Soil moisture sensors can be used to determine the appropriate interval between irrigation, depth of wetting, depth of extraction by roots and adequacy of wetting. We tested the performance of soil moisture sensors in different crops. Sensors that read on a continuous basis, such as the Enviroscan device, can provide valuable information that may not be readily evident from periodic measurements. The Watermark blocks responded well throughout the wetting and drying cycles, indicating that they function more consistently over a wider range of soil moisture contents compared with tensiometers and gypsum blocks. Irrigation scheduling addresses the questions of when to irrigate and how much water to apply. Determining when to irrigate requires estimating the irrigation timing so that yield reductions will not occur due to excessive soil moisture depletions. One method for irrigation scheduling is to measure or monitor soil moisture content. This paper discusses methods for estimating when to irrigate and presents case studies of using various soil moisture sensors for irrigation scheduling. When should you irrigate? Available soil moisture is the water that plants can use. It is the difference between the field capacity moisture content and that at 15 bars (1,500 centibars), sometimes referred to as the permanent wilting point. Normally, recommended soil moisture tensions and allowable depletions are presented independent of climate and soil texture. However, research has shown that for cool, humid conditions, relatively large allowable depletions can occur before transpiration and yield are reduced. For warm, dry conditions, allowable depletions may be relatively small for the same soil type. Some incompatibility may exist between the two methods. A recommended tension of 70 centibars may deplete 60% to 70% of the available soil moisture in sandy soil, based on generic soil moisture release curves (not shown). In contrast, the 70-centibar recommendation may deplete only 15% to 20% of the available soil moisture in clay soil. However, a 50% allowable depletion in this soil may cause a soil moisture tension of 150 centibars. Which irrigation scheduling method is the best? The best method is that which maximizes crop yield. The recommended values in tables 2 and 3 reflect site-specific conditions, and thus some adjustment may be necessary for other soil types, salinity, climate, cultivars and cultural practices. Site-specific conditions under which recommendations were developed are not known, and thus any adjustments may require some trial-and-error. We developed the following guidelines using the recommended values in tables 2 and 3 and the generic moisture release characteristic curves: • For sand/loamy sand, consider using allowable depletions as the criterion for irrigating. Use of soil moisture tension may result in soil moisture depletions greater than allowable depletions. • For sandy loam/loam/silty loam soils, either method may be appropriate. Compatibility between the two methods is more likely for these soils. • For clay loam/clay soils, consider using allowable depletion as the criterion for irrigating. Use of soil moisture tension may result in small depletions. These recommendations are appropriate for low-frequency surface and sprinkler irrigation, where irrigation intervals are such that large soil-moisture depletions occur between irrigations. For high-frequency irrigation (microsprinklers, drip emitters and drip tape), small irrigation intervals recommended for these systems result in very small soil moisture depletions between irrigations. Therefore these recommendations do not apply for high-frequency irrigation. Methods of monitoring/measuring soil moisture include tensiometers, electrical resistance blocks, neutron moisture meter and dielectric soil moisture devices. Advantages of measuring/monitoring soil moisture include determining soil moisture depletions, adequacy of wetting from irrigation, patterns of soil moisture extraction due to root uptake of water and trends in soil moisture content with time during the irrigation season. This information can also be used to validate other irrigation scheduling techniques. The following examples illustrate the type of information that can be obtained from monitoring/measuring soil moisture. Flood-irrigated walnut orchard We used Watermark blocks, gypsum blocks and tensiometers to evaluate flood or border irrigations of a walnut orchard planted on sandy loam
