Hidrosalinity balance to monitor soil salinity at field scale due to brackish irrigation water

Irrigation and drainage are essential technical measures to control soil-salinity due to irrigated agriculture in arid and semi-arid environments, especially in case of poor water quality. A “hydrosalinity balance” could be considered as a straightforward method to detect the conditions that cause salt increase into the root soil layer. A permanent experimental field-unit was established in autumn 2006; three plots of 100 m2 each (6.4 x 15.6 m) were delimited; at the center of each plot an artificial draining basin was arranged digging the soil out of a trench, 3.2 m wide, along the entire plot length; the bottom of each trench was covered with a plastic sheet in order to prevent water percolation; a set of drains (two groups per trench, three drains per group) were displaced over the plastic cover to collect the percolating water and conveying it into thanks placed at the edge of each plot (two thanks per plot). The trenches were then filled with the same soil obtained by the excavating procedure, trying to correctly reproduce the original soil stratification. In this very first cropping season (summer 2007) the aim of the trial was to detect the risk of soil salinity build up due to the ordinary irrigation practice applied by the farmer, according to the real context of a farm of the Apulian Tavoliere, close to the coast of the Manfredonia gulf (Adriatic sea), where brackish irrigation is a diffused and frequent practice due to seawater intrusion into the groundwater. Tomato was transplanted on 20th April and harvested on 26 th July. The electrical conductivity (EC) of the irrigation water increased from 4.7 to 5.3 dS m-1 along the crop cycle. Soil ECe (electrical conductivity of soil saturated water extract) was periodically measured (approximately every 10 days) by soil samplings. Seasonal farmer irrigation was 544 mm while rain amounts were equal to 117 mm; thus the total water supply was 661 mm. Along the crop cycle, an increasing soil salinization occurred, starting from a value of 2.4 dS m-1 up to a maximum value of 6.6 dS m-1. Due to leaching, at harvest the soil ECe decreased to an average value of 4.8 dS m-1. Approximately, a 25% of yield reduction was recorded as compared to contiguous field irrigated with good water quality. The irrigation water supplied by the farmer exceeded the plant water consumptions, particularly in the closing part of the crop cycle, thus producing drainage that, on average, was equal to 61 mm. A leaching fraction of approximately 10% resulted from the water balance while almost the 20% of salt was removed from the soil profile with respect to the salt load bring about with irrigation. That means an actual leaching efficiency of approximately 220 kg ha-1 per each percent point of leached water