Use of sub-hourly soil water content measured with a frequency-domain reflectometer to schedule irrigation of cabbages

Abstract.

Laboratory and field measurements of soil water content were obtained using gravimetric and Theta-Probe methods, the latter a frequency-domain reflectometry method. We obtained real-time in situ measurements of soil water content at depths of 30, 80, 160, 240 and 350 mm to evaluate irrigation practice. A datalogger recording the change in soil water content (and cumulative irrigation) at 20-min intervals was used, with appropriate calculations and graphical presentation, to predict the time and amount of irrigation water required for soil water content to reach field capacity. Measurements at three instead of five depths yielded a different depth-averaged soil water content under relatively dry conditions (less than 0.273 m³ m⁻³), and predicted a later start for irrigation and early crop water stress. Investing in additional sensores for scheduling irrigation would be compensated by financial resources saved through avoiding excess or deficit irrigation, and the associated application cost, loss of nutrients and soil due to deep percolation and erosion, and loss of crop production.