Dew point hygrometers for irrigation scheduling in fine-textured soils

The competition for water in irrigated agriculture with other water users has increased demand for accurate determination of soil water status for efficient irrigation. There has been much interest in measuring topsoil hydraulic properties, including soil matric potential (SMP), as a tool for irrigation scheduling. Currently, many methods are available to measure SMP, including tensiometers, electrical resistance units, heat dissipation, and filter paper techniques. However, their acceptance is limited because of limitations in different applications (i.e., soil type, measurement range, accuracy, etc.). Soil hygrometers may be better-suited instruments to measure SMP when they are used in fine-textured soils (i.e., clay). The objectives of this experiment were: (1) to determine the mean SMP in topsoil (0-0.30 m) at the onset of irrigation, resulting in maximum corn (Zea mays L.) grain yield using a dew point soil hygrometer; (2) to relate these SMP values to the full profile (0-0.90 m) available water (AW) to determine the optimum value of SMP for scheduling irrigations for summer-grown corn under a Mediterranean climate; and (3) to determine the number of soil samples required for a valid representation of SMP measured with a dew point hygrometer under field conditions. An experiment with three irrigation treatments (S1, S2, and S3) was conducted on a clay soil (47% clay). Three irrigation treatments were based on replenishing the 0.90 m root zone to the field capacity when the soil water dropped to 75% (S1), 50% (S2), and 25% (S3) of available water holding capacity (AWHC). A dryland treatment (S4) was also included. The mean SMPs for topsoil at the onset of irrigation were -303, -406, and -635 kPa for S1, S2, and S3 treatments, with grain yields of 5333, 6058, and 4570 kg/ha, respectively. The dryland treatment resulted in the minimum yield of 740 kg/ha. Results showed that the mean SMP value of -406 kPa can be a reliable point to apply irrigations for corn grown in this soil. The relationship between 0-0.30 m depth SMP versus full profile (crop root zone) AW was found to be: SMP = 2150 - 4.35AW (r2 = 0.80, n = 47, SMP in kPa, AW in mm). This equation can be used to estimate full profile AW using 0-0.30 m depth SMP. For this soil and no-rainfall condition, monitoring one layer (topsoil) SMP can be used to estimate full profile AW. Results also showed that 28 +/- 8 or 20 +/- 5 soil samples are needed to obtain +/- -10 kPa precision of SMP measurements for the maximum yielding treatment (S2) at 95% and 90% confidence levels, respectively. For these soil conditions, the dew point soil hygrometer is a well-suited device that can be used to measure SMP for irrigation scheduling. It is sensitive and consistent, and easy to calibrate and use.