Effect of wetting and drying on P availability in calcareous arid region soils

Laboratory incubation studies were carried out on four soils from the Gezira region of the Sudan to measure the effect of wetting and drying cycles, similar to those experienced by irrigated field soils, on P availability, as measured by the sodium bicarbonate (NaHCO3) and resin extraction methods. These soils, which are of low P status, were used both with and without amendments of P. In general, for unamended soils, wetting and drying increased NaHCO3-extractable P compared to that from soils maintained dry and, for prolonged wetting and drying treatments, increased NaHCO3-extractable P compared to soils maintained for the same period at a constant water content. In contrast, resin extraction of unamended soils showed that the wetting and drying treatments decreased the extractability of P compared to soils maintained dry and there was no difference when wetting and drying was compared to constant moisture. For soils with P added, wetting and drying increased NaHCO3-extractable P compared to soils maintained moist; a similar effect was seen with resin P extraction, except that the differences were generally not significant. Thus, wetting and drying does increase P extractability in these arid region soils and NaHCO3 extraction was more sensitive than resin extraction in measuring the effect.