What is soil?

The soil under our feet has been produced over time by the weathering of the parent rock; that is the rock underneath the layers of soil. The parent rock is broken down by physical weathering (eg water penetrating cracks in the rock then splitting it when the water freezes and expands) or chemical weathering (eg by the minerals in the rock absorbing water and breaking down much as pasta does when soaked in water). The rock is ultimately broken down into mineral particles. About 45% of soil is made up of these particles.

Some types of parent rock can have a greater influence on the soil. Chalk soils, for example, tend to be found on slopes and, because of the continuous erosion, are very shallow. Because of the pourous nature of chalk these tend to be very well draining soils, which makes them prone to drought. The high lime content tends to make the soil alkaline, although where the slopes are less severe, and the soil is a bit deeper, chalk soils can become acidic due to the leaching of the lime.

The other main ingredient in soil is organic matter. Some of this organic matter is living (eg plant roots and worms) but the key part for the make up of soil is the residue of dead plants or animals. This contains nutrients which plants can use and also helps control the structure of the soil. Organic matter makes up about 5% of the soil.

The other 50% of the soil is made up of differing proportions of air and water.