Age-hardening phenomena in an oxisol from the subtropical region of Brazil

Soil strength is not only affected by water content and bulk density, but also by the age-hardening phenomena, which plays a key role in increasing the soil strength as a function of time. It has been demonstrated that soil penetration resistance in no-tillage is higher when compared with other tillage systems at the same bulk density and water content. The objectives of this study was to investigate the effects of the age-hardening phenomena on soil penetration resistance in a long-term soil management system, running since 1988 in a very clayey Oxisol, in southern Brazil. Soil samples were collected from three soil layers (0.0-0.10 m; 0.10-0.20 m and 0.20-0.30 m) and five soil tillage systems: conventional tillage; minimum tillage with chiselling performed every year or every three years; and no-tillage for 11 or 24 years. Age-hardening was investigated using soil penetration resistance analysis and modelling. We used the area under the soil resistance to penetration curve to compare the age-hardening phenomena under the different tillage systems. For the same bulk density and water content, the soil resistance to penetration increased with time under no-tillage or without soil chiselling. For the same bulk density, no differences were found for macroporosity and microporosity among the tillage systems. Higher soil penetration resistance values in long-term no-tillage at the same soil bulk density and water content were attributed to the age-hardening phenomena, which increased the number and strength of bonds among soil particles, leading to higher soil cohesion. It is necessary to establish critical limits of soil penetration resistance as a function of the soil tillage system, and the time without soil chiselling or under no-tillage system.