Short-term variations in chemical properties of Vertisols as affected by amounts, carbon/nitrogen ratio, and nutrient concentration of crop residues

Wheat (Triticum aestivum L.), faba bean (Vicia faba L.), and soybean (Glycine max L.) are sown in rotation with cotton (Gossypium hirsutum L.) on eastern Australian Vertisols. Several long-term studies, where soil was sampled annually, have reported that rotation crops affected only soil structure and soil nitrogen (N). The long time interval between sampling times in the field studies may have resulted in changes in soil quality remaining undetected. The objectives of this study were to (1) determine if mineralization of crop residues in Vertisols previously sown with cotton resulted in short-term changes in soil properties, and (2) describe their nature. Short-term changes in properties of Vertisols were studied in two laboratory experiments by adding milled residues to a sodic, heavy clay soil (635 g/kg clay, exchangeable sodium percentage, ESP, of 12) and a nonsodic medium clay soil (540 g/kg clay, ESP of 2). The crop materials used were vegetative and seed material from faba bean, vegetative material from wheat at anthesis ("green wheat"), cotton, wheat straw ("mature wheat"), and soybean stubble, which were added to the soils at five rates: 0% (untreated control), 2%, 5%, 10%, and 20% of the air-dried weight of the soil. Soils were analyzed for pH (in CaCl2), EC(1:5), exchangeable calcium (Ca), magnesium (Mg), potassium (K), and sodium (Na) and were measured with an atomic absorption spectrophotometer after washing with aqueous alcohol and aqueous glycerol to remove soluble salts followed by extraction with alcoholic 1 M NH4Cl at a pH of 8.5. The sodicity indices ESP and EC(1:5)/exchangeable Na were then calculated. Most soil improvement, viz. distinct increases in exchangeable K and lower sodicity, occurred with green wheat residues, due to its high K concentration and low C:N ratio. Variation in soil changes between crop residues was directly related to the amounts added, their C:N ratios, and nutrient concentrations. The primary cause of the inability of rotations to cause extensive soil changes appears to be the small amounts and low mineral content of crop residues returned to the soil. Significant improvements of soil quality in Vertisols sown with cotton-based farming systems are more likely if large amounts of green cereal crop residues are added.