EXCHANGEABLE SODIUM AND SOIL WATER BEHAVIOR UNDER FIELD CONDITIONS

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Abstract
The effect of exchangeable Na on soil water behavior under field conditions was investigated in 5 differentially gypsum treated plots of a highly sodic soil. The soil is representative of the sodic soils of the Indo-gangetic plains in northern India. The soil texture is sandy loam in the surface 0-15 cm and the clay fraction is dominantly illitic. Exchangeable sodium percentage (ESP) varied from 4-38 in 0-15 cm and 28-88 in the 15-30 cm layer. Basic intake rate of the soil increased from 8.7 mm per day in soil of highest ESP to about 25 mm per day in soil of lowest ESP. Higher basic intake rate and increased retention at low suctions resulted in greater profile water storage. Soil water diffusivity values (D) calculated from the water distribution following evaporation showed that at a water content of 0.22 cm3/cm3, while D (cm2/day) decreased from 34-26 when ESP increased from 4-11, the D value sharply decreased to 8 with a further rise in ESP to 16. Improvement in transmission characteristics resulting from lowered ESP was reflected in the drying pattern of soils. The 1st stage of drying was delayed in low ESP treatments compared to the high ESP treatment because of regular movement of water from lower layers to the surface to meet the evaporation demand. The effect of treatments on water movement was noticeable to a depth of 90 cm studied in these experiments, although the improvement in layers below 15 cm was very small in treatments receiving lower doses of gypsum.

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