SHRINKAGE OF SOME NEW-ZEALAND SOILS AND ITS IMPLICATIONS FOR SOIL PHYSICS

Aggregates from three New Zealand soils were used to produce shrinkage curves. Each soil had a clay mineralogy dominated by a different mineral, namely allophane, halloysite and kaolinite.The curves showed marked differences. Only the allophanic soil showed structural shrinkage, and only the halloysitic soil showed residual shrinkage. When the slope of the normal shrinkage line is about unity, this indicates the soil is liable to crack- this was only shown by the allophanic soil. The implication for soil physics is that, since all soils with at least a moderate clay content shrink, bulk densities change with moisture and this must be allowed for in such calculations as soil moisture on a volume basis.