0041-3216/2005/030001-07 ©2005 Trop. Agric. (Trinidad) Trop. Agric. (Trinidad) Vol. 84 No. 3 January 2007 1 Calcium, magnesium, and potassium interaction on groundnut (Arachis hypogaea L.) yield, nutrient uptake, and soil nutrient levels of an acid sandy soil W.T. Mupangwa International Crops Research Institute for the Semi-Arid Tropics, P O Box 776, Bulawayo, Zimbabwe F. Tagwira Faculty of Agriculture and Natural Resources, Africa University, P.O. Box 1320, Mutare, Zimbabwe Research efforts to examine groundnut (Arachis hypogaea L.) response to fertilizer have been frus- trated by the erratic nature of responses to individual nutrients. An experiment was established on acid sandy soil to determine the effects of Ca, Mg, and K on groundnut yield, nutrient uptake, and soil chemical properties. Field experiments were laid out in a 3 × 3 × 2 factorial structure in a randomized complete block design with three replicates. A greenhouse experiment was laid out in a 3 × 3 × 3 factorial combination of Ca, Mg, and K. In the ield experiment, 3 rates of Ca and Mg, and 2 of K were combined factorially to give 18 treatments. The nutrient rates were: 0, 160, and 320 kg Ca ha −1 ; 0, 9, and 18 kg Mg ha −1 ; and 0, and 50 kg K ha −1 K. Calcium signiicantly increased groundnut kernel yield, but decreased dry matter production. Negative interaction of Ca and K resulted in suppressed uptake of K. Soil Ca, Mg, and pH levels increased following applica- tion of calcitic lime (40% Ca, 4.5% Mg). Keywords: Calcium; Magnesium; Potassium; Groundnut; Interaction Nutrient depletion is one of the major causes of soil degradation in sub-Saharan Africa (FAO, 2003). There is widespread nutrient mining and soil fertility decline in the small- holder farming sector of southern Africa. Ex- port of nutrients through crop residues fed to livestock and harvested grain, is rampant in the smallholder sector. The majority of small- holder farmers are cropping on acid granitic sandy soils. Although granitic sandy soils are inherently infertile, poor soil fertility manage- ment by the smallholder farmers exacerbates the situation. The farmers add little or insig- niicant quantities of nutrient inputs as organic or inorganic fertilizers. The yield potential of cropping systems on acid soils is restrained by deficiencies of P, Ca, Mg, and K, and toxicities of Al, Mn, and Fe (Salazar et al., 1997). Sub-Saharan Africa accounts for <1.8% of global fertilizer use and <0.1% of global fertilizer production (Bationo et al., 2004). The current fertilizer use in the smallholder farm- ing sector is conducive to the development of soil acidity and is biased towards N, K, and P use. There is need for emphasis to be also placed on the application of micronutrients and secondary nutrients such as Ca and Mg (Bationo et al., 2004). In granitic sandy soils, the greatest variation in cation ratios occurs between Ca and K. Uptake of K in acid soils is influenced by quantities of exchangeable Ca and Mg. A K supply in the rooting environment that is far in excess of Ca and Mg supplies,