Native Range and Cultivated Soils Compared

Roger Veseth

Chapter 1 – Erosion Impacts, No. 9, Spring 1988

Much of the Northwest cropland has sustained substantial topsoil loss by water and wind erosion, and the mechanical movement of soil downslope by tillage. On some Palouse hilltops, it has been estimated that up to 4 feet or more of topsoil and underlying subsoil have been lost over the last 100 years of farming. Considering that it has taken hundreds to thousands of years to form each inch of productive topsoil, allowing this high rate of erosion to continue jeopardizes a productive sustainable agriculture for future generations.

There is very little documentation of original topsoil depth and other soil properties when the land was first plowed near the turn of the century. Consequently, it is often difficult to accurately determine the decline in topsoil depth and degradation of other soil properties affecting productivity. Few remnants of virgin land, typical of the surrounding cropland, exist for comparison. Where they can be found, these tracts of land offer an opportunity for potential comparisons with nearby cropland to evaluate the impacts of farming practices.

One such virgin site of native range near Colton in southeastern Washington was used for this type of comparative study with a nearby field cultivated for at least 60 years. The cultivated field had been conventionally farmed, with the exception of uphill plowing for the past 10 years. The research was conducted by Ann Rodman, a graduate research assistant in soils at Washington State University in Pullman, in cooperation with WSU soil scientists Bruce Frazier and Alan Busacca. It was conducted in 1986 and 1987 as part of the STEEP conservation research program in the Northwest.

Sampling and Analysis

Soils were described and sampled on transects which crossed a ridge with north- and south-facing slopes on the native and nearby cultivated sites. Sample locations were grouped into specific slope positions on the landscape for comparison (Fig. 1). A 30 percent slope was measured on the most steeply sloping portions of the ridge on both slope aspects of the cultivated site and the south aspect of the native site. The north aspect of the native site had a 60 percent slope, the main reason it was not under cultivation,

Although a number of soil properties were described and analyzed, data on two properties will be presented here. These include: (1) percent organic carbon content of the surface 6 inches and (2) depth to 1 percent organic carbon. Percent organic matter may be more familiar to most people, however, percent organic carbon is a more exact measurement for scientific purposes.

For an approximate conversion of percent organic carbon to percent organic matter, multiply the organic carbon percentage by 1.7, The depth to 1 percent organic carbon is an approximation of the thickness of the organic matter-rich surface layer of soil, commonly referred to as A horizon (including transition AB horizons). This method removes some of the subjective variation in visual descriptions of soil horizon boundaries.

Organic Carbon Content

Organic carbon content of the soil, in the form of soil organic matter, is an important factor in fertility, plant available water-holding capacity, aeration, aggregation and tilth and other soil properties affecting productivity. A comparison of the percent organic carbon content of the surface 6 inches of soil for specific landscape positions on north- and south- facing slopes on the native and cultivated sites is shown in Fig, 2. No data are available for the footslope and lower backslope positions on the south facing slope of the native site because these slope positions were under cultivation.

Percent organic carbon was substantially higher on the native site than the cultivated site on all comparative slope positions sampled. On the summit and shoulder positions of the south-facing slope, organic carbon content averaged 2,6 percent on the native site compared to 0.7 percent on the cultivated site. On the north-facing slope, comparative figures were about 3.4 and 1.9 percent on the native and cultivated sites, respectively. The researchers point out that this reduction in organic carbon content is due largely to the erosion of organic matter-rich top soil from the cultivated site. As topsoil depth is reduced, organic carbon content is further reduced by mixing of underlying B horizon (low organic carbon content) with the surface soil in the tillage zone. Past crop rotations, particularly the use of conventional summer fallow, and other farming practices also probably influenced organic carbon content over time.

Depth to 1% Organic Carbon

A comparison of the depth to 1 percent organic carbon as a measure of the thickness of the organic matter-rich soil layer (the approximate thickness of the A horizon) is shown in Fig. 3. Summit, shoulder and upper backslope positions on the south-facing slope of the cultivated site had lost all the A horizon. In some cases, the upper portion of the underlying B horizon was also missing. The percent organic carbon content of the surface soils at these positions was less than 1 percent, consequently negative values are shown in Fig. 3.

Based on the depth to 1 percent organic carbon, the summit of the south-facing slope on the cultivated site has lost an average of 25 inches of topsoil compared to the native site. This estimate, however, is based on an average depth to 1 percent organic carbon on the summit of the native site, which ranged from 11 to 42 inches. The summit and upper slopes of the cultivated north-facing slope appear to have lost about 11 to 14 inches of topsoil (about 50%) compared to the native site. On the cultivated site, the depth to 1 percent organic carbon content is greater on southfacing lower-backslopes and footslopes, and north-facing footslopes than on upper slope positions.

The researchers explain that this is a result of soil material which was eroded from upper slope positions and redeposited on lower slope positions. This may be more an effect of tillage erosion, since water-eroded material is often carried to flat bottomland areas and into the drainageways. They also point out that where subsoils are exposed on the summit and upper slopes, this downslope transportation process carries soil material which can cause a significant reduction in production potential on the formerly highly-productive footslope and lower-backslope positions. This is because the subsoil material typically provides a less desirable seedbed and plant growth-medium. It often has a higher clay or calcium carbonate (lime) content, lower organic matter content, lower fertility level, slower water infiltration rates, poorer structure and aggregation and other qualities resulting in lower production potential when compared to the surface A horizon.

On the cultivated north-facing summit, shoulder and backslope positions, an abrupt boundary existed between the A horizon and underlying B horizon. Soils on the native site, in contrast, had a diffuse boundary between the A and B horizons on these slope positions. The researchers concluded that the abrupt horizon boundary indicates that sufficient A horizon has been eroded to allow mixing of the subsoil B horizon in the plow layer. The upper layer of the B horizon could also have been eroded as the surface mixture of the A and B horizons was removed.

Conclusion

The 60-plus years of conventional farming has sharply reduced the organic carbon content of the surface 6 inches of topsoil and topsoil thickness compared to the native range site. Organic carbon content declined about 1 to 2 percent on all slope positions. South-facing summits and upper slopes had lost all of the A horizon topsoil, estimated to be 11 to 42 inches in thickness, and probably some of the underlying B horizon. Upper north-facing slope positions had lost an estimated 11 to 14 inches of topsoil. Abrupt boundaries between the A and B horizons indicate that sufficient A horizon had been lost to allow incorporation of B horizon material in the tillage layer. Part of the soil lost from upper slope positions was deposited on lower slopes, as evidenced by thicker A horizons and higher organic carbon contents.