Early in the season, annual crops have shadow root zones and approach the values in Table 3 only when they reach full cover. Plants will show signs of wilting and drought stress before they use all available water stored in the soil. Table 3 shows percent of total available moisture that different crops can withdraw without suffering yield loss. Soil moisture should be measured initially and monitored regularly to determine the available soil moisture. Soil moisture blocks, neutron probes, tensiometers, or the feel method with soil probing will all work.
Some methods work better than others with different soil types.
Because soil moisture monitoring entails a lot of work and an irrigation cannot be completed instantly, this is not a practical approach. The root zone should be filled with moisture just before the period of peak crop water use. The amount of usable water available in the root zone and the rate at which water is being used determine irrigation timing. The maximum number of days before the next irrigation must be applied is calculated by dividing available soil moisture by the estimated daily crop water use. The next irrigation must be applied within 18 days.