SOILS AND LAND USE
The Changing Soil
Since soil is the product of its environment, the soils of New Zealand are being changed continually by land use. They have been modified by cultivation, fertiliser practices, and introduced animals and, in less obvious but important ways, by the introduced pastures and exotic forests that have replaced much of the native vegetation. Some of these changes involve the destruction or exhaustion of the soil (for example, by soil erosion or excessive cropping), but others reflect its improvement or enrichment.
The complex soil changes that occur under present-day grassland management are illustrated by those associated with the improvement of pastures on many soils. Low-producing pastures are mostly “sod bound” or underlaid by a tight strong sod; in places under paspalum, browntop, or Yorkshire fog, the residues of the slowly decomposing leaf bases of the grasses produce a yellowish, greasy, peatlike layer of organic matter (diagram 8A). Somewhat more productive pastures also tend to have a tight fibrous sod, but, owing to more rapid decomposition, the peatlike layer is absent and the organic matter has accumulated within the upper part of the mineral soil to form a well defined dark grey or greyish brown topsoil (diagram 8B). High-producing pastures tend to have a weaker sod. They are obtained by adding fertilisers and animal manures and by lowering soil acidity, all of which favour organisms that decompose organic matter. Under these conditions earthworms multiply, break up the sod, and bury the crowns of pasture plants with worm casts. In long-established pastures of this kind the perennial grasses and clovers have rerooted year by year in the worm casts and hence are young and vigorous, but the sod is no longer tight and strong (diagram 8C).
Where the improvement of pastures has reduced the strength of the soil in this way, puddling under animal treading during wet weather has tended to increase and place a limit on the number of animals that can graze on the pasture. The understanding and control of such changes in the soil has an important place in the future of New Zealand soils and land use.