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

Puddling is used to prepare soil for irrigated rice (Oryza sativa L.) throu ghout SE Asia creating a soft mud often over a plough pan. Whilst these con ditions 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 o r erratic yields. The physical properties of soils puddled for rice were me asured as they changed in the period after draining flood water for rice ha rvest. The experiments were run at four sites In Indonesia and the Philippi nes 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 effect ive rainfall during the experiment, Soil moisture content and hydraulic pot ential in the upper 40 cm were measured regularly in five replicate plots. Evaporation from the soil was measured using mini-lysimeters, Strength prop erties were measured using a penetrometer. Cracks were measured using an in tercept technique. Hydraulic conductivity was calculated using a modificati on of the instantaneous profile technique. The sites behaved in a similar f ashion, with initial loss of water by drainage, followed by loss by evapora tion from the surface, However, the low conductivities limited the upwards supply of water for evaporation so that evaporation from the soil surface d ecreased. This resulted in strong drying of the upper 5-10 cm, but much sma ller decreases in moisture content lower down, The strength of the upper la yers increased as they dried. The low conductivities suggest that waterlogg ing 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 i n the sowing dates for dry season crops to avoid periods of heavy rain or s hortage of labour. (C) 2000 Published by Elsevier Science B.V.