Page 7 – Wet and rock type-dominated soils
Prolonged wetness affects the colour of soils, the types of plants that grow in them, and the rate of plant growth. In New Zealand there were extensive wetlands before European settlement, but large areas of Gley and Organic soils have now been drained for farming.
Gley soils, which cover 3% of New Zealand, are saturated with water for most of the year. Their subsoils are grey with rust-coloured spots or mottles. Gley soils are usually on low-lying land where there are high groundwater tables, or on hill slopes where there are seepages. They can be very productive when drained.
Organic soils, covering 1% of the country, are peat soils that are wet throughout the year. Located in wetlands, they formed from the decomposed remains of wetland plants (peat) or forest litter. The peat or litter accumulated because the processes that decompose fresh organic matter happened very slowly.
Organic soils can be productive when drained and fertilised. However, their use is not likely to be sustainable in the long term because drainage dries out the organic matter. It then decomposes more rapidly and the soils begin to shrink.
Rock type-dominated soils
Soil parent materials, whether they are loess, ash, alluvium or weathered rock, are influenced by the nature of the original rock from which they formed. The parent rock influences the release of nutrients (for example calcium), the formation of clay minerals, and the amount of iron oxides, stones and clay.
Allophanic soils cover 5% of the country and are found mostly in the central North Island. They are predominantly made up of volcanic ash. The ash contains natural volcanic glass, which weathers in the soil to become allophane. Allophane is a stiff, jelly-like mineral that produces a porous, low-density soil. Allophanic soils have a wide range of uses.
Pumice soils cover 7% of New Zealand. They are dominated by pumice, or pumice sand high in volcanic glass. These soils are found mainly in the central North Island, where most of the pumice originated from the powerful Taupō eruption, 1,800 years ago. Many Pumice soils are used in commercial forestry. They were unsuitable for animal farming until they were found to be deficient in cobalt. Now that their deficiencies have been corrected by fertilisers, they are very productive.
Melanic soils, which cover 1% of the country, come from rocks that are rich in calcium (including limestone) or magnesium and iron (from dark volcanic rocks). Calcium and magnesium stabilise organic matter and promote the development of firm soil aggregates. Consequently, Melanic soils are fertile, with organic-rich black topsoils.