Effects of straw burning and drying-rewetting cycles on the fertility of some soils of North-East Nigeria
The effects of straw burning and drying-rewetting cycles on nutrient availability were studied in an attempt to assess the practical importance of the yearly precropping practice by farmers of burning off excess crop residues and of the seasonal climate of North-East Nigeria. Changes in soil properties resulting from heating three soils up to 700°C and from field burning of straw on three soils were determined. Heating to 250° did not affect soil texture but increased exchangeable NH4+-N, available P, Fe, Mn and phosphate adsorption. A peak of NH4+-N representing decomposed organic N, retained by cation exchange sites was observed after heating at 250°. Soil physical and chemical properties were changed by heating to ≥500^o, resulting in fusion of clay particles into sand-sized particles, decomposition of soil organic matter and alteration of soil minerals with a corresponding increase in non-exchangeable cation status of the soils. The changes observed after field burning of straw were mainly due to deposition of ash; these include a dramatic increase in phosphate adsorption and addition of considerable amounts of inorganic P and exchangeable cations. The availability to maize of the inorganic P returned in ash after field burning or released as a result of laboratory heating of soils was assessed over a 7 day period. The increased P status of the soils after straw burning or heating was not significantly reflected in plant uptake of P but K adsorption was enhanced from the ash. Fertilization of heated and field burnt + overlying ash soil samples resulted in adsorption of P with a corresponding reduction of P concentration in the soil solution and P uptake. The N mineralized after rewetting air-dried clay soils or those containing very low organic matter was insignificant. Investigations also showed that, when dried soils were rewetted, the flush of mineral N was derived mainly from soil organic matter and not from killed microbial cells.