On the Origin of a Gradient in Productivity of Taupo Volcanic Ash Soil on Sloping Ground

Abstract

Rhyolitic volcanic ash deposits of recent origin constitute the parent soil material over several million acres in the central part of the North Island of New Zealand. This part is often referred to as "the pumice country". The ash has been deposited in a number of successive showers and the material of the youngest shower or showers is considered to be in a relatively unweathered state (Van't Woudt, 1952). The surface soil formed from it generally occurs as a sandy silt with many embedded pumice lumps varying in diameter from a fractino of an inch to several inches. Large-scale agricultural development of this land did not start till after 1938, at which time it was discovered that a serious stock disease could be overcome by cobalt application. Since then the pumic country has received much attention from research workers, partly because of an academic interest in the development of soil and plant cover on volcanic ash which has been deposited within the last few thousand years. This study attempts to elucidate one of the problems encountered, that of the variation in productivity of the soil in relation to relief.