Acidification of a kaolinitic Alfisol under continuous cropping with nitrogen fertilization in West Africa

Increased use of N fertilizer and more intensive cropping due to the rising food demand in the tropics requires design and evaluation of sustainable cropping systems with minimum soil acidification. The objectives of this study were to quantify acidification of an Oxic Kandiustalf with different types of N fertilizer in two cropping systems under no-tillage and its effect on crop performance. Chemical soil properties in continuous maize (Zea mays L.) and maize-cowpea (Vigna unguiculata (L.) Walp) rotation were determined with three N sources (urea (UA), ammonium sulfate (AS) and calcium ammonium nitrate (CAN)) in Nigeria, West Africa, during five years. Chemical soil properties were related to grain yield and diagnostic plant nutrient concentrations. For the three N sources, the rate of decline in soil pH in maize-cowpea rotation was 57±7.5% of that in continuous maize, where double the amount of N fertilizer was applied. The rate of soil acidification during the five years was greater for AS than for UA or CAN in continuous maize, and not different for UA and CAN in both cropping systems. With AS, soil pH decreased from 5.8 to 4.5 during five years of continuous maize cropping. Exchangeable acidity increased with N fertilization, but did not reach levels limiting maize or cowpea growth. Return of residues to the soil surface may have reduced soluble and exchangeable Al levels by providing a source of organic ligands. Soil solution Mn concentrations increased with N fertilization to levels likely detrimental for crop growth. Symptoms of Mn toxicity were observed on cowpea leaves where AS was applied to the preceding maize crop, but not on maize plants. Soil acidification caused significant reductions in exchangeable Ca and effective CEC. Main season maize yield with N fertilization was lower with AS than with UA or CAN, but not different between UA and CAN during the six years of cropping. The lower maize grain yield with AS than with the other N sources was attributed to lower pH and a greater extractable Mn concentration with AS. When kaolinitic Alfisols are used for continuous maize cropping, even under no-tillage with crop residues returned as mulch, the soil may become acidifed to pH values of 5.0 and below after a few years. The no-till cereal-legume rotation with judicial use of urea or CAN as N sources for the cereal crop is a more suitable system for these poorly buffered, kaolinitic soils than continuous maize cropping. The use of AS as N source should be avoided. H Marschner Section editor