Cover Crops
Soil Management-Cover Crops
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| Planting no-till corn into soybean residue improves water infiltration and slows evaporation. Photo by USDA-NRCS |
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Cover crops, seeded between or amid cash crops, contribute a variety of conservation benefits. For water conservation, they offer a triple bonus. A living cover crop traps surface water. When killed and left on the surface, cover crop residue increases water infiltration, lessens erosion and reduces evaporation. Finally, when incorporated into the soil, residue adds organic matter that increases infiltration to the root zone.
Palm date growers in California’s dry Coachella Valley asked USDA Agricultural Research Service (ARS) researchers to help them improve their soil, which is both stratified with clay layers and compacted by frequent cultivation with deep plows. Led by now retired ARS researcher Aref Abdul-Baki, more than 40 growers helped test lana vetch, a heat-tolerant legume cover crop, in their orchards, despite concerns that the cover crop would out-compete the trees for water.
The growers, flood-irrigating from the Colorado River, applied six inches of water every two weeks. Yet, much of that evaporated rather than infiltrated.
Research over a decade proved that adding lana vetch improved the soil and thus water availability, to the date trees. The study helped convince growers that cover crops aren’t water hogs.
“Population growth in Los Angeles will demand that the water be channeled to the city of LA rather than to irrigate dates,” said Abdul-Baki, whose lifetime of research on the soil-building benefits of hairy vetch cover crops attracted the date growers’ attention. “We proved to them that the cover crop wasn’t taking water.”
Instead, by shading the soil, the vetch lowered the soil temperature by 7 degrees, reducing evaporation. Lana vetch also bound the soil at the surface, preventing erosion and evaporation-promoting cracks, while its root system opened up channels for infiltration. Perhaps most important, the vetch also increased date yield by 15 percent and, growers said, improved fruit quality.
Soil compaction, and the water loss it allows, occurs all over the country. In Illinois, Ralph “Junior” Upton farmed poorly drained land that was constrained by a “plow pan,” a tough clay layer six to eight inches deep. The plow pan was so thick, crop roots couldn’t penetrate. It also affected his drainage, causing even moderate downpours to saturate his topsoil and run off. Crops quickly used up the small amount of moisture in the shallow top layer above the plow pan.
To break through, Upton began planting cover crops – rye grass, cereal rye and hairy vetch – after harvesting beans and corn. Following soybeans he now seeds rye grass, which breaks up his soil with its deep roots.
Combined with no-till planting, Upton’s cover crops have enhanced the soil’s ability to store water, and the additional water available to the crop during the growing season has improved grain yields. Short-term drought matters less, crop health has improved, and when water does leave the farm, it isn’t carrying much soil with it.
To improve soil aggregation, consider adding grass, either as a hay crop or forage, into your rotation, since the complex root systems of grass loosen soil. Some grass cover crops have especially deep roots that do a yeoman’s job of breaking up compacted soil. David Wolfe, a Cornell plant ecology researcher, studied the effectiveness of sudangrass to improve soil in vegetable systems. “As our research showed, the roots are relatively good at proliferating into soil that is moderately compacted,” said Wolfe, who received a SARE grant. “This would tend to improve both water infiltration and drainage for subsequent crops.”
University of California-Davis researchers funded by SARE measured as much as 50 percent higher water infiltration and 35 percent lower runoff in the cover crop-heavy organic plots in a long-term trial comparing organic and conventional cropping systems.
“Nobody could have possibly predicted such a dramatic difference in the water runoff and infiltration between the organic and conventional systems,” said project leader Steve Temple. “It’s given us a new appreciation of the importance of cover cropping and residue management.”
While Florida enjoys bountiful rainfall, the challenge for growers is to capture precipitation for plant growth before it percolates through sandy soils. Vegetable farmers like Gainesville grower Rose Koenig and citrus farmer Lynn Steward in Arcadia use cover crops such as sunn hemp with a lot of biomass to build the soil.
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