Spatial distribution of soil mechanical strength in a controlled traffic farming system as determined by cone index and geostatistical techniques

Controlled traffic farming (CTF) is a mechanisation system in which all load-bearingwheels are confined to the least possible area of permanent traffic lanes and where crops aregrown in permanent, non-trafficked beds. In well-designed systems, the area affected by trafficrepresents less than 15% of the total field cropped area. The extent and distribution of soilcompaction at locations laterally outboard of the permanent traffic lanes may explain theperformance of the crop on the rows located either side of the wheeling. This compaction is dueto lateral displacement of soil caused by repetitive wheeling, the effect of soil-tyre interaction andthe soil conditions (strength) at the time of traffic. The impact of compaction on crop rowsadjacent to permanent traffic lanes is also dependent on the seasonal effect of weather, becauseof changes in soil water availability. This work was conducted to model the spatial distributionof soil mechanical strength under increasing number of tractor passes to simulate the soilconditions that may be encountered in CTF systems at locations near-permanent traffic lanes. Thestudy was conducted on a Typic Argiudoll (26% clay, 72% silt, 2% sand) with four traffic intensities(0, 6, 12 and 18 passes) using a 120 HP tractor (overall mass: 6.3 Mg). Traffic treatments wereapplied to experimental plots using a completely randomized block design with three replicationsper treatment. The spatial distribution of soil strength within wheeled and non-wheeled zones wasdetermined using a cone penetrometer (depth range: 0–300 mm) and geostatistical techniques. Inall treatments, cone index showed a quadratic response with depth, which explained between 67%and 88% of the variation in soil strength. The number of tractor passes had no effect on the rangeof spatial dependence of residuals. No differences were observed in the proportion of grid cellswhere penetration resistance was greater than 2 MPa (considered to be the soil strength limit forroot growth of most arable crops) between-traffic treatments, or wheeled and non-wheeled zones,respectively. The overall mean proportion (± 95% confidence interval) of grid cells (4.9 ± 4.5%)suggested that this measure has a relatively high variability and therefore may not be a reliableparameter to be used in the design of future experimental work.