Larry G. Heatherly - Crop Doctor

Producers depend on soil water to sustain crop growth and provide water for an economical yield of forage, grain, seed, and/or fiber.

Texture is the major soil component governing the amount of water available to plants during the growing season. Textural classes range from coarse sand (coarsest) to clay (finest).

Soils of the different textural classes differ in their ability to supply water to plants. The accompanying table provides a general categorization of plant available water related to soil texture. Available water per foot of soil ranges from an average 0.7 inch in coarse sand to an average 2.25 inches in very fine sandy loam and silt loam.

Crops such as corn and soybeans that are grown on sandy soils are more prone to yield-reducing effects of drought before flowering than are crops grown on finer-textured soils. Thus, crops growing on sandy soils should be monitored for pre-flowering drought stress that may limit vegetative growth. Severe water deficits before flowering on these soils can render crops unable to realize maximum yield potential because of too little vegetative growth to produce an economical yield even when adequate water is available during the reproductive period.

Scheduling surface irrigations that recharge the soil profile should be governed by how much water a particular soil textural class will provide between irrigations. Therefore, sustaining production potential in a crop that uses 0.25 inch of water per day from an effective rooting zone of 18 inches requires surface irrigation of a fine sand every 5 to 6 days, of fine sandy and silty clay loams every 9 to 12 days, and of very fine sandy and silt loams every 12 to 15 days.

Re-irrigation with overhead systems is typically timed to system capabilities rather than to available soil water. Thus, regardless of soil texture, an overhead system designed to apply a net 1 inch per acre per day should be scheduled to irrigate every four days in the absence of rain when estimated crop water use is 0.25 inch per day.

The coarser the soil texture, the later the last irrigation should be for most crops. For corn, the general rule is that the last surface irrigation should occur 10 to 20 days before maturity. The shortest and longest times between the last irrigation and maturity should be planned for coarse- and fine-textured soils, respectively.

Soybeans planted in April on clay soils should receive a last surface irrigation (in the absence of rain) 12 to 14 days before R7 or beginning maturity, which coincides with R6 or full seed. Assuming a water use rate of 0.12 to 0.15 inch per day from R6 to R7, this last irrigation will provide enough available water in an 18-inch rooting zone to carry the soybean crop to beginning maturity.