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Fluid Journal : Fluid Journal 1999-2001
1 Fluid Journal Winter 1999 Agronomists often talk about the importance of water use efficiency (WUE), which is measured in bushels of grain per inch of precipitation. Imagine a giant sponge 24 inches thick that covers an acre of ground. A cellulose sponge this size would absorb 10 inches of rain and store it. What does this have to do with farming? Plenty. You can rebuild your soil structure to do the same thing. A reservoir of water will be there when the dry summer months hit and hot winds begin to buffet thirsty crops. The big payoff is increased WUE that can produce higher yields and increase profits. Forty years ago, U.S. farmers produced an average of 60 bushels of corn per acre. At an annual precipitation rate of 30 inches or so per year that means they achieved an average WUE of only two bushels per inch of rain. In those days farmers, who planted corn in rows up and down hillsides, used very little fertilizer and couldn't control weeds and insects. Fact is they wasted more rainfall than they used to grow the row crop. Establishing a reservoir It did not take long for farmers to smarten up, however, and begin to concentrate on how to make the most of rain and snow. Table 1 shows how two farmers within the last couple of decades have used good management to increase WUE. Because the single most precious natural resource in the Corn Belt that determines crop yield is water, its availability and amount through rainfall, and the frequency of that rainfall, is critical to our future. Ways to make the most of this resource and improve WUE are several: 1) use a balanced fertility program to grow a FFF Review Water Use Efficiency Key to MSP Requires rebuilding soil to spongy structure over a period of years. healthy plant with a vigorous root system that explores the deep soil profile for water, 2) use conservation- till, leaving residue on the surface to catch water and reduce erosion, 3) control weeds and insects that compete TABLE 1. Good management raises WUE to produce 300+ bu/A corn yields on 2 U.S. farms. Yield Rainfall WUE State bu/A Inches bu/inch South Dakota 145 21.0 6.9 Iowa 108 17.0 6.3 Illinois 338 30.0 11.2 Michigan 352 32.2 10.9 TABLE 2. Average available water on different U.S. soils and at different depths. Inches Water Soil Texture 2ft* 4ft* Loamy 2.5 4.8 Sandy Loam 3.2 6.2 Silt Loam 5.1 10.1 Silty Clay Loam 4.9 9.4 *soil depth with crops, 4) use narrow rows, select a good hybrid, and plant in time to reduce stress at pollination. Modifying soil structure The secret to WUE? Build a deep soil structure high in organic matter that will absorb and hold water until needed in the peak usage period of July and August. Why must we do this? Because precipitation does not occur evenly during the growing season to coincide with plant usage. The problem is further compounded by those "gullywashers" (thunderstorms---40% of the time) and the 10 to 15 percent that falls as snow. Although total annual precipitation increases as we move east from the Corn Belt---from 29 inches in Omaha, Nebraska, to 38 inches in Columbus, Ohio---every farm has the same water deficit when only about 8 inches falls (that's right---Omaha to Columbus) in the July-August period when a corn crop needs 12 to 18 inches. Thus, a corn crop must draw from a reservoir to stay healthy and produce a maximum yield. Table 2 shows that an absorbent, spongy soil, whose structure has been modified and is high in organic matter, can store up to 10 inches of water in the top four feet. Figure 1 shows what kind of water management is required to raise 300
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