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

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 realisticfield traffic on soil penetrationresistance and barley crop yield for three Luvisols developed from glacial till. Undisturbed soil cores wereused for quantifying the precompression stress (spc) of non-compacted soil. Tractor-trailer combinationsfor slurry application with wheel loads of3,6 and8 Mg (treatments M3, M6, M8) were used for theexperimental traffic in the spring atfield-capacity. For one additional treatment (labelled M8-1), the soilwas loaded only in thefirst year. A tricycle-like machine with a single pass of wide tyres each carrying12 Mg (treatment S12) was included at one site. Traffic treatments were applied in a randomized blockdesign with four replicates and with treatments repeated in four consecutive years (2010–2013). Aftertwo 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. Theresults did not support our hypothesis of spc as a soil strength measure predicting resistance to subsoilcompaction. The tyre inflation pressure and/or the mean ground pressure were the main predictors of PRin the upper soil layers. For deeper soil layers, PR correlated better to the wheel load. The number ofwheel passes (M-treatments vs the S12 treatment) modified this general pattern, indicating a very strongimpact of repeated wheel passes. Our data indicate that a single traffic event may mechanically weakenthe soil without inducing major compaction but with influence on the effect of subsequent traffic evenafter as long an interval as a year (treatments M8 vs M8-1). Crop yields were much influenced bycompaction of the plough layer. Due to the repeated wheel passes for the M-treatments, significant yieldpenalties were observed, while the single-pass treatment with 12 Mg wheel load in S12 did not havesignificant effects on crop yield. Our hypothesis of 3 Mg wheel load as an upper threshold for not inducingsubsoil compaction was confirmed for the tractor-trailer treatments with repeated wheel passes but notsupported for the single-pass machinery. The results call for further studies of the potential for carryinghigh loads using wide, low-pressure tyres by crab steering/dog-walk machinery.