Estimation of reference bulk density from soil particle size distribution and soil
organic matter content
Thomas Keller a,b,c,⁎, Inge Håkansson
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.