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Calculating the effect to soil organic matter concentration on soil bulk density.

Soil bulk density ({rho}b) is required to estimate, evaluate, and calculate many physical soil properties and processes and is essential to convert data from weight-based to volume- and area-related data. One of the dominating factors changing {rho}b is the soil’s organic matter (SOM) concentration that alters the soil’s compressibility; {rho}b is an important soil structure attribute. Currently, no parameter for characterizing soil compactness giving directly comparable values for all soils is available. Therefore, our aim was to develop a general approach to calculate the effect of SOM concentration on {rho}b that would be universally valid for soils different in their genesis, compaction, and type of land use. To describe the effect of SOM on {rho}b mathematically, we used a nonlinear regression model that was parameterized and validated using published data from experiments where SOM concentration was the main {rho}b–affecting factor (long-term fertilization and proctor experiments, wetlands, reclaimed soils, and volcanic soils). To obtain a standardized parameter describing the present compaction status of a site, we introduced the standardized bulk density s{rho}b. Mathematically, s{rho}b is the intercept parameter of the used nonlinear regression model, and ranged between 0.7 and 2.1 Mg m–3 and was very simple to estimate. Another distinct advantage of this novel concept is that only one representative pair of {rho}b and SOM has to be known to calculate s{rho}b as well as the bulk densities corresponding to other SOM concentrations measured on the site. This concept might also be helpful for identifying similar universal approaches to standardize the effect of other {rho}b affecting parameters (e.g., texture, soil depth, tillage regime), however, reassessed from the SOM effect.

Rühlmann, R.; Körschens, M. 2009. Calculating the effect to soil organic matter concentration on soil bulk density. Soil Science Society of America Journal 73, 876-885.