Quick Dips

Leaching Concerns

Figures suggest that New Zealand's pastoral soils are becoming increasingly acid and deficient in potassium. Comparisons of mineral inputs and outputs for 1989-90 suggest soil potassium reserves are being depleted by about 226,000 tonnes annually, an average loss of 11 kilograms of potassium per hectare per year.

About 1.8 million tonnes of lime per year is needed to counteract increasing soil acidity, caused by the leaching of nitrates from the soil. Currently, less than half this quantity of lime is applied.

Massey University soil scientist Professor Bob White describes this as "somewhat alarming". White says that more research is urgently required to refine estimates of soil potassium and rates of release. More research is also needed to check the accuracy of the soil and animal loss factors used in calculations.

Some combinations of soil type, pasture and animals may not be sustainable over the next 10-20 years. Leaching of potassium and nitrates from soils under intensively managed dairy farms may be greater than that of the average farm.

White says, however, that soil levels of nitrogen, phosphorus and sulphur appear to be in balance. Despite fertiliser cutbacks in the last five years, the grassland system seemed in surprisingly good heart.

Georgina Hall, Massey University