ADDRESSING SOIL COMPACTION MANAGEMENT KNOWLEDGE GAPS IN EASTERN WASHINGTON CROPPING SYSTEMS

Natalie Sturm

Farmers and researchers have been contending with soil compaction, one of the most widespread forms of soil degradation, for over a century. Especially in modern large-scale agriculture, the use of increasingly heavier equipment has begun to exceed critical soil strength limits, leading to potentially irreversible compaction and threatening soil functions and crop productivity around the world. Thus, avoiding the creation of compaction is promoted as one of the best ways to conserve soils. However, completely avoiding soil compaction in mechanized agriculture is rarely possible. Intentional management to remediate soil compaction is often necessary. Typically, compaction is addressed by mechanically loosening soil layers with tillage. More recently, controlled traffic farming systems have gained popularity for their ability to limit the area of a field exposed to compactive forces. However, both of these soil compaction management practices have significant barriers that limit their adoption in some important agricultural regions. One such region is eastern Washington state, US. This area is one of the most productive wheat-producing regions in the world; Whitman County, WA is the number one wheat-producing county in the US. Comprising part of the Inland Pacific Northwest’s Palouse region, eastern WA is characterized by steep complex topography, a Mediterranean-type climate with almost all precipitation occurring from November through May, and deep silty soils derived from loess. While these features are extremely conducive to the growing of wheat, they also cause unique barriers for soil compaction management. Highly silty soils tend to be at greatest risk of structural damage and crop yield loss from deep tillage. In addition, implementing tillage on steep hills can be economically and ecologically costly. In systems with complex topography, issues with controlled traffic farming technologies have not yet been overcome. In this dissertation, knowledge gaps regarding soil compaction management in eastern WA are addressed to assist in improving soil conservation and agricultural production in the region. In Chapter One, an overview of the impacts of compaction on soil processes and crop performance is provided, followed by a discussion of the need for regionally relevant research on soil compaction management practices. In Chapter Two, the published article Informing soil compaction research priorities with farmer focus groups in the United States by Sturm et al. is presented. In it, the current state of the soil compaction problem for farmers in eastern WA, and other growing US growing regions, is described and farmer-driven research questions are identified. In Chapter Three, a cropping system trial to investigate some of those farmer-driven questions, and gaps in scientific literature, is presented. In this project, a silt loam soil near Pullman, WA was purposely compacted to examine the potential of three crop sequences to remediate damaged soil physical properties. In addition, this provided an opportunity to enhance the understanding of the impacts of an intense compaction event on crop and soil responses. Finally, in Chapter Four, on-farm trials of tillage and compost application to manage soil compaction at field entrances and hill knobs or slopes are described. The results presented in this dissertation confirm that soil compaction is a complex issue in need of management from a full system perspective. In the short-term, crops such as wheat, sunflower, and canola appear to have little to no impact on reducing surface soil compaction, while perennial crops, such as tall fescue, may be effective at recovering soil porosity and limiting subsoil re-compaction. On two farms in eastern WA, deep tillage reduced soil compaction but had no impact on crop yield. By one year and five months post-treatment, re-compaction of the tilled soils was observed. Compost application did not improve the effectiveness of deep tillage, but it did lead to crop yield increases in some cases. In conclusion, this dissertation posits that management practices in eastern WA cropping systems should focus on improving plant and soil health in the context of compaction rather than targeting reductions to compaction directly.

ADDRESSING SOIL COMPACTION MANAGEMENT KNOWLEDGE GAPS IN EASTERN WASHINGTON CROPPING SYSTEMS

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