Hardsetting soils: a review

Abstract

Hardsetting soils have been defined as soils that set to a hard, structureless mass during drying and are thereafter difficult or impossible to cultivate until the profile is rewetted. Soil strength increases rapidly as the soil dries, and so seedlings must grow quickly before soil strength becomes too high for root growth or shoot emergence. Recent work on the mechanisms of hardsetting confirms that aggregate disruption through slaking and dispersion on wetting leads to coalescence. Bridging by dispersed particles under matric potential makes a soil hardset. Failure to recover from a coalesced state as the soil dries leaves it with a massive structure. This paper reviews the worldwide occurrence of hardsetting soils, the evolution of definitions of hardsetting, and the use of those definitions in soil classification with particular emphasis on Australia. Measurement of hardsetting includes methods such as visual score of slaking and dispersion, penetration resistance, fall-cone penetration, dispersion, fractions of soil organic matter, friability index, modulus of rupture, and a particular use of the soil water retention curve. Overcoming problems associated with hardsetting soils and their ongoing management is difficult. Further work is needed on the reasons for variable responses to tillage, no tillage, and pasture. Modifying soil texture has limited application, and increasing soil organic matter under cropping is difficult in low-rainfall areas. Polymers have been shown to be beneficial. Mulching maintains higher soil moisture and therefore a softer surface, while biochar shows inconsistent effects. Controlled traffic is a key to reducing recompaction. Management of a hardsetting soil must include the whole rotation, including when to till, when to crop, and when to graze or not.