AMOUNTS AND SOURCES OF PHOSPHATE IN HILL COUNTRY ROCKS, SOUTH-EASTERN NORTH ISLAND, NEW-ZEALAND

The phosphate (P) present in a range of parent rocks for hill country soils in Wairarapa region, New Zealand, has been studied. In general the highest P concentrations occur in the mudstones which contain 50% more P than the fine to medium sandstones, and at least 10 times more P than the coarse sandstones. Most of the P is in the 20-2-mu-m and <2-mu-m size fractions of the rocks. The P is present both in apatite and calcite mineral grains. Although the highest P concentrations occur in the heavy fraction (containing apatite), most of the total P is held in the light fraction in which calcite is abundant and where some apatite also occurs within aggregates. Since both calcite and apatite are relatively soluble in this soil environment, and since both have large reactive surfaces, they may readily release P for plant growth.