Using Cover Crops to Help Increase Soil Moisture Retention

Cover crops are typically used locally to help reduce wind erosion, but there are additional benefits to planting a cover crop this fall. Soil moisture is a particularly important resource that cover crops can help capture and retain, especially in residue-limited cotton fields.

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Both wheat and cereal rye can be used effectively as winter cover crops throughout areas of the Texas High Plains. High winds and blowing sand in these areas can cause extensive soil erosion and damage to crop seedlings. Because cotton produces minimal crop residue (Figure 1), soil is often highly erodible when fields experience regular high winds during late winter and spring. Cover crops planted in the fall can help anchor soils during periods of high winds. In the spring, cover crop residue can shelter cotton seedlings from high winds and damage from blowing soil.

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One of the major questions about cover crops is their effect on soil moisture. Cover crops are extensively used in areas that generally receive adequate seasonal rainfall for dryland production. However, in semi-arid environments, soil moisture can be the most limiting factor in crop production, and cover crops can extract a portion of the soil moisture for their growth and development. A study conducted in West Texas used wheat as a cover crop following cotton or sorghum and terminated at the boot stage. The following spring, cotton was planted into the wheat stubble. Results of this test revealed lower soil water use rates for the fields with a cover crop than fields with bare soil. It was concluded that tillage operations performed on the bare soil led to an almost 30% increase in evaporation.1 The cover crop reduced soil water evaporation, improved soil water infiltration, and decreased water extraction for the soil which for water-limited environments is a benefit for subsequent crop growth.

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Dr. Paul DeLaune, a Texas A&M AgriLife Research environmental soil scientist, compared no-till, strip-till, and conventional till with cotton following a terminated wheat cover crop. He has not seen a significant difference in water infiltration rates between tillage systems, but does see a large advantage to planting a cover crop. In fields planted with a cover, water infiltration rates increased by up to two to three times the amount of fields without a cover crop. Although more water may be used, DeLaune recommends waiting for the wheat crop to form a head before termination to increase the amount of residue produced, thus further protecting the soil surface.2

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In addition to helping increase water retention, cover crops help maintain and improve soil characteristics like aggregate stability, organic matter, nutrient retention, and nitrogen (N) assimilation. The ability of cover crops to capture and recycle N can help to reduce N fertilizer inputs and the potential for nonpoint source water pollution caused by nitrate leaching and soil erosion. Winter cover crops can minimize nitrate leaching by sequestering residual nitrate left over from the summer crop. Nitrate can be mined from the soil by the cover crop roots, recycled, and then made available for the following summer crop.3

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Figure 1

Figure 1. Limited residue remaining after a cotton crop.

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Wheat and cereal rye are considered the most dependable of the winter annual cover crops for producing biomass for soil cover.4 Planting cereal rye as a cover crop in semi-arid environments may have several advantages over wheat:5

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  • Earlier and greater biomass production
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  • Higher drought tolerance
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  • Increased winter hardiness and ability to perform under adverse growing conditions.
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Seeding an entire field with cereal rye may not be necessary due to the increased biomass production. Interseeding between rows of the previous summer crop to cover only 25 to 50% of the land area may offer many of the benefits of a complete cover crop with lower costs and presumably less soil moisture extraction.

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Timing the termination of a cover crop is important to help cover crops persist during cotton seedling development. If termination of the cover crop is too early, there may be little standing biomass or stubble to help protect seedlings from damaging winds and to conserve soil moisture. Late termination of a cover crop may result in excessive use of valuable soil moisture that will not be available to the crop.5

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Planting wheat or cereal rye as a cover crop in the fall can help your cotton crop get off to a good start next year. Some rainfall is needed for adequate germination and survival of the cover crop. Irrigation could be used to reduce the risk of stand failure when there is a lack of rainfall.