Addendum to August 31, 2001

Worksheet to determine last irrigation Field 1. Water needed to reach crop maturity, in inches (Table 1)   Crop 2. Current soil water balance, in inches (estimated in field)   Soil type 3. Minimum allowable balance, in inches (Table 2)   Date 4. Remaining usable moisture, in inches (Line 2 minus Line 3)   Present stage of growth 5. Irrigation requirement assuming no rainfall, in inches (Line 1 minus Line 4)   Note: If Line 4 is greater than or equal to Line 1, another irrigation is not needed.

Table 1. Normal water requirements for corn, grain sorghum, and soybeans between various stages of growth and maturity. Crop growth stage Approximate days to maturity Water use to maturity (inches) Corn   Beginning dent 24 5.0   Full dent 13 2.5   Black layer 0 0.0 Grain sorghum   Soft dough 23 5.0   Hard dough 12 2.0   Black layer 0 0.0 Soybeans   Beginning seed fill 29 6.5   Full seed fill 18 3.5   Leaves begin to yellow 10 1.9   Beginning maturity 0 0.0

Table 2. Available water capacity for various soil types and minimum allowable balances at physiological maturity. Soil type Available water capacity (inches/foot) Minimum allowable balance in top 4 feet of soil profile* (inches) Silty clay loam 1.6 2.6 Upland silt loam 2.0 3.2 Bottomland silt loam 2.5 4.0 Very fine sandy loam 1.8 2.9 Sandy loam 1.4 2.2 Fine sands 1.0 1.6 *Based on depletion of 60% of the available water.