Field study of soil compaction due to traffic in northern France: pore space and morphological analysis of the compacted zones

Soil compaction can have important impacts in cropping systems but little i s known about the geometry and volumes of compacted zones. This paper exami nes the change in soil compaction due to traffic under a wide range of soil conditions, describing its intensity and the soil volumes affected. Soil c ompaction after several types of wheeling operations, namely seedbed prepar ation, sowing or harvesting, was characterised for a wide range of moisture conditions in a loamy soil (Haplic Luvisol) in a long-term field experimen t in northern France. Soil compaction was described by pore space and morph ological analysis. Pore space analysis was characterised using the structur al porosity under wheel tracks estimated from held bulk density measurement s and laboratory textural porosity measurements. Morphological analysis was characterised using the relative percentage of the compacted zones (massiv e zones without visible macropores) of the soil profile under wheel tracks. Soil compactness varied greatly under wheel tracks, from 0.25 to 0 m(3) m( -3) for structural porosity, from 100% to 0% for the percentage of the comp acted zones. Structural porosity depended on the type of held operation and the corresponding tractor size: it remained higher after seed-bed preparat ion (5 Mg tractor with wide tyres inflated to 80 kPa) than after sowing (4 Mg tractor with narrow tyres inflated to 200 kPa) and harvesting (10 Mg har vester with wide tyres inflated to 250 kPa). Structural porosity decreased linearly when the soil water content at wheeling increased. It increased as the structural porosity before wheeling increased. The same effects of the soil conditions and the type of held operation were obtained using the per centage of compacted zones to describe soil compaction. The percentage of c ompacted zones increased for soil moisture >0.15 g g(-1) at harvesting, >0. 16 g g(-1) at sowing, and >0.21 g g(-1) at seed-bed preparation. Morphologi cal analysis indicated that the change in the percentage of compacted zones resulted from changes in both their width and depth. The geometry of the c ompacted zones could be approximated by a half-ellipse. These results show how a morphological method is useful for describing the soil volumes affect ed by soil compaction under field conditions. (C) 1999 Elsevier Science B.V . All rights reserved.