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Phosphate Rock Fertilization in Tilled and No-Till Low-Input Systems in the Humid Tropics

Int. Inst. for Tropical Agric., Ibadan, Nigeria
Sanchez, Dep. of Soil Sci., North Carolina State Univ., Raleigh, NC 27695-7619.

^* Corresponding author.

Application of phosphate rock is of interest to low-input systems in the humid tropics, because phosphate rock is less expensive than ordinary superphosphate and releases P quickly in acid soils. However, the lack of incorporation in zero-tillage systems may result in low availability of surface-applied P. This study was directed toward determining the effects of these two P sources under no-till and rotovation tillage systems on crop production in a fine-loamy siliceous, isohyperthermic Typic Paleudult of Yurimaguas, Peru. A rotation of Al-tolerant cultivars of rice (Oryza sativa L.)-rice-cowpea (Vigna unguiculata) was followed for seven consecutive harvests. Grain yields increased with rotovation in the first crop, were not affected by tillage methods during the second and third crops, but decreased with rotovation from the third to the seventh crops. Sechura phosphate rock at a soil pH of 4.5 was as effective as superphosphate in supplying available P. A total of 13.9 Mg ha^–1 of rice and 2.5 Mg ha^–1 of cowpea grain was produced in seven harvests in newly cleared fields without lime or P application. There were significant responses to P fertilization in one rice crop and in both cowpea crops. On the average, however, rice yield did not respond to P. A single application of 22 kg P ha-' was sufficient to produce 85% of the maximum yield of cowpea for 2 yr. The results with cowpea indicate that broadcast phosphate rock is a good source of P for low-input systems on acid soils where acid-tolerant cultivars are used.

Received for publication October 7, 1987.

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