Soil precompression stress, penetration resistance and crop yields in relation to differently-trafficked, temperate-region sandy loam soils

• Soil precompression stress was a poor predictor of sensitivity to compaction.
• The number of wheel passes had a strong effect on penetration resistance and yield.
• Multiple passes with >6 Mg wheel loads resulted in persistent subsoil compaction.
• Traffic with 8 Mg wheel load a single year had no aftereffect on crop yields.

Compaction of the subsoil due to heavy traffic in moist and wet soil is widespread in modern agriculture. The objective of this study was to quantify the effects from realistic field traffic on soil penetration resistance and barley crop yield for three Luvisols developed from glacial till. Undisturbed soil cores were used for quantifying the precompression stress (σpc) of non-compacted soil. Tractor-trailer combinations for slurry application with wheel loads of ∼3, ∼6 and ∼8 Mg (treatments M3, M6, M8) were used for the experimental traffic in the spring at field-capacity. For one additional treatment (labelled M8-1), the soil was loaded only in the first year. A tricycle-like machine with a single pass of wide tyres each carrying ∼12 Mg (treatment S12) was included at one site. Traffic treatments were applied in a randomized block design with four replicates and with treatments repeated in four consecutive years (2010–2013). After two years of repeated experimental traffic, penetration resistance (PR) was measured to a depth of 1 m. The yield of a spring barley crop (Hordeum vulgare L.) was recorded in all four years of the experiment. The results did not support our hypothesis of σpc as a soil strength measure predicting resistance to subsoil compaction. The tyre inflation pressure and/or the mean ground pressure were the main predictors of PR in the upper soil layers. For deeper soil layers, PR correlated better to the wheel load. The number of wheel passes (M-treatments vs the S12 treatment) modified this general pattern, indicating a very strong impact of repeated wheel passes. Our data indicate that a single traffic event may mechanically weaken the soil without inducing major compaction but with influence on the effect of subsequent traffic even after as long an interval as a year (treatments M8 vs M8-1). Crop yields were much influenced by compaction of the plough layer. Due to the repeated wheel passes for the M-treatments, significant yield penalties were observed, while the single-pass treatment with 12 Mg wheel load in S12 did not have significant effects on crop yield. Our hypothesis of 3 Mg wheel load as an upper threshold for not inducing subsoil compaction was confirmed for the tractor-trailer treatments with repeated wheel passes but not supported for the single-pass machinery. The results call for further studies of the potential for carrying high loads using wide, low-pressure tyres by crab steering/dog-walk machinery.