Keeping Our Soil in Place with the Right Crop Rotation

Jan. 1, 2002
The kinetic energy exerted by wind and water forces contributes to the erosion processes that remove fine particles and transport nutrients and soil organic matter off-site, reducing productivity of a given field. Rain events break exposed aggregates at the surface, detaching soil particles that move into suspension and transporting them with runoff from the fields. This runoff, as it moves through the system, helps generate additional kinetic forces that increase the movement and transport of other suspended particles. At the other extreme are dry regions where wind provides the needed kinetic energy that drives the erosion processes and transport of unsheltered particles. These erosion processes can significantly contribute to the decline of soil productivity across different agroecosystems. Erosion processes are contributing to the degradation of lands and reduction of yields in many areas of the world (Lal, 1999; Tato and Hurni, 1992). These authors have presented and discussed the relationship between soil erosion processes, land degradation, reduction of yields, poverty, and malnutrition for many regions. Because erosion is a precursor to so many environmental and social problems, it is imperative in this new millennium, as the world population continues growing and increasing the demands for soil and water resources, that we maintain soil quality and sustainability of agroecosystems. Erosion significantly reduces the soil productivity by exposing roots to acidic subsoils, aluminum toxicity, and compact layers and by other mechanisms. The transport of fine particles (one of the important components needed for nutrient retention and water-holding capacity) by wind or water is also a significant mechanism that contributes to reduction of yields and of soil quality. It has been established by several authors that soil erosion processes reduce soil fertility and contribute to a decline in yield (Lal, 1999; Tato and Hurni, 1992). Wind erosion can contribute to sand blasting, resulting in abrasive damage to seedlings and reducing crop yield and quality. Wind and water erosion can significantly affect growth development if they occur when seedlings are more susceptible, exposing their rooting systems and damaging the crop. The opposite problem can also occur: Sand accumulating on areas of the field can damage the crop. Figure 1 is a remote-sensing near-infrared picture of a center-pivot—irrigated barley field in south central Colorado. The areas absent of the near-infrared color were damaged by deposition and accumulation of sand, caused by wind erosion, around the tires of the sprinkler irrigation systems. This accumulation reduced barley germination and damaged seedlings. Such environmental factors as moisture can interact with soil and affect the rate of wind erosion because moist soils have greater cohesion and are therefore more stable than drier and looser soil particles that can offer less resistance.Although wind and rain erosion can occur in all regions, the main mechanism by which erosion takes place in humid regions is surface transport, while wind transport is the principal mechanism in drier regions. Rain contributes to different kinds of erosion: sheet erosion, the uniform runoff and transport of fine particles over a slope, and rill erosion, which starts forming well-defined channels. The channels are usually more visible in depressed areas but can be smoothed out by traditional tillage practices. Gully erosion, the most severe type, forms channels greater than 0.5 m in depth, which cannot be smoothed by soil land management and tillage operations. Gully channels can be up to 25-30 m deep and still be defined as a gully (SSSA, 1996).In 1813 Thomas Jefferson was one of the first Americans to report how anthropogenic activities and changes in land use contribute to soil erosion. He also was among the first to recommend soil and water conservation practices, such as plowing perpendicular to the gradient rather than uphill and downhill. Several authors have discussed the history of soil conservation in the United States (Pierce and Frye, 1998). Among the great steps in soil conservation we have made–in response to the Dust Bowl of the 1930s–was establishing the US Department of Agriculture Soil Conservation Service, today called the Natural Resource Conservation Service, which provides technical assistance to farmers regarding soil conservation. Other national research programs also contribute to the conservation of soil and water quality, led by the USDA Agricultural Research Service in cooperation with contributing partners from land-grant universities, Soil and Water Conservation Service chapters, conservation districts, state agencies, and others dedicated to soil conservation and maintaining soil sustainability. With continuous population growth and the increasing demand for natural resources, we will need to maximize soil and water conservation to sustain high efficiency and productivity of our agroecosystems throughout future generations.
