Soil spatio-temporal distribution of water, salts and nutrients in greenhouse, drip-irrigated tomato crops using lysimetry and dielectric methods

This work was mainly aimed at studying the spatio-temporal distribution of water content (θw), bulk (ECa) and soil solution (ECSS) electrical conductivity measured with a dielectric sensor (GS3) and a tension lysimeter (suction cup) throughout three drip-irrigated tomato crops in Mediterranean greenhouses. The mean θw (GS3) for the wet bulb was well estimated by measuring at representative positions, especially at the centre of the wet bulb. The ECSS substantially increased in the wet bulb, irrespective of the soil position, reaching relatively high values (6-7 dS m−1) in the second half of the cycles, mostly due to sodium and chloride accumulation. The mean ECSS for the wet bulb was narrowly and linearly related to that measured at any of four representative positions in the wet bulb, which presented similar seasonal dynamics and absolute values throughout most of the crops. The mean ECSS for the wet bulb can be well estimated by measuring at one of these positions, since the errors of using measurements from these positions were relatively low. The relationship between the ECSS estimated from GS3 and that measured with suction cup varied depending mostly on soil position and cropping year, but the GS3 did not generally provide accurate ECSS estimates, especially in the second half of the cycles, when salts accumulated in the soil. Despite this, measurements of ECa and ECSS from GS3 at the centre of the wet bulb might be useful for identifying tendencies or relevant salinity changes for automated irrigation systems. The solution concentration for main salts and nutrients can be fairly well monitored by sampling at any of the four representative positions of the wet bulb. However, it appears advisable to measure at the centre of the wet bulb, as samples from this position might respond faster to changes in the nutrient solution supply or the root activity, especially for very mobile elements, such as nitrate.