Effects of compact layer on salt accumulation in plastic film house soils

Abstract

A compact layer was detected below the plow layer of old plastic film house soils (Eutrudepts) cultivated for more than 10 y in Yousung and Yesan districts, in the central part of Korea. In addition, salts had accumulated in the surface layers of these soils. One representative site was irrigated continuously for 96 h and redistribution patterns of the salts and water were investigated. Initially, the salt concentration was highest in the uppermost soil layer and decreased with increasing depth, due to the high evaporation rates inside the plastic film house. Continuous irrigation moved the peak point of electrical conductivity (EC) and the wetting point to a 20-30 cm depth after 48 h. Additional irrigation (96 h) did not move significantly the EC peak point or change the water regime. In order to analyze the effects of the compact layer on the inhibition of water flow and salt movements in the soil profile, saturated hydraulic conductivity (Ksat) and salt elution time were determined in soil columns with varying bulk densities. Ksat significantly decreased with increasing soil bulk density up to 1.4 Mg m-3. Salt elution time was markedly delayed by about 100 h when the Ksat rate at a 10 cm depth of the soil profile in a 1.4 Mg m-3 soil column was compared with that in a 1.3 Mg m-3 soil column. The point of zero net charge (PZNC) was used to monitor the effects of soil charge characteristics on the retardation of salt elution. PZNC was observed at lower pH values than the intrinsic pH of the Ap and Bw horizons. It was expected that anions with non-specific adsorption properties would be scarcely adsorbed. In batch tests, the soil materials in the Ap and Bw horizons only slightly adsorbed 3 anions (NO3-, Cl-, and SO42-). As a result, the compact layer was the most important factor in accelerating salt accumulation in the surface soil of old plastic film houses. Chemical reactions had a limited or minor impact on the salt movement and salt infiltration in the soil profile. It is considered that it may be difficult to effectively improve old plastic film house soils with salt accumulation without reclaiming the compact layer.