Irrigation management in Mediterranean salt affected agriculture: how leaching operates

In the frame of a crop rotation currently applied in a farm of the Apulian Tavoliere (Southern Italy), this paper reports the effect of brackish water irrigation on soil, outlines the corresponding salinity balance, formulates quantitative relations to model salt outflow below the soil root-layer and defines operational criteria to optimize irrigation management at farm level in order to control soil salinity through leaching. The general aim is to contribute to a sustainable use of the available water resources and a proper soil fertility conservation. A three-year trial (2007-2010) was carried out on a farm located close to the coast of the Manfredonia gulf (Mediterranean - Adriatic sea), where irrigation with brackish water is frequently practiced due to seawater intrusion into the groundwater. An especially designed experimental field-unit was set-up: the bottom of three hydraulically insulated plots was covered with a plastic sheet to intercept the percolating water and collect it into tanks by means of drain tubes. Each year a double crop cycle was applied to the soil; a spring-summer crop (tomato, zucchini and pepper, respectively) was followed by a fall-winter crop (spinach, broccoli and wheat). Short “fallow” periods (completely bare soil) were inserted between two crop cycles. Irrigation or rain completely restored crop water consumptions (with the exception of wheat, considered a rainfed crop) and leaching was performed both unintentionally (by rainfalls) or intentionally (supplying higher irrigation volumes whenever the soil electrical conductivity exceeded a fixed threshold). The soil electrical conductivity was periodically measured together with volume and electrical conductivity of irrigation and drainage water. All these measures allowed to draw-up the salt-balance of the soil, respectively at the beginning and the end of each crop cycle. Absolute and relative variations in soil salt content were interpreted with respect to absolute and relative drainage volumes according to a three steps procedure of covariance analysis. A simple, general and comprehensive leaching model is thus presented. Results showed that salt build up into the soil can be very rapid, generally occurring within a single irrigated summer crop cycle. Rainfalls of the autumn-winter period had a crucial role in the removal of salts brought into the soil by summer irrigation. This paper strongly emphasises that additional fresh water supply is of great importance to establish acceptable soil conditions. Two suitable periods for intentional leaching were identified