Organic matter stability and nutrient availability in Taupo Pumice

Summary Soils derived from rhyolitic Taupo pumice when under permanent pasture accumulate organically bound nutrients in the top soil to a high degree. The available nutrient supply is thereby reduced. Respirometry studies show that this accumulation is not due to a reduced microbial activity resulting from a deficiency of a major or minor nutrient element. Raising the pH increases the microbial activity of the soil and the rate of breakdown of the organic matter. These increases are due to a release of carbon into an available form. Available carbon is the limiting factor to microbial activity though the soil may contain 12 % total carbon. The inference is made that the stability of the organic matter in Taupo pumice soils is due to the formation of a stable complex. The possible nature of this complex is discussed. A greenhouse experiment showed that raising the pH releases useful amounts of nitrogen into available form but that a minor‐element deficiency can be easily induced. The effect of raising the pH on the mineralisation of organic phosphorus is uncertain. The agronomic significance of these findings is discussed.