Changes to the physical properties of soils puddled for rice during drying

Puddling is used to prepare soil for irrigated rice (Oryza sativa L.) throughout SE Asia creating a soft mud often over a plough pan. Whilst these conditions are favourable for the rice crop, they are less so for any dryland crop, such as mungbean (Vigna radiata (L.) Wilzek) or soybean (Glycine max L. Merr.), grown in the dry season (DS) after rice harvest leading to low or erratic yields. The physical properties of soils puddled for rice were measured as they changed in the period after draining flood water for rice harvest. The experiments were run at four sites In Indonesia and the Philippines for 4-6 weeks, during which they were kept free from weeds. Apart from one site, where there were heavy rain showers, the sites received no effective rainfall during the experiment, Soil moisture content and hydraulic potential in the upper 40 cm were measured regularly in five replicate plots. Evaporation from the soil was measured using mini-lysimeters, Strength properties were measured using a penetrometer. Cracks were measured using an intercept technique. Hydraulic conductivity was calculated using a modification of the instantaneous profile technique. The sites behaved in a similar fashion, with initial loss of water by drainage, followed by loss by evaporation from the surface, However, the low conductivities limited the upwards supply of water for evaporation so that evaporation from the soil surface decreased. This resulted in strong drying of the upper 5-10 cm, but much smaller decreases in moisture content lower down, The strength of the upper layers increased as they dried. The low conductivities suggest that waterlogging due to heavy rain is likely to cause problems for dry season cropping. However, the preservation or. water in the sub-soil may allow flexibility in the sowing dates for dry season crops to avoid periods of heavy rain or shortage of labour. (C) 2000 Published by Elsevier Science B.V.