8. Organic Soils

Organic Soils form from partly decomposed remains of plants in peat-forming wetlands, or in deep accumulations of forest litter. Organic Soils comprise about 1% of New Zealand with the largest areas in the Waikato, on the high rainfall South Island West Coast, and in some moist mountainous areas where soils are often saturated. Organic Soils are acidic (pH ≤  ~4), have a low dry bulk density, high C:N ratios, high shrinkage potential, and exceptionally high soil water holding capacity. Organic Soils have been developed mainly for pasture, maize, potato, and blueberry production, and are also mined to extract peat. Organic Soils have naturally low nutrient contents so need inputs of fertiliser and lime, along with drainage, for productive farming. Such inputs hasten the biodegradation of Organic Soils. To slow the rate of degradation, water tables need to be kept high to prevent oxygen from entering the soil. Although about 70% of Organic Soils have been heavily impacted by drainage and development for farming, or by peat mining, some natural areas remain in reserves. The Whangamarino wetland and the rainfed Kopouatai and Moanatuatua bogs are recognised as wetlands of international importance with unique indigenous vegetation, including the tall cane rush (Sporodanthus ferrugineus).