘unreactive’. The ‘reactivity’ of PRs is influenced strongly by the extent of carbonate substitution for phosphate
in the apatite minerals. Under certain soil and climatic conditions reactive PRs (RPRs) can be used as a source of P for direct
application.
Partially acidulated phosphate rocks (PAPRs) are produced either by direct partial acidulation of PRs with mineral acids or
by mixing PRs with fully acidulated superphosphate reaction mixtures. Partial acidulation of PRs with H3PO4 generally results in higher water soluble P contents than those acidulated with H2SO4. Mixing of RPRs with superphosphate reaction mixtures sometimes results in the preferential consumption of free acid and
thereby increases the amounts of residual unreacted PRs.
Thermal phosphates are produced by either heating PRs below melting point both in the presence and the absence of silica (calcined
phosphates) or heating PRs with silica above melting point (fused phosphate). These phosphates are alkaline in nature and
hence suitable for acidic soils. |
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Keywords: | Calcined phosphate fused phosphate partially acidulated phosphate rocks phosphate rock thermal phosphate |
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