Several management practices can reduce cropland erosion. A rough surface and/or shortening the length of the surface can contribute to reduced soil erosion. Crop barriers such as grass, crops, or trees along the prevailing wind direction can reduce potential wind erosion. For humid systems, vegetative barriers can also be used to reduce water runoff and increase infiltration. Vegetation and crop residue is the most effective way to protect against soil erosion since it maintains the soil cover and protects unsheltered particles. Conservation tillage is a system that leaves more than 30% of the surface area covered by residue. Among other management practices that contribute to soil and water conservation are minimum tillage, winter cover crops, elimination of summer fallow, no till, buffers, and several other practices discussed in more detail by several authors (Pierce and Frye, 1998; Lal, 1999).Irrigation can also contribute to moving fine particles and nutrients off the fields. New alternatives that are being used are the applications of polyacrylamide,1 a high-molecular-weight anionic long-chain organic polymer (Sojka and Lentz, 1997). These compounds have been tested and are widely used, reducing as much as 99% of the irrigated furrow erosion. Additional techniques that can be used to evaluate the effects of best management practices (BMPs) to reduce potential erosion include the use of new computer models. A series of simulation models has been developed to test the relationships between soil erosion and soil productivity, such as the Erosion Productivity Impact Calculator (EPIC) (Williams et al., 1983). This model has been developed, calibrated, and validated for several regions of the US.The US Department of Agriculture’s Natural Resources Conservation Service (USDA-NRCS) Resource Assessment Division used the Universal Soil Loss Equation (USLE) and the wind erosion equation (WEQ) to calculate wind, sheet, and rill erosion from cropland during 1997 (USDA-NRCS, 1999). The division estimated that 24.5 million ha of highly erodible land and 20.9 million ha of non—highly erodible land was eroding excessively for a total estimate of 1.2 billion megagrams (Mg) of erosion from US cropland. The USDA-NRCS data did not included information from federal lands, Alaska, the Pacific Basin, and Puerto Rico. Although there was a potential 1.2 billion Mg of erosion from cropland during 1997, the potential wind and water erosion from cropland and Conservation Reserve Program lands has been reduced in the US by almost 40% during the last 15 years (Figure 2). The mean sheet and rill erosion for cropland was decreased from 9.2 Mg ha-1 y-1 in 1982 to 6.3 in 1997. Similarly, the mean cropland rate of wind erosion was reduced from 7.4 Mg ha-1 y-1 in 1982 to 4.9 in 1997. The USDA-NRCS (1999) reported that this combined wind and water mean erosion reduction from 1982 to 1997 is equivalent to savings of 1.1 billion Mg of soil per year on cropland. These significant reductions in soil erosion achieved during the last 15 years have been the result of the implementation of BMPs that reduced the rate of erosion significantly across the nation. Although this is a great success story, there are still 45.4 million ha eroding excessively, and there is the need to continue improving research and developing new methods in the area of soil and water conservation to continue the transfer of new viable BMPs that can continue reducing this potential soil erosion.As an example of some of these efforts and cooperation, the USDA-ARS Soil Plant Nutrient Research Unit, USDA-NRCS, Colorado State University Research Center, Colorado State University Cooperative Extension, farmers, and private industry have cooperated since 1992 in soil and water-quality studies conducted across different cropping systems of south central Colorado. The typical rotation for this region is potato—small grain rotation. Such vegetables as lettuce or spinach, however, are also grown in rotation with small grains. Lettuce, spinach, and potato leave low quantities of surface-crop residue after harvesting. We studied the potential to use new crop rotations to reduce potential wind erosion in this region. Figure 3 shows the amount of minimum soil cover after harvesting potatoes, compared to how the system looks with stubble after harvesting small grain. For potatoes and small vegetables, the bare soil surface is highly susceptible to soil wind erosion (Figure 4). During the same windstorm, erosion from small-grain areas was minimal, almost not detected. Leaving the small-grain stubble undisturbed until spring when tillage operations could start would significantly reduce wind erosion. When winter-cover rye was planted between a lettuce-potato rotation, the losses of soil particles, organic matter, and nutrients were significantly reduced for the rotation (Figures 4 and 5). Similarly, if we plant winter wheat immediately after harvesting potatoes and don’t leave the soil fallow until the spring for planting of spring wheat, the potential soil erosion and losses of organic C (soil organic matter) and nutrients will also be significantly reduced for the rotation.
These studies show that winter cover crops can significantly contribute to improved air quality in south central Colorado (Delgado et al., 2001 a). They can reduce potential wind erosion from vegetables by an average of 28.3 Mg ha-1 y-1. For 2,430 ha, this will be equivalent to maintaining 68,769 Mg of topsoil in the fields. Additionally, if we assume that 15% of the area in potato (29,039 ha) is in a potato—potato—small grain rotation, then the winter cover crops can reduce the potential wind erosion by 123,270 Mg of topsoil per year. By reducing potential wind erosion, the winter cover crops are also increasing the nutrient-use efficiency in the systems and conserving soil quality, because fine particles and soil organic matter can also be lost from wind erosion. There is potential to reduce the wind erosion losses of soil organic matter by 2.2 million kg and the losses of nitrogen and P2O5 by 119,000 and 21,000 kg, respectively, for this region.These significant reductions in wind erosion losses of soil particles and nutrients can be achieved by planting such winter cover crops as rye between lettuce and potato and potato-potato rotations. If winter wheat is used instead of spring wheat, and other BMPs are implemented, such as leaving the stubble undisturbed until spring, the potential for keeping soil in place and conserving soil quality is further improved. Actually, in a winter cover crop rotation, there are about 50% vegetables and 10-15% potatoes. This research in soil and water quality has contributed significantly to the technologic transfer of information and to the conservation of soil and water quality. Additionally, the rotation with these winter cover crops significantly contributes to increased nitrogen-use efficiency of these systems, to recycling nutrients, and to recovering and mining NO3-N from the underground irrigation water (Delgado, 1998; Delgado, 2001; Delgado et al., 2001 b).Rangeland ecosystems with brush, grasses, and bare soils were also studied for south central Colorado. The soil texture was measured for bare soil, brush, and grass, and the bare soil had a significantly lower content of fine particles, which suggests that after decades of being exposed, these bare-soil surfaces were susceptible to significant losses of fine particles (Figure 6). If bare soils are not covered, the constant effects of wind erosion can affect the texture of the soil by transporting a significant amount of fine particles out of the system. These data show that innovative BMPs can significantly contribute to reducing the transport of fine particles, soil organic matter, nitrogen, and phosphorus out of the system. The data also suggest that if cultivated sandy soils with sandier coarse substratum are left uncovered and unsheltered for decades, the physical and chemical properties may change, affecting the quality and potential soil productivity of the systems. Using the right crop rotation and winter cover crops can contribute to maintaining soil cover and reducing the potential soil erosion from these systems, increasing nitrogen use efficiency, protecting soil and water quality, and mining NO3-N from underground water.Note1 The USDA neither guarantees nor warrants the standard of the product, and the use of a given name by the USDA does not imply approval of that product to the exclusion of others that may be suitable.