Facts about rocks and soils

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Soil, the stuff plants dig their roots into for stability as well as nutrition, is the end product of many different processes, biological as well as physical.

A new landscape of raw stone begins to develop soil as the rock weathers, fragmenting into smaller and smaller pieces, helped along by the presence of lichens and other tough plants. Soil may include minerals transported by glaciers or rivers as well as decayed plant material.

Soil particle size

Gravel is any particle over 0.2 mm (0.008 inches), including grit, pebbles and small rocks. Many plants grow well in gravelly soils because of the excellent drainage if there is enough silt and humus to hold water. Sand particles are 0.02 mm (0.0008 inches) to 0.2 mm (0.008 inches) in size and a sandy soil has less than 20 per cent of silt and clay. Sandy soils drain well but are poor in nutrients. Silt particles are even smaller and, if looked at with a magnifying glass, seem like tiny pieces of rock. Clay is made up of tiny flat particles, the remains of certain types of minerals including feldspar. A clay-like soil contains more than 30 per cent clay and holds water and nutrients well, but may drain slowly.

Vegetation and soil

On meadows and fields, the dense, fibrous roots of the grasses die and decay each year, contributing a regular bonus of dark organic matter to the soil that helps sandy soils hold water and clay-like soils drain more quickly. In wooded areas, tree roots are more diffuse and the soils lighter. Evergreen, coniferous forests have shallow, acidic soils because of the shallow root systems and acid needles of the trees.

Rock and soil type

The original rock that weathered to produce the sand, silt or clay is called the parent material. Limestone weathers to a soil high in lime and calcium carbonate, often giving it an alkaline pH. Shale is a sedimentary rock, formed from mud that hardened under pressure into stone. Shale soils tend to be shallow, acidic and low in nutrients.

Climate and soil

In a rainy climate, soluble substances such as lime and nitrogen compounds leach downwards out of the reach of plant roots. Climates with widely varying temperatures encourage the development of an open structure easy for roots to penetrate. Freezing and thawing speeds up the breakdown of rocks into their mineral components.