Erosion Impacts on the Palouse Misunderstood
Roger Veseth
Chapter 1 – Erosion Impacts, No. 1, October-November 1985
The Palouse region in eastern Washington and northern Idaho is one of the most productive and one of the most rapidly eroding areas in the country. Since this land was first cultivated about 100 years ago, all of the original topsoil has been lost from about 10 percent of the cropland, and from one-fourth to three-fourths of the original topsoil has been lost from another 60 percent of the cultivated cropland.
Combinations of winter precipitation and snowmelt on frozen soils, steep slopes and convention tillage with little or no surface residue create a high rate of water erosion which is changing the Palouse landscape and productivity. In a single winter season, erosion rates up to 200 tons per acre (more than 1 inch of topsoil) have been measured on steep slopes. Tillage erosion, particularly downslope plowing, has removed over 3 feet of topsoil from hilltops and ridges.
For many years, farmers and agronomists have spoken of the’ ‘Palouse soil, ” conjuring an image of a homogeneous, equally fertile deposit of silty, wind-deposited loess 100 to 200 feet thick. However, research by Allan Busacca and Bruce Frazier, Washington State University soil scientists, and other STEEP researchers has shown that this image is false. Layers of buried ancient soils, called paleosols, are common within the loess. They often differ markedly from the more recent overlying 10CSS deposit on the surface. Exposure of these paleosols by erosion dramatically impacts soil productivity.
Loess History
The loess deposit in the Palouse and surrounding region was carried from the Columbia Basin by southwesterly winds over the past 1 to 2 million years. Fluctuations in climate, along with geologic events, led to periods of rapid loess accumulation alternating with periods of landscape stability and soil formation. These earlier episodes of soil formation led to the development of 10 to 20 or more soils interlayered with repeated loess accumulations (Fig. 1).
Some of these paleosols have strongly developed layers, or horizons. In the dryer, western portion of the Palouse, calcium carbonate (lime) and silica-cemented hardpan horizons developed. Soil formation progressed further in the eastern Palouse under higher precipitation, with clay-rich subsoil horizons being formed. Remnants of these paleosol horizons now play an important role in the decline of productivity as they are exposed by continued erosion.
Overlying these paleosols, and forming a relatively thin mantle over the Palouse landscape, is the last major loess deposit. It is the result of the most recent episode of loess accumulation and soil formation during the last 13,000 years. The source of the tremendous productivity for Palouse agriculture lies in the fertile topsoil (A horizon) and, in some cases, the weakly developed subsoil (B horizon) that are formed in this recent mantle of loess.
Paleosol Distribution on the Landscape
On hilltops, and generally on south-facing slopes, the recent loess was originally thin over the paleosol horizons. On other parts of each landscape, such as the north slopes and bottomlands, it was several yards deep. Approximately one-half of the soil series mapped in Whitman County, WA, and as much as 30 percent of the acres have paleosol horizons in the root zone. The paleosol horizons have a complex and poorly understood distribution with respect to the modern landscape and its covering of recent loess.
Busacca and Frazier currently have research underway to help estimate the areas in the Palouse where paleosol horizons are exposed and will be exposed in the future if high rates of erosion continue. The researchers presently estimate that in 10 to 20 percent of the Palouse landscape, clay or hardpan paleosol layers are exposed at the surface. Soil map units in the Whitman County Soil Survey, delineated during mapping in the 1960’s and 1970’s, in many cases no longer reflect the soils that exist today in those delineations because of erosion.
Paleosol soil material is moved downslope from ridges and knobs by water erosion and tillage erosion and mantles the topsoil below. This further increases the area affected by the paleosol horizon exposure, causing additional crop loss.
Impacts of Paleosols at or Near the Soil Surface
As water erosion and tillage erosion reduce the thickness of the recent loess deposit, the clay-rich and hardpan paleosol horizons are closer to the surface, and exposed areas are growing in size. Soil productivity can be seriously reduced by these paleosol horizons. These horizons affect crop production in many different ways.
