Salt leaching due to rain in Mediterranean climate: is it enough?

The increasing limitation of available water resources for agriculture raises the issue of an appropriate use of low quality water (particularly brackish or saline water) for agricultural productivity without jeopardizing the quality of soil and its productive capacity. Referring to typical Mediterranean climate conditions and assuming a systematic irrigation use of brackish groundwater, this paper analyzes the capability of yearly rainfall, particularly in fall-winter period, to leach the salts accumulated in the soil during the previous spring-summer irrigation season. The leaching capability of water supplies exceeding the soil water holding capacity has undergone direct evaluation through a particular experimental arrangement: under a rain shelter, soil columns (inside special cylindrical containers), previously salinized and bare at the surface, were treated with repeated irrigations. Fresh water was used for this purpose, in order to simulate rainwater. The amounts and proportions of salt removed from the soil as well as the relative quantity of salt left in the soil were monitored. An appropriate statistical data analysis led to the interpretation of the observed process by developing a leaching curve able to predict the fraction of salts remaining along the soil profile according to the height of leaching water added to the soil, expressed as a fraction of the depth of the soil layer considered. According to the experimentally determined “leaching curve†(related to a silt-loam textured soil, basically unstructured and compacted as a result of a prolonged salinization), the following rule of thumb can be taken: the application of a defined height of leaching fresh water reduces by 70% (i.e. reduces to 30%) the salt content of a soil layer of equal depth. The elaboration of this conveniently parameterized “leaching curve†prompted an attempt to extend what had been experimentally observed to a larger time and spatial scale. Therefore, different scenarios were elaborated, regarding soil salinity in relation to particular hypotheses of irrigation management and crop rotation aimed, respectively, at promoting salt leaching and minimize salt load into the soil. This has been done on the basis of historical rain-gauge series reported by the Foggia weather observatory, with reference to the years 1951-2000. Very critical scenarios follow from the performed simulations. If only brackish groundwater is used for irrigation, the annual cultivation of a spring-summer irrigated crop without any additional leaching (apart from rain) leads to a saline buildup. This compromises agricultural soil quality. Therefore, a recommended technical choice is the cultivation of irrigated summer crops not more than once every two years. At the same time, a limited (100-200 mm) but influential fresh water applications for leaching purposes could be usefully associated.