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Soils : Soil composition

Rock material | Organic material

Soil is composed of a complex variety of materials, some of which come from the actions of weather on rock, and some from living sources.

Good fertile soil is necessary for quick growth

Good fertile soil is necessary for quick growth

Rock material

The material that comes from rock includes stones, sand, silt and clay. Stones and sand are fairly obvious and easily understood components; silt and clay less so. Silt is made of very fine particles and compares in texture to the finest powder.

Clay is composed of very fine particles too, even finer that silt, and it is the result of the chemical decay of rocks as much as their physical breakdown. Clay is chemically active, which means that it can bind strongly to itself, and to other soil constituents. It bonds sand and silt together into tiny soil particles. The presence of a lot of clay causes too much binding, and the soil becomes a hard, lumpy mass.

Organic material

The material from the decay of once-living organisms – both plant and animal – arises when plants and animals die and the remains are food for bacteria, fungi and algae in the soil. These, in turn, die and become food for other tiny organisms.

The cycle continues until the original dead plant tissue is broken down into tough, insoluble materials such as waxes, resins and gums. Mixed closely together, those materials, dark-brown in colour, are collectively called ‘humus’.

Humus, like clay, is chemically active and bonds itself to other soil constituents almost like an adhesive. Apart from stones, sand, silt, clay, dead plant and animal material, living organisms and humus, soil also contains air and water, and plant nutrients released by the decay of rock material and organic material.


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