Restricted Root Development
Deep soils such as the Palouse and Walla Walla series are fairly uniform, having formed almost entirely in the recent loess deposit. They generally do not have paleosol horizons in the rooting zone. Subsoil bulk densities are only slightly higher than in the topsoil, so crop roots encounter little restriction with depth. Soil series such as the Risbeck, Endicott, Naff, Garfield and Thatuna have paleosol horizons in the rooting zone. In Risbeck soils, erosion increases the percentage of cemented hardpan fragments in the rooting zone, effectively reducing the rooting volume. Endicott soils have an intact hardpan at 20 to 40 inches depth, which acts as a complete barrier to roots. In the rooting zone of Naff, Garfield and Thatuna are clayey paleosol horizons (32 to 48 percent clay) that have 20 to 35 percent higher bulk densities than the overlying topsoil. When these lie near the surface, they physically restrict rooting depth. Surface exposure of paleosol horizons can severely limit plant growth.
Water Infiltration
Exceptionally high soil water storage has been one of the major factors in the success of dryland farming in the Palouse region. However, researchers are finding that doughtiness is becoming an increasing problem in soils that have paleosol horizons at or near the soil surface. This is largely because of low water infiltration rates.
Soils without paleosol horizons, such as the Palouse and Walla Walla series, have infiltration rates of 15 to 50 inches per day for both the A and subsoil or B horizons. In contrast, infiltration rates of the clayey paleosol horizons in the Naff, Garfield, Thatuna and similar soils are only 1.5 to 15 inches per day. Because of these slow infiltration rates, surface runoff and evaporation are increased and soil water storage decreased. As erosion causes the paleosol horizons to be closer to the surface, runoff and soil erosion potential are increased.
Seedbed Preparation and Emergence
The exposure of clayey and hardpan paleosol horizons has created new problems in crop management. Seedbed preparation is often more difficult because these subsoils tend to form larger, more resistant clods with tillage than the former topsoil, Power requirements for tillage operations are usually increased. Seed-soil contact and soil moisture levels are often less than desirable. Soil crusting also commonly results in poor seedling emergence.
Fertility
Deficiencies of nitrogen, phosphorus, potassium, sulfur and zinc on exposed paleosol horizons have been reported at an increasing rate. These deficiencies are difficult to treat because small areas in the irregular Palouse landscape cannot easily be managed separately. Also, additional fertilizer alone will generally not correct the loss in soil productivity.
Organic Matter Content Declining with Topsoil Loss
Besides the direct impact of these paleosol horizons or other restrictive soil layers at or near the soil surface, loss of the overlying topsoil itself is detrimental to crop production. One of the reasons is the loss in soil organic matter. Even on sites where erosion has been minimal, conventional tillage has significantly reduced organic matter levels below their original level.
In the dryer western Palouse, the combined effects of tillage erosion and water erosion has reduced topsoil organic matter content from the original 2 percent to the present level of 1 percent. With the low residue production in this area, increasing the organic matter level will be extremely slow. In the eastern Palouse, soils originally had a higher organic matter level, but higher soil erosion rates also prevail. In these soils, the organic matter content has declined from 4.5 percent to less than 2 percent today, The percent organic matter in the paleosol and other subsoil horizon now exposed at the surface on ridges and knobs is less than 0.5 percent.
Organic matter is important to soil productivity. Some benefits are that soil organic matter increases water infiltration into the soil and soil water-holding capacity; promotes soil aggregation and reduces surface crusting; increases soil aeration; reduces soil resistance to root growth, and can provide a substantial portion of the crop’s requirement of nitrogen, sulfur and other nutrients. Loss of the organic matter-rich portion of the topsoil can seriously reduce production potential.
Erosion Principles Apply in PNW
The common occurrence of paleosol horizons in the Palouse soils creates a unique situation for soil erosion. The same principles that limit crop production potential with these paleosol horizons, however, also apply to other restrictive subsoil horizons within the root zone of soils throughout the Northwest. Whether the horizons are paleosols or not, clay-rich horizons and other restrictive layers can impact rooting depth, water infiltration, soil water storage capacity, soil fertility and other soil productivity factors as topsoil depth is decreased by erosion. Attempts to make productive soils from these exposed paleosol and other unproductive subsoils have generally not been successful. Shallow soils over bedrock are particularly vulnerable. In all soils, loss of the organic matter-rich topsoil also reduces soil productivity.
Productive topsoil is a limited resource which must be managed wisely in the Palouse and all Northwest cropland. Conservation tillage systems, such as no-till and minimum tillage, which maintain a higher level of surface residue, can effectively reduce water runoff and soil erosion. With the use of up-to-date production technology, efficient conservation tillage systems can maintain or increase yields andconserve our soil and water resources.