| Abstrakt: |
The possible effect of urea hydrolysis on the availability of phosphorus (P) from phosphate rock (PR) was evaluated in two greenhouse experiments with maize, using two sources of PR — Pesca (Colombia) and Bayovar (Peru) — representing low and high chemical reactivity, respectively. In Experiment I, on a neutral Josephine silty clay loam (pH 6.2) (Typic Haplozerult), Bayovar PR compacted with urea (Bayovar PR + urea) performed better than Bayovar PR compacted with NH4Cl (Bayovar PR + NH4Cl) in increasing dry-matter yield at a rate of 100 mg P kg-1but not at rates of 50 and 200 mg P kg-1. It was also found that the dry-matter yield obtained with compacted Bayovar PR products was significantly higher when the N ratios of urea: NH4Cl were 1:1 or higher than when the ratios were below 1:1. Although Bray I–P can overestimate available P from PR with respect to that from TSP, a good relationship was observed between Bray I–P and dry-matter yield from various compacted Bayover PR products with a small particle size (-0.43 + 0.15 mm). In Experiment II an acid Bladen sandy loam (pH 4.5) (Typic Albaquult) was used. Finely ground Bayovar PR (- 0.07 mm) was about 66% as effective as TSP in increasing dry-matter yield, whereas Pesca PR was ineffective. When Pesca PR was partially acidulated with H3PO4at 20% level (PAPR), it became 70% as effective as TSP. Granulated PAPR and Pesca PR compacted with TSP (Pesca PR + TSP) were found to be equally effective in increasing dry-matter yield when both products had the same particle size and the same water-soluble and citrate-soluble P as percent of total P, and when prilled urea was used as the N source. However, when urea was compacted with Pesca PR and TSP, the product's effectiveness was further increased by 30% and to the same level as TSP. In summary, the results tend to support the suggestion that urea hydrolysis can be beneficial in increasing the availability of P from PR to plants in soils having medium to high organic matter contents. |
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