Submitted to: Soil Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/1/2001
Publication Date: 8/1/2001
Citation: Sojka, R.E., Busscher, W.J., Lehrsch, G.A. 2001. In situ strength, bulk density, and water content relationships of a durinodic xeric haplocalcid soil. Soil Science. 166(8):520-529. Interpretive Summary: Soil compaction (bulk density) affects the ability of plant roots to penetrate the soil to reach for water and nutrients and anchor the plant. The degree to which a soil restricts root growth is related to a measurement of soil strength called penetration resistence or cone index. Soil cone index has been shown to be affected by bulk density (compaction) and water content. The nature of these bulk density x water content x strength relationships varies with other soil properties, and therefore is unique from soil to soil. The Portneuf soil is an important soil of the US Pacific Northwest and has properties similar to many soils in the region. The relationships between soil water content, bulk density, and cone index have not been defined for this soil. Knowing the relationship would make it easier to assess true soil strength status in terms of potential root penetration, with less chance of misinterpretation because of confounding affects of bulk density or water content. Our studies showed that bulk density had a smaller affect than water content in these soils for determining soil strength. Our data suggests that this may be partly the result of calcium carbonate (lime) cementation changes that occur with soil wetting and drying. The data point to the value of not allowing soil layers do become excessively dry in order to allow strength to remain low enough for root penetration. Rewetting layers may not immediately allow root penetration, but may require time for lime cementation to weaken as the lime cementing soil particles slowly dissolves.
Technical Abstract: Compaction reduces yield, quality, and profitability of irrigated crops largely because of root penetration (soil strength) limitations. Assessment is usually via bulk density (BD). For most soils, strength is primarily related to interaction of soil water content (SWC) and BD. We hypothesized that soil strength (cone index) of Portneuf silt loam could be predicted for a given BD or SWC by knowing the missing factor's value and soil strength would increase with increasing BD and decreasing SWC. Cone index, BD and SWC profile of a 1.5 ha field was intensively characterized to produce soil water-strength-bulk density response surface relationships using curve fitting. Cone index relationships were poor when derived from full-profile data sets but improved when data were segregated by depths. When separated by depth intervals, cone index of individual layers was always strongly correlated with SWC, but not always with BD. The high calcium carbonate content of this soil may have produced hysteretically variable cementation affects on cone index. Spatial variability between in situ strength penetrations and BD corings was also believed to reduce model accuracy. The difficulties in developing comprehensive relationships of soil strength to BD, and the overriding dependency of strength on the dynamic variable of SWC, suggest great difficulty for using BD sampling as a realistic assessment component of overall soil status affecting root restriction or crop performance, unless the sampling is highly extensive and the relationships between strength, BD, and SWC have been intensively documented for an individual soil.