Drainage and desalinization of heavy clay soil in Portugal

Abstract

The Leziria Grande area consists mainly of poorly drained, saline clay soils of marine origin. Three experimental fields were laid out to find whether subsurface drainage can be effective in lowering the groundwater table and improving desalinization. Subsurface drainage results in a lower groundwater table than does surface drainage. With increasing spacing, the groundwater remains at a higher level for longer periods, which is expressed here by the sum of exceedances of the groundwater table above 30 cm during winter. Soil salinity, expressed as EC 1:2, and sodicity, expressed as E.S.P., decreased during the first 3 years, in which precipitation varied between 600 and 750 mm and the average drain outflow was about 250 mm. The leaching efficiency decreased with time, indicating that the removal of salt is a slow process in fine-textured soil. Application of gypsum lowered the E.S.P. The infiltration rate and the drain outflow increased. Although the total amount of salts in the drainwater was 40% higher than for the untreated plots, no lower EC 1:2 values were found. This is ascribed to spatial variability in soil salinity.