We are most familiar with topsoil – the dark layer (also called the horizon) at the ground’s surface. Digging deeper yields surprising colours and structures. The underlying subsoil gives clues to the history of the soil and past environmental conditions. It also shows the conditions that plant roots must explore, and how this affects land use choices.
What soils are made of
Weather, micro-organisms (or microbes) and chemicals all act to break down gravels, sands and silts into clay. However this is balanced in many soils by gradual accumulation of layers of loess (fine, wind-blown soil), volcanic ash, pumice and alluvium (clay, silt and gravel left behind by a stream or river). Ash and pumice ejected from volcanoes cover the central North Island. Loess covers large areas of the South Island and the south and east of the North Island.
The material from which soil is formed is called parent material.
Soil colour is caused by its age, wetness, the climate in which it formed, and its parent material. In topsoils, the true colour is masked by dark organic matter, but it can be seen clearly in subsoils.
- Brighter colours indicate relatively old soil, which is usually moist but not wet.
- Rust-coloured or reddish spots or mottles show seasonal periods of water saturation, drying out in between.
- Mottled grey is a sign of long periods of wetness.
Some brightly-coloured soils have been used to make paints.
Soil texture depends on the proportion of clay, silt, sand, stones (or gravel) and boulders. Clay particles are the smallest, then silt, then sand. Clay surfaces have an electrical charge and attract nutrient atoms and molecules such as potassium and calcium. Sand’s comparatively larger particles create spaces in the soil which drain water and allow air to reach plant roots.
A mixture of sand, silt and clay is called loam. A loam can be named after the type of soil particle that dominates, for example silt loam, clay loam or sandy loam.
- Silt loams are common in New Zealand, especially in soils made of loess and river-deposited alluvium.
- Clay loam is common where soils have formed from clay-rich rocks, and where the land is very old and the rocks have weathered to clay.
- Sand and sandy loam soils are most common on sand dunes, river alluvium deposits, and pumice deposits.
When a spade full of soil is dropped on the ground it shatters into aggregates – clusters of soil particles, also called peds. The shape, size and strength of aggregates show how porous the soil is. This controls water storage and drainage, air circulation to plant roots, and how deep plant roots can penetrate.