April 5, 2013

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.

Evaporation

When the soil surface is wet, evaporation from a bare soil will occur at a rate controlled by atmospheric demand (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.

A four-year study was initiated 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 dethatcher 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. This difference is attributed to reduced soil water evaporation on the residue-covered plots. It is equivalent to a 2-3 inch difference in available water to the crop since each inch of irrigation water yields an additional 8-10 bushels of corn and 3-4 bushels of soybeans.

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 2.0 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 2.5-5.0 inches per year for the residue-covered plots compared to bare-soil plots.

Research conducted in southwest Kansas on this topic showed similar results. The Garden City study showed that with a bare, fine sand and silt loam soil, surface soil water evaporation can be as much as 30 percent of the total evapotranspiration (ET) during the irrigation season. Evaporation from a no-till corn or wheat straw-covered soil surface amounted to 15 percent of the total ET. This difference translated into a water savings of 2.5 to 3.0 inches per growing season (mid-June to mid-September).

In addition to reducing residue, tilled soils often dry to the depth of tillage. Each tillage operation can cause 0.5 to 0.75 inches of soil water evaporation. With multiple tillage events, soil water may not be adequate for uniform germination and emergence.
 

Infiltration

Long-term no-till management leads to better soil structure, reduced soil crusting, higher infiltration rates, and less surface runoff. Crop residue reduces the energy of water droplets impacting the soil surface and reduces detachment of the fine soil particles that tend to seal the surface. In a UNL rainfall simulator demonstration near Sidney, more than 3.75 inches of water was applied in 90 minutes to no-till soils before runoff started. Compare this to only 20 minutes before runoff started when 1.0 inch of water was applied on a plowed soil.

Summary

Both the North Platte and Garden City studies indicate that avoiding tillage and leaving residue in place may conserve up to 5 inches of water per year. If water is a limiting factor, saving this much water will contribute significantly to crop yield.

Resources

For more information, see the NebGuide, Tillage and Crop Residue Affect Irrigation Requirements (G2000).

Simon van Donk
UNL Extension Irrigation Specialist
Chuck Burr
UNL Water/Cropping Systems Extension Educator