Reducing tillage and retaining more crop residue on the soil surface can significantly reduce the amount of irrigation water needed to grow a crop. These practices reduce evaporation of soil water and increase the amount of soil water by increasing infiltration and decreasing runoff, according to University of Nebraska-Lincoln irrigation engineers.
When the soil surface is wet, evaporation from a bare soil will occur at a rate controlled by weather. The evaporation rate decreases as the soil surface dries over time. If the soil surface is covered with residue, it is shielded from solar radiation and air movement just above the soil surface is reduced. This reduces the evaporation rate from a residue-covered surface compared to a bare surface, say UNL specialists Simon van Donk and Chuck Burr.
A four-year study was begun in 2007 at North Platte to determine the effect of crop residue on evaporation, soil water content, and yield. The two treatments included residue-covered plots and bare-soil plots. In April, bare-soil plots were created by using a de-thatcher and subsequent hand raking to remove most of the residue. This practice was repeated each year in April.
The crop was purposely water-stressed so that any water conservation in the residue-covered plots might translate into higher yields.
Results of the study showed that in the residue-covered plots corn yields were 17-25 bushels higher and soybean yields were 8-10 bushels higher than in the bare-soil plots, van Donk says.
"This difference is attributed to reduced soil water evaporation on the residue-covered plots," he says. "It is equivalent to a 2- to 3-inch difference in available water to the crop, since each inch of irrigation water yields an additional 8 to 10 bushels of corn and 3 to 4 bushels of soybeans," he adds.
In addition to recording the yields for each plot, soil water differences were measured. In two years of the study, there was 1.5 to 20 inches more moisture in the soil profile for the residue-covered plots at the end of the growing season. For the four years of the study, total estimated water savings was 2.5 to 5 inches per year for the residue-covered plots compared to bare-soil plots.