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Cut Tillage to Cut Water
It’s often thought that the best way to increase water infiltration is to perform some type of deep tillage. That may seem true temporarily, but according to Verlon Barnes, NRCS Missouri River Basin Coordinator based in Omaha, Nebraska, it quickly produces a negative effect, which hinders infiltration and increases runoff. That, in turn, results in water loss, reduced irrigation efficiency, and often lower yields than would otherwise be attainable.
“No-till is actually the best tillage practice for conserving water, including under an irrigation pivot,” Barnes says, noting that he would put a strip-till close behind it. “When you perform tillage or even deep ripping, you destroy the pores created by earthworms and microorganisms that serve as a conduit for water to go down deep into the soil.”
According to studies at the University of Nebraska, tillage actually breaks up soil structure and pulverizes the soil surface, creating a condition that seals the soil, resulting in more runoff and less effective rainfall or irrigation. Nebraska research with a rainfall simulator demonstrated that effect in a wheat-fallow rotation. More than 3.75 inches of water were applied in 90 minutes on continuous no-till before runoff started, compared with only 1 inch of water applied in 20 minutes on plowed ground.
“With the improved soil structure of continuous no-till, infiltration also improves, reducing runoff even more,” says Paul Jasa, University of Nebraska. “Our field research showed a much greater infiltration rate for no-till (over 4 inches per hour) than for tilled conditions (only 0.4 inch per hour) after 25 years of continuous tillage system evaluation.”
The presence of crop residue also reduces evaporation, even though it never eliminates it, since moisture evaporates from the soil, the residue itself, and from the crop canopy every time they get wet.
“This loss has been estimated at 0.08 to 0.1 inch each time rain falls or irrigation water is applied,” Jasa explains. “This is why light, frequent irrigations are less effective than longer, soaking ones. Many center pivot irrigators have problems with runoff on sloping, tilled soils, so they apply small amounts frequently, typically only .5 inch at a time; .10 inch evaporation from .5 inch applied is a 20% loss, even more if runoff occurs. When adopting continuous no-till under center pivot irrigation, the pivot can apply more water before runoff occurs. With more water applied less often, the evaporation and runoff losses are reduced, and irrigations can be scheduled to make better use of naturally occurring rainfall,” Jasa says.
Use of cover crops commonly results in greater water infiltration, as well, Barnes explains. This is due to the direct effects of improved canopy cover, improvement in soil aggregation, and the formation of macropores by cover crop roots, which are left intact after termination. In fact, a cover crop alone can improve porosity and infiltration from 3% to over 50%.
“However, if you disk that cover crop under before you plant, you can nullify the benefits pretty quickly,” Barnes adds.
Boost Organic Matter
Another way to improve soil structure is to increase the organic matter. As an example, the addition of solid manure as part of one study increased infiltration by up to four times in the surface layer after one year. The improvement was partly due to straw in the manure, as well as to aggregate development.
“Research has shown that when you increase carbon in the soil by just 1%, you can increase the soil’s capacity to hold water by anywhere from 2,500 to 12,000 gallons or more per acre,” Barnes concludes. “Anytime you increase water-holding capacity, you reduce the need for irrigation and the chance of runoff.”