Bypass flow, salinization and sodication in a cracking clay soil

Abstract

In Sicily, the increasing scarcity of good-quality waters is spreading irrigation with saline–sodic waters, thus enhancing the risk of soil secondary salinization and sodication. Sustainable management strategies are urgently needed in Sicily to prevent extent of salinization and sodication, thus preserving soil quality. Since irrigation is performed in cracking soils using irrigation systems involving high application rates, bypass flow of water and solutes occurs during irrigation. The objectives of this paper were (i) to investigate the process of Na–Ca exchange and subsequent salinization/sodication during bypass flow and (ii) to explore possibilities of using cyclic strategies, based on alternating good-quality waters to saline solutions, to prevent salinization. Six NaCl–CaCl 2 solutions (named 1–6) with the following values of sodium adsorption ratio (SAR, [mmol c/l] 0.5) and of cationic concentration ( C, mmol c/l)—(1) SAR=5, C=20; (2) SAR=5, C=1; (3) SAR=15, C=25; (4) SAR=15, C=2; (5) SAR=30, C=30; and (6) SAR=30, C=5—were prepared in laboratory. Two undisturbed soil columns were sampled in a Sicilian irrigated area where a risk of secondary salinization and sodication was indicated by previous investigations. Bypass flow experiments were performed by supplying the six solutions to each soil column in the order from 1 to 6 at an almost constant initial value of the cracking volume, using an application intensity determining occurrence of bypass flow. Results of experiments showed that in concomitance with bypass flow, (1) a process of Na–Ca exchange occurred, with Na adsorption and accumulation in the soil. In both the two columns, the exchangeable sodium percentage (ESP) was found to increase by about 100% at the end of the six bypass flow experiments. Being the volume of the supplied saline/sodic solutions much smaller than the pore volume, this result indicates that a considerable risk of sodication may occur under bypass flow conditions; (2) cyclic strategies are useful for preventing accumulation of solutes when solutions with SAR close to the soil ESP are used. Instead, when solutions with SAR higher than the initial ESP are supplied to the same soil, due to the Na–Ca exchange and subsequent sodication, alternating waters having not only lower C but also lower SAR could be necessary to prevent salinization and sodication.