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SOIL PHYSICS

Calculating the Effect of Soil Organic Matter Concentration on Soil Bulk Density

^a Inst. of Vegetable and Ornamental Crops, Theodor- Echtermeyer-Weg 1, 14979 Grossbeeren, Germany
^b Inst. of Plant Nutrition and Soil Science, Martin-Luther-University Halle-Wittenberg, Adam-Kuckhoff-Strasse 17b, 06108 Halle, Germany

^* Corresponding author (ruehlmann{at}igzev.de).

Soil bulk density (_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 _b is the soil's organic matter (SOM) concentration that alters the soil's compressibility; _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 _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 _b mathematically, we used a nonlinear regression model that was parameterized and validated using published data from experiments where SOM concentration was the main _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_b. Mathematically, s_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 _b and SOM has to be known to calculate s_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 _b affecting parameters (e.g., texture, soil depth, tillage regime), however, reassessed from the SOM effect.

Abbreviations: OM, organic matter • _b, bulk density • RMSE, root-mean-square error • RC, relative compaction • SD, standard deviation • SE, standard error • SOC, soil organic carbon • SOM, soil organic matter • s_b, standardized bulk density