A simple model to predict soil penetrometer resistance as a function of density, drying and depth in the field

• A model was developed to explain the effects of soil bulk density, water status and depth on penetrometer resistance.
• Influence of soil depth on penetrometer resistance was related to overburden pressure.
• No tillage increased soil bulk density and reduced macropores in the 0–20 cm layer.

Penetrometer resistance is one of the important soil physical properties that can determine, or be strongly correlated with, both root elongation and yield. We used a field experiment, which was designed to explore the effects of no-tillage (NT) and ridge tillage (RT) on a Mollisols in Northeast China, to test an approach for predicting the effects of soil bulk density, matric potential of water, and depth on penetrometer resistance. The field experiment was initiated in the fall 2010 on a clay loam soil. Soil bulk density, water content, water release curve, and penetrometer resistance were measured during the corn growing seasons in 2012 and 2013. Compared with the RT treatment, NT significantly increased soil bulk density and reduced the fraction of macropores in the 0–20 cm soil layer. The increase in density under NT resulted in a higher soil penetrometer resistance, which was associated with a lower water uptake by roots compared with the RT treatment. It was possible to use a relatively simple model for penetrometer resistance to explain the effects of soil bulk density, water status and depth on penetrometer resistance. Depth was included by relating it to overburden pressure. In a penetrometer resistance model that excluded overburden pressure, or depth empirically, penetrometer resistance in deeper layers (>35 cm) could not be fitted accurately. The effects of depth on penetrometer resistance from this study were consistent with published data. Our data suggests that at the assessment of soil conditions for root growth, ignoring the effects of depth is likely to give misleading penetrometer resistance.