Trends in soil fertility

Abstract

G. W. Robinson once defined soil fertility as the ability of a given soil to produce satisfactorily some or all of those crops permitted to grow by reason of the climate. This statement implies a relationship between fertility and productivity which is widely accepted as axiomatic. It is a good definition because it covers the fact that there are wide variations in the nutritional and environmental requirements of plants, and there can be no fixed critical values of certain soil properties which can be used as criteria of fertility, no ideal pH, no ideal level of nitrogen or even drainage, for all crops. A certain acid soil may grow magnificent potatoes but miserable lucerne, a soil low in organic matter and therefore nitrogen may grow a fine crop of legumes but very poor grass. In New Zealand our complete dependence upon legumes in a system dominated by pastoral farming, enforces a rather different approach to problems of soil fertility, than in countries where arable cropping reigns supreme. The history of man in agriculture is much more closely concerned with the maintenance of fertility for arable farming than for the improvement of grasslands, which are widely accepted as gifts of God, in much the same way as we still tend to accept our forests. An examination of the desirable soil properties for arable cropping and for herbage production will provide a suitable basis for assessing trends in soil fertility.