Structural vulnerability of a sandy loam exposed to intensive tillage and traffic in wet conditions

Summary

Sustainable soil management requires that the structural degradation is balanced or exceeded by regeneration. Our objective was to investigate the vulnerability of topsoil structure to stress exerted by intensive tillage or traffic. The study addressed the short-term stability to disturbance as well as the recovery (resilience) within a year. A field experiment was conducted in a randomized block design on a humid sandy loam in 1997-1999. Each year, either compaction from a heavy tractor (PAC) or puddling by intensive rotary cultivation (PUD) produced a severe impact on topsoil structure. The PAC and PUD treatments were carried out on wet soil in early spring. The mechanical treatments were referenced by plots (REF), which were left undisturbed until the soil had dried to a friable condition and ready for seedbed preparation. The PAC and PUD treatments were prepared for sowing at the same time as the reference plots. Penetration resistance was recorded in the spring of 1998 and 1999 to a depth of 200 mm. Soil was sampled from the 0-40 mm layer in May 1998 and in March, May and November 1999. The soil was air-dried and separated into four aggregate size fractions. The aggregates were subjected to tensile strength and density measurements. The penetration resistance in the 0-200 mm layer ranked in the order PAC>>PUD>REF. Both mechanical treatments significantly increased the density of 4-8 mm aggregates. One or two months after the mechanical treatments, they had increased tensile strength relative to REF by 44% and 33% in 1998 and by 13% and 33% in 1999 for the PAC and PUD treatments, respectively. Thus, our result showed substantial topsoil degradation when exposed to the PAC and PUD treatments, i.e. the sandy loam showed low stability. In November 1999, the PUD-treated aggregates were still markedly stronger than those found in the REF soil. Hence, the PUD-treated soil showed little resilience within a six-month summer period. There was no significant difference in aggregate tensile strength between the treatments in March 1999 after a winter with cycles of freezing and thawing and a mouldboard ploughing operation in early spring. Our results thus imply that soil degradation induced by soil compaction or intensive rotary cultivation early spring may reduce the ease of tillage in the following autumn whereas little residual effect can be expected in the following spring.