A sandy soil will hold an inch or less of plant available water per foot of soil. A loam may hold as much as 1.5 inches per foot. Another factor is the strategy to be used for starting an irrigation application. The manager must decide how dry the soil should be before irrigation. Irrigation in a subhumid area such as Missouri needs to take into account the possibility of rain, and a reserve in the water balance in the soil should be held for the rain which may occur.
Weather forecasts can provide a guide.
The relationship is as follows. Here SMi and SMf are the initial and final soil moisture estimates and the rainfall and evapotranspiration are the values for the period between the initial and final days. A final consideration should be taken by the irrigator for decisions made late in the growing season. The value of added water during the late grain filling and maturation period is not as great as when the crop is going through its flowering and early grain filling periods. This conservation practice is particularly important in the southwest and southeast regions of the state where cool season precipitation is likely in Missouri.