Potential Agronomic Value of Calcium in Some Phosphate Rocks from South America and West Africa
Hellums, D. T. | Chien, S. H. | Touchton, J. T.
Phosphate rock (PR) has been used for decades as a P fertilizer for plants. Apatite is a basic P mineral that could have agronomic value as a source of Ca. Little information is available in literature on this possibility, however. A greenhouse experiment (6 wk) was conducted to compare the Ca value of six PRs (Bahia Inglesa, Chile; Bayovar, Peru; Capinota, Bolivia; Tilemsi Valley, Mali; Tahoua, Niger; and Hahotoe, Togo) with respect to CaCO₃. The soil used was an acid (pH 4.5) Bladen sandy loam (Typic Albaquult). Calcium was applied at rates of 0, 200, 400, and 800 mg kg⁻¹. All treatments received 400 mg P kg⁻¹ as KH₂PO₄ so that P was not a limiting factor on plant (maize) growth. In general, the effectiveness of Ca response by maize to various PRs followed the order of the PRs' reactivity except in the case of Capinota PR which had a substantial amount of free carbonates. Dry-matter yield, Ca uptake, and soil exchangeable Ca increased with increasing Ca rates from the various Ca sources. The relative agronomic effectiveness (RAE) of various PRs with respect to CaCO₃ (100%) in terms of increasing dry-matter yield and Ca uptake ranged from 28 to 88% and from 8 to 58%, respectively. The results showed that PRs with medium or high reactivity have potential Ca value, in addition to their use as a P source, when directly applied to acid soils.
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