Estimation of reference bulk density from soil particle size distribution and soil organic matter content

Crop growth and yield are affected by soil compactness. Rather than using bulk density, it has been suggested that soil compactness be described by relative bulk density, e.g. the degree of compactness, previously defined as the ratio of bulk density (ρ) to reference bulk density (ρref). This study investigated relationships between ρref as defined by Håkansson (1990) and soil particle size distribution (PSD) by analysing soil data from 171 experimental sites in Sweden, two in Poland and three in Finland. PSD was characterised either by the common size fractions (i.e. clay, silt and sand content) or by fitting a (continuous) mathematical function to the experimental PSD data. The Rosin–Rammler equation was used for the latter, and the PSD was characterised by the Rosin–Rammler parameters α and β. We present equations for estimation of ρref from either soil textural classes or from α and β in combination with soil organic matter content (OM). It was shown that ρref is largely controlled by OM. The best model (i.e. the model with the smallest value of Akaike Information Criterion) was found to be one that estimates ρref from α, β and OM. The regression models for calculation of ρref presented here could be incorporated into models for calculation of crop yield losses due to soil compaction. Furthermore, we found good agreement between ρref and values for critical bulk density for root growth reported in the literature, indicating that ρref is a critical bulk density for root growth.