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Management and Dynamics of Potassium in a Humid Tropical Ultisol under a Rice-Cowpea Rotation

Soil Science Dep., North Carolina State Univ., Raleigh, NC 27695-7619.

^* Corresponding author.

Little is known about the role of K fertilization, stover management, and tillage methods on soil K availability as they affect rice (Oryza sativa L.) and cowpea [Vagina unguiculata (L.) Walp.] productivity on Ultisols of the humid tropics. The effects of five K rates (0-120 kg K ha^–1), returning or removing stover, and three tillage methods (notill, strip, and conventional) were evaluated during 12 crops of rice and cowpea grown for a 4-yr period. Fertilizer K was applied to the first seven crops. The site was a recently cleared, 18-yr-old secondary forest in the Peruvian Amazon Basin. The soil was a fine-loamy, siliceous, isohyperthermic Typic Paleudult. Soils samples were collected at each crop harvest to 90 em in 15-cm increments. Potassium fertilizer always increased grain yields when stover was removed. Conversely, responses to K additions were seldom obtained when the stover was returned. The extractable K (Modified Olsen) critical level for both upland rice and cowpeas was calculated to be 0.10 cmol L^–1. Returning stover with no K fertilization maintained soil K concentrations above critical levels for both species up to the last crop of the rotation. Residual effects of fertilizer K were prolonged by returning the stover. When stover was returned, subsoil exchangeable K increased with increasing rate of K fertilization. Removal of stover resulted in greater increases in subsoil exchangeable K at the 40 kg K ha^–1 rate than at 120 kg K ha^–1, apparently because the higher rate resulted in K fixation. Tillage methods did not affect crop yields.

Received for publication March 27, 1991.

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