Advancing Sustainable Agriculture in the Pacific Northwest

Conservation Tillage Systems

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PNW CONSERVATION TILLAGE HANDBOOK SERIES
Chapter 1 - Erosion Impacts, No. 5, Fall 1986



Erosion Reduces Water

Storage and Yield Potential


Roger Veseth

Dryland crop production is highly dependent on plant-available soil water throughout the growing season. This soil water is stored from precipitation during the fall and winter as well as precipitation during the growing season. In the Pacific Northwest, about 65 to 75 percent of the annual precipitation occurs during the fall and winter, whereas only about 20 to 30 percent occurs during the spring and early summer growing season. This precipitation distribution dictates where Northwest producers must concentrate their tillage and residue management efforts for efficient soil water storage in order to have the greatest impact on yield potential.

One factor which impacts soil water storage and yield potential has been the loss of topsoil through water and tillage erosion. Keith Saxton, USDA-ARS hydrologist and STEEP researcher at Pullman, WA, emphasizes that soil erosion almost always reduces soil water storage potential and subsequent yields. Two different impacts are involved. One of the most frequent is a reduced rate of water infiltration into the soil. This increases the potential for water loss through surface runoff and evaporation. A second impact of erosion is the reduced water storage capacity as topsoil depth declines.

Reduced Water Infiltration Rate
Saxton points out that the rate of water infiltration into the soil is largely determined by the presence of large pores (macropores) in the soil. This porosity can be spaces between soil particles or aggregates, created naturally or by tillage, as well as worm and insect holes, and old root channels. Once created, however, these macropores can readily become sealed by fine soil particles if the stability of the soil aggregates is insufficient to resist weathering or destruction by tillage. Aggregate stability has generally declined over the years with erosion and intensive tillage.

Organic matter plays a key role in maintaining stable soil structure and thus is important for keeping macropores open for water infiltration. The organic matter-rich surface soil is the first to erode. Consequently, as erosion continues, aggregate strength and the rate of water infiltration into the soil declines. Through erosion and intensive tillage, soil organic matter content has typically declined 50 percent or more on most Northwest cropland over the past century of cultivation.

Higher clay content, often typical of subsoil layers, also restricts water infiltration. The increasing exposure of subsoil layers by erosion has seriously reduced water storage potential. As soil clay content increases, pore size decreases and soil particles disperse more readily, sealing surface pores. Subsoil layers also have a lower organic matter content than the topsoil, and consequently have lower soil aggregate stability,

The eroded "clay-knobs" of the Palouse region of eastern Washington and northern Idaho are prime examples of lost productivity due in part to reduced soil water storage from lowered infiltration rates and increased runoff and evaporation. For comparison, the water infiltration rate into uneroded Palouse area soils is commonly about 0.6 to 2 inches per hour compared to 0.06 to 0.6 inches per hour where clayey subsoil layers are exposed on hills and ridges.

Saxton estimates that average annual runoff in the Palouse region varies from over 5 inches, where annual precipitation is 25 to 30 inches, to about 1 inch in the 10to 12-inch precipitation zone of eastern Washington. This 10 to 20 percent loss of potential stored soil water can directly limit yield where water is a yield-limiting factor. A generalized soil water-yield correlation is that each 1 inch of available water over and above the 4 inches required to produce the wheat plant results in approximately 7 bushels of wheat. Consequently, he points out that this lack of infiltration and increased runoff can translate directly to yield losses of 5 to 20 bushels per acre.

Besides reducing soil water storage potential, the exposed subsoils commonly present other crop production problems. These include a lower fertility status, difficult seedbed preparation, restricted root growth and poor seedling emergence because of soil crusting.

Reduced Water Storage Capacity

Saxton lists reduced storage capacity as another erosion impact affecting soil water storage potential. In some areas, soil erosion has reduced the depth of soil available for storing water. Generally this is less of an impact than the reduction of infiltration rate from exposed subsoil layers, unless the soil has a shallow root-restricting layer. Lime or silica cemented soil layers, called duripans, and soils shallow to bedrock are common throughout the Northwest. Reduced soil depth can significantly reduce water storage.

For example, a Walla Walla silt loam soil 6 feet deep over bedrock or other restricting layer has a plant-available soil water storage capacity of about 14 inches. Eroding 1 foot of soil, to make a 5-foot-deep profile, would reduce this storage capacity to 12 inches, a 15 percent reduction. For an Endicott silt loam soil, 20 inches deep over a root restricting, lime/silica cemented duripan, a 1 foot soil loss would reduce plant-available water capacity from about 4 inches to 1.6 inches, a 67 percent reduction.

The loss of 1 foot of topsoil would require a severe erosion rate of 50 tons per acre per year for about 35 years

(150 tons per acre equals approximately 1 inch of soil). Saxton points out that this amount of soil loss and more has occurred through years of water erosion and tillage erosion on a substantial portion of the Palouse and other Northwest cropland areas. Furthermore, yield potentials have declined on these eroded soils, due in part to the lower soil water storage.

Consider Water Storage Efficiency

Water is one of the most important yield-limiting factors on much of the Northwest dryland farming region. Producers need to evaluate the water storage efficiency of their crop production practices and the potential yield impact on the subsequent crop, as well as long-term crop production. Crop residue management for soil water storage and erosion prevention makes good farming sense. Water is a "free" crop production input that must be managed wisely to help maintain profitability in today's dryland agriculture.

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Hans Kok, WSU/UI Extension Conservation Tillage Specialist, UI Ag Science 231, PO Box 442339, Moscow, ID 83844 USA (208)885-5971
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