Effect of Liming and N and P Fertilization on the Growth of Soybean in an Acid Soil in Zimbabwe.
Justice Nyamangara (Dr), Memory Matokwe, and Chipo Musharo. University of Zimbabwe, Department of Soil Science & Agricultural Engineering, Mount Pleasant Drive, Harare, Zimbabwe
Soybean (Glycine max L. Merr.) accounts for 40% of the national edible oil and seed cake output in Zimbabwe. The inclusion of soybean in cropping systems significantly improves the availability of N, a nutrient inherently deficient in soils in the smallholder farming sector of Zimbabwe. A 2-yr field study was conducted to assess the effect of liming, N and P fertilization on the performance (nodulation and grain yield) of soybean variety (Solitaire) grown on an acid soil (pH 0.01 CaCl2 = 4.6), limed or unlimed. Liming significantly (p<0.05) improved nodulation in both the first (21% nodule number; 81% nodule weight) and second (56% nodule number; 81% nodule weight) seasons, while the application of 30 kg N ha-1 ammonium nitrate depressed (p<0.05) nodulation in both the first (34% nodule number; 60% nodule weight) and second seasons (96% nodule number; 96% nodule weight). P fertilization only improved nodulation in unlimed treatments in the second season. There was no liming effect on grain yield (average 3.6 and 3.7 t ha-1, respectively). N fertilization had no significant (p<0.05) effect on grain yield, and 30 kg N ha-1 depressed yield by 13% in unlimed treatments, but increased stover yield by 10-19%. P fertilization only increased grain yield in the second season in unlimed treatments at 7.5 kg P ha-1, and no further increases were observed at higher P rates (up to 22.5 kg P ha-1). It was concluded that although the study variety was relatively tolerant to the prevailing acidity (no lime effect), the acidity was high enough to affect biological nitrogen fixation (nodulation) and this can significantly affect grain yield in poorer soils. Increased P uptake due to liming implied that P uptake efficiency was improved at higher more favourable pH values.