Compaction significantly reduces yield, quality, and profitability of irrigated
crops in the US Pacific Northwest (PNW). Compaction assessment
is usually done via bulk density measurement, even though crops
respond negatively to excessive compaction largely because of root penetration
(soil strength) limitations, not because of bulk density per se. For
most soils, strength is thought to depend primarily on the interaction of
water content and bulk density. We hypothesized that the soil strength
(expressed as cone index) of an important PNW soil, Portneuf silt loam
(Durinodic Xeric Haplocalcid), could be predicted for a given bulk density
or water content and that it would increase with increasing bulk density
and decreasing water content. To test this, the in situ cone index, the bulk
density and water content profile of a 1.5-ha field was intensively sampled
three times over a 2-year period, producing 688 data triplets. These
data were used to produce soil water strength-bulk density response surface
relationships using robust curve fitting. Cone index relationships
were poor when derived from full-profile data sets but improved when
data were segregated by depths. When grouped by depth intervals, cone
indices of individual layers were always correlated strongly with soil water
content, but not always with bulk density. The high calcium carbonate
content of this soil was thought to have produced cementation effects
on the cone index that varied with prolonged wetting versus prolonged
drying. Variability among in situ strength penetrations and bulk density
cores was also thought to reduce model accuracy. The difficulties inherent
in developing the comprehensive relationships of soil strength to bulk
density, and the overriding dependency of strength on the dynamic variable
of water content, suggest great uncertainty when using bulk density
sampling for realistic assessment of overall soil status affecting root restriction
or crop performance unless sampling is extensive and the relationships
between strength, bulk density, and water content have been
intensively documented for an individual soil
