Using plant water status to define threshold values for irrigation management of vegetable crops using soil moisture sensors

Thresholds of soil matric potential (SMP) and available soil water content (AWC) required to prevent water limitations between irrigations were determined for bell pepper, melon, and spring and winter tomato grown in Mediterranean-type greenhouses on the south-eastern coast of Spain. Thresholds were identified by measuring the divergence of leaf water potential of un-watered plants from that of well-watered plants. Soil matric potential thresholds were −58 kPa for pepper, −35 kPa for melon, and −38 to −58 kPa for tomato. In general, SMP thresholds were more negative under lower evaporative demand conditions such as during autumn and winter months. Available soil water content thresholds, for a given crop and drying cycle, differed appreciably depending on soil depth and the method used to calculate the values. For the four crops studied, AWC thresholds calculated at 0–40 cm were 13–15% higher than those calculated at 0–20 cm. Each AWC threshold for 0–20 cm depth was 21–29% lower when AWC was based on laboratory rather than field determinations of field capacity and permanent wilting point. For a given method of calculating AWC, AWC threshold values were similar for different crops and drying cycles, suggesting limited sensitivity of the AWC approach. Using the manufacturer's calibration, the capacitance sensor used for SWC measurements overestimated SWC by an average of 36%. An in situ calibration provided generally good agreement with the actual SWC between 0.15 and 0.22 cm3 cm−3; however, for higher SWC values, the in situ calibration underestimated SWC. The results of this study demonstrated the uncertainty of using recommended fixed AWC threshold values for irrigation management, using SWC sensors, because of issues related to the definition of rooting depth, measurement of FC and PWP, sensor calibration, and sensor accuracy across the relevant range of water contents. These data suggest that SMP thresholds are much more reliable than AWC thresholds for scheduling irrigations in greenhouse-grown vegetable crops. Technical issues regarding on-farm measurement of SMP and SWC are discussed.