Erosion Makes Soils More Erodible

Roger Veseth

Chapter 1 – Erosion Impacts, No. 10, Fall 1989

Loss of topsoil by water, wind and tillage erosion has severely reduced the productivity of a significant portion of the Northwest cropland, as it has around the world, Besides the off-site impacts of sedimentation and associated water quality problems, erosion can result in significant changes in surface soil properties affecting the sustainability of production. David Mulla, Washington State University soil scientist, points out that organic matter content is one of several soil properties particularly impacted by erosion. Organic matter is not only important for good soil fertility, improved soil permeability and water holding capacity, resistance to surface soil crusting and other factors related to crop production potential, but it is also important to the soil’s ability to resist erosion.

Topsoil with an appreciable organic matter content generally has what producers call good soil ”tilth” and also what soil scientists describe as being “well aggregated. ” Mulla explains that soil with stable aggregates is better able to resist dispersion and detachment of soil particles by rainfall, runoff, wind, or from wetting and drying cycles. The stability of those aggregates determines, in part, the erodibility of the soil. The resistance of a soil to water erosion also increases as the water infiltration capacity of the soil increases because surface runoff is reduced, Infiltration capacity is again related to the degree of soil aggregation and stability of those aggregates.

Mulla gives the common example in the Palouse region of exposured clayey subsoils with topsoil erosion. Compared to the original topsoil, the higher clay content and lower organic matter content of the subsoil typically result in reduced aggregate stability and water infiltration, and increased potential for surface sealing, crusting and surface runoff. These changes in turn accelerate the rate of soil erosion, Fertility of the subsoil is also lower than the overlying topsoil. This further reduces production potential and future contributions to the soil’s organic matter content.

STEEP Research Focus

Developing the technology to improve prediction of the erodibility of Northwest cropland soil has been a part of the STEEP (Solutions To Environmental and Economic Problems) research program. Since 1975, this research program has involved more than 100 scientists in Idaho, Washington and Oregon from at least 14 different disciplines with USDA-Agricultural Research Service (ARS) and universities. It has been a cooperative effort to address all aspects of controlling the soil erosion problem. The STEEP program has greatly accelerated in the development of new production technologies for conservation farming.

Research on aggregate stability relationships, with soil properties and landscape slope positions, has been the focus of a STEEP research effort near Pullman by Mulla and research assistant Frederick Pierson, now a USDA ARS hydrologist in Boise, Idaho. Their research points out some strong predictive relationships with organic carbon content and clay content. These properties, and the degree of aggregate stability, were associated with particular slope positions, and amounts of topsoil erosion and subsoil exposure. Their results not only help to better predict soil erodibility on the variable cropland areas in the Northwest, but also help to point out another serious potential impact of topsoil erosion; as soils are eroded, they become even more erodible – a vicious cycle that strongly impacts crop production.

Research Effort

A study site was selected in fall 1986 on rolling Palouse cropland about 2 miles north of Pullman. Four north-south parallel transects 2,625 feet (800 m) long and 150 feet (45 m) apart were established for sampling. The transects cut across three east-west ridges which were sloping to the east (see Fig, 2). The slope positions sampled on the transects were divided into four slope position categories (Fig. 1) for data analysis.

On each transect, soil samples were collected every 66 feet (20 m) to a depth of 6 inches (15 cm). The samples were analyzed for organic carbon content, amorphous iron content, water content by weight, clay content and aggregate stability as measured by a slow wetting structural index. Percent organic matter may be more familiar to most people, however, percent organic carbon is a more exact measurement for scientific purposes. For an approximated conversion of percent organic carbon to percent organic matter, multiply the organic carbon percentage by 1.7.

The site has silt loam soils of loessial (wind deposited) origin. Soil on the summit and some shoulder slope positions were primarily the Naff series, which has an argillic (clay enriched) horizon at or near the surface. Exposure of the argillic horizon impacted infiltration rate, rooting depth and many other factors affecting production potential and erodibility. Where it has not been extensively eroded, depth to the argillic horizon in the Naff series is normally about 18 inches (45 cm). The Palouse series, which does not have an argillic horizon, was the predominant soil on the other slope positions.

Research Results

Specific soil properties were often associated with slope position (Table 1). Organic carbon and water contents were generally lowest on summit and shoulder positions, which were the more eroded areas, and highest on the footslope and toeslope positions, which had deeper soil of the Palouse series. Similarly, aggregate stability was lowest on upper slope positions and highest on lower positions. In contrast, amorphous iron and clay contents were highest on summit and shoulder positions, and lowest on lower positions, again reflecting greater topsoil loss and subsoil exposure on summit and shoulder positions.

The researchers determined that there was a significant positive correlation (r = .66) between aggregate stability and organic carbon content. In other words, as organic carbon content increased, aggregate stability also increased. The close relationship between aggregate stability index and organic carbon content along the transects is shown in Fig. 2. Both are highest on foot and toe slopes (low elevations) and lowest on summits (high elevations).

Although organic carbon content was highest on the foot and toe slopes, sharply lower values occur in these positions at about the 200 and 550 m locations on the south-north transects. This was because of extensive gully erosion at the bottoms of the two major drainages resulting in loss of organic carbon-rich topsoil and/or deposition of eroded subsoil material with low organic carbon content from upslope. The gullies are not apparent on the elevation diagram.

As clay content increased, aggregate stability decreased – a significant negative (inverse) correlation (r = -.54) was found between the two. Again, the highest clay contents and lowest aggregate stability were associated with ridge summit positions where extensive loss of topsoil had occurred. Only weak correlations were found between aggregate stability and either amorphous iron content (r =-.20) or soil water content (r = .27), although their variation was also related to slope positions (Table 1).

Table 1. Mean values of organic carbon, amorphous Iron, soil water and clay contents and aggregate stability Index for four slope positions on cropland near Pullman, WA, 1986 (Mulla and Pierson, WSU).

Conclusions

The researchers concluded that relative changes in aggregate stability can be predicted from changes in organic carbon and clay contents on the field landscape. Their findings should be useful in revising and adapting the Universal Soil Loss Equation (USLE), and more recent predictive models of soil erosion, to Northwest conditions. In addition, their results also point out that soil becomes more erodible as erosion of topsoil progresses. Yield potential also declines because of the degradation of soil properties, such as organic matter content and aggregate stability, which influence soil productivity.