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Potassium in Two Humid Tropical Ultisols under a Corn and Soybean Cropping System: I. Management

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

^* Corresponding author.

Sub-optimum soil K levels often limit corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] production in humid tropical Ultisols. The objectives of this study were to determine soil and plant critical levels of K and optimum rates of K fertilization for corn and soybean grown in the Amazon Basin of Peru. Two field experiments were conducted on Typic Paleudults, a loam and a sandy loam. Five K rates, ranging from 0 to 120 kg ha^–1, were broadcast and incorporated prior to planting the first crop of a corn, corn, soybean rotation at both sites. At the loam site, K treatments were reapplied at the end of this rotation and three more corn crops grown. The corn stover was returned while the soybean stover was removed. Critical exchangeable K levels for corn were 110 kg ha^–1 on the loam and 90 kg ha^–1 on the sandy loam, while for soybean it was 75 kg ha^–1 for both soils. The critical levels of K in plant tissue at flowering were 13 g kg^–1 in corn and 12 g kg^–1 in soybean. Soybean seemed to lower exchangeable K to where Kwas released h m non-exchangeahle sources. An annual cropping system of corn and soybean with an intervening cover crop during the rainy season is proposed for the region. Potassium fertilization is recommended only prior to growing corn at 90 kg ha^–1 on a loam and 60 kg ha^–1 on a sandy loam.

Received for publication March 27, 1991.