Soil salinity under traditional and improved irrigation schedules in central Spain

Traditional irrigation management in some semiarid zones waste water relative to crop evapotranspiration requirements. Improved irrigation schedules (irrigation depths were adjusted to seasonal crop consumption), however, may reduce deep percolation and fail to provide adequate leaching of salts. We hypothesized that salt discharge induced by rainfall may partially offset the need for a larger leaching fraction from irrigation water. The experiment was conducted near Madrid, Spain, on a sandy loam Typic Xerofluvent, between February 1993 and May 1997. The crop sequence was corn-wheat-corn-oat (Zea mays L-Triticum aestivum L.-Z. mays L-Avena sativa L.), and the irrigation treatments were applied only to corn. The salt concentration of the irrigation water was monitored 23 times in 24 plots and that of the soil solution at 0.5-m, 0.9-m, and 1.4-m depths was monitored 61 times. Net salt losses (salt input through irrigation minus salt removed through drainage) were I and 0.64 kg m(-2), respectively. In plots under the improved irrigation schedule, 88% of salt discharge occurred after periods of heavier-than-usual rainfall against 55% in traditionally irrigated plots. During the experiment, mean electrical conductivity (EC) of the soil solution in the top 0.5 rn of soil was 0.61 +/- 0.26 S m(-1) in plots with the improved irrigation schedule. The corresponding estimate of the EC of the saturated paste extract was 0.34 S m(-1), lower than the reported threshold value for corn (0.5 S m(-1)). These methods can therefore be recommended for semiarid areas where occasional heavy rainfall will remove some of the accumulated salt.