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Unacidulated and Partially acidulated phosphate rock: Agronomic effectiveness and the rates of dissolution of phosphate rock

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Abstract

The agronomic effectiveness of unground North Carolina phosphate rock (PR) and partially acidulated phosphate rocks (PAPR) prepared by acidulation of the PR with 30%, 40% and 50% of the phosphoric acid needed for complete acidulation, was determined in a 4 year field experiment on permanent pastures. The soil developed from volcanic ash, and was highly P retentive. The rate of dissolution in soil of the PR component in PAPR and of PR applied directly was measured, together with bicarbonate extractable P. The priming effect of the monocalcium phosphate (MCP) component of PAPR on root growth was also investigated.

Pasture yields showed that even the 30% acidulated PAPR was as effective as fully acidulated triple superphosphate (TSP), mainly due to the high reactivity of the PR used. The 50% acidulated PAPR tended to be superior to TSP. Soluble P in PAPR caused a marked increase in root proliferation, and dry matter yields were greater than predicted from the amounts of MCP and PR in PAPR. Directly applied PR was inferior to TSP in years 1 and 2 but was equal in year 4. (There was no pasture response to application of P fertilizers in year 3.)

Dissolution rates of the PRs were determined applying a cubic model to PR dissolution data. The rate of dissolution increased with increasing acidulation and this is tentatively ascribed to increased root proliferation around PAPR granules and acidification of the clover rhizosphere during nitrogen fixation.

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Authors and Affiliations

  1. Ruakura Agricultural Research Centre, Private Bag, Hamilton, New Zealand

    S. S. S. Rajan & J. H. Watkinson

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  1. S. S. S. Rajan
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  2. J. H. Watkinson
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Rajan, S.S.S., Watkinson, J.H. Unacidulated and Partially acidulated phosphate rock: Agronomic effectiveness and the rates of dissolution of phosphate rock. Fertilizer Research 33, 267–277 (1992). https://doi.org/10.1007/BF01050882

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  • DOI: https://doi.org/10.1007/BF01050882

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