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Determining the Last Irrigation Date in 2006

With irrigation costs at all-time highs, now is the time for irrigators to be looking for an answer to a very important question: Do I need to irrigate any longer in 2006? Due to the warm temperatures, crops are maturing quickly so the decision about the last irrigation is near. Furrow irrigators need to be making that decision soon while pivot irrigators can hold off some and try to catch a rainfall. With the range in maturities grown across the state, the best way to determine if more irrigation is needed is to go through a step-by-step procedure.

Step 1: Determine the crop stage for each field. Develop a field average stage of growth by evaluating the crop at 4 to 5 locations just like during insect scouting routines. Table 1 below gives some targets to look for. Depending on the maturity range and planting date there likely will be a range in maturities across a county.

Step 2: The toughest part is estimating how much water remains in the soil. An excellent discussion of how to use the hand-feel method is presented in an USDA-NRCS publication available at local Natural Resources Conservation Service Offices. Fields with soil water sensors installed should use them to determine how much water is left in the soil.

Using the hand-feel method requires a lot of practice to get it right. Begin by squeezing the soil as if making a fist; if a wet outline of the ball of soil remains on your hand, the soil is very near field capacity or 100% of available soil water content. As the soil dries, the wet outline disappears and the soil will crumble when pressure is applied to the ball of soil with your thumb. At 50% available water content, sandy soils tend to form a ball, but crumble when your hand pressure is released. Silt loams will tend to stick together after pressure is released. Clay loams will stick together, and form a short ribbon when the soil is squeezed between your thumb and forefinger. The NRCS publication does a good job of depicting the way the soil looks at different soil water contents.

Step 3: Multiply the active rootzone depth in feet by the total soil available water capacity and the current percent soil water content. For example, a sandy loam soil, 4 feet deep can hold up to 5.6 inches of plant available water (4 x 1.4 inches per foot). If the average current percent soil water content is 75%, then 4.2 inches of plant available water remain in the soil (0.75 x 5.6 inches).

Step 4: Subtract the minimum balance from the current available water content to get the remaining useable water in the soil. Crops need adequate water through to maturity to ensure that the grain is filled to full size. To prevent yield losses, the soil water content should be kept above the 40% of available level as the crop nears maturity. So, the minimum balance for a sandy loam soil would be 2.2 inches (0.4 x 5.6 inches). The remaining useable water for this sandy loam soil is 2.0 inches (4.2 – 2.2 inches).

Step 5: Determine the amount of water needed to carry the crop through to maturity. Table 1 summarizes the estimated crop water requirements for corn, grain sorghum, and soybean. Each year and variety will be slightly different so field

scouting is a very important part of making a final decision. For example, corn is near the full dent stage in much of the area. The crop water requirement to reach maturity is around 2.5 inches and typically requires about 13 days.

Step 6: Finally, subtract the depth of water needed to take the crop to maturity (Step 5) from the remaining useable water (Step 4). If the result is positive, no further irrigation is needed. If not, keep track of crop water use rates and effective rainfall. Irrigate only if rainfall does not meet crop needs. In the example above, an additional 0.5 inches of water would be needed to take the crop to maturity without significant stress (2.0 – 2.5 = -0.50 inches).

The last irrigation decision is always a very important one, however, if these steps are followed, we can minimize the amount of irrigation water applied to the field, save on energy costs, and have good yields. (BK)

Table I. Normal water requirements for corn, grain sorghum and soybeans between various stages of growth and maturity in Nebraska .

1 The R-number corresponds to the soybean growth stage number.

David P. Shelton
Extension Agricultural Engineer

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