Page 9 – What makes a good soil?
A versatile soil capable of many uses needs to be deep, fine-textured, moist, free-draining, loamy, and have an organic-rich topsoil. These properties best enable plant roots to take up nutrients, water and oxygen, and get enough support for rapid growth. Fertility is highest in soils young enough not to have been leached and old enough to have built up organic matter. They are also derived from parent rocks that are well supplied with essential nutrients.
New Zealand’s best soils
New Zealand’s best soils are called ‘versatile’ or ‘high-class’. They supply the nutrients required for optimum plant growth, and are good for growing food. Their area is limited (about 5.5% of New Zealand). High-class soils are most common among the Recent and Allophanic soils. They are also found among the other soil orders, but least among Podzol, Raw, Anthropic, and Brown soils that are acidic, lie on steep hills, or where the earth is frequently disturbed.
Because high-class soils are rare in New Zealand, it has been argued that they should be reserved for horticulture and agriculture, and not used for towns.
The effects of agriculture
The last century of farming in New Zealand has changed many soils that were originally poor. Acidity has been corrected by adding lime, low fertility by using fertilisers, drought by implementing irrigation, wetness by drainage, and poor drainage by breaking up subsoil ‘pans’ with deep-pronged equipment. These and other interventions have contributed to the country’s wealth, and an increasing intensity of land use. More intensive production, whether by horticulture, cropping or dairying, raises the issue of sustainability. Can we continue to use soils without losing quality or harming the environment? A knowledge of soils, and the opportunities and constraints they present, is essential for managing the land.