Soil normally contains three types of particles; sand, silt and clay. Sand particles are quite large, between 0,5mm and 0,05mm in diameter. If there is too much sand in the soil water will drain through the soil too quickly. This will mean that plant roots will not have enough time to absorb the water.
Clay is made of much smaller particles, below 0,002mm in diameter. Too much clay in the soil will not allow water to drain through it very well. This means that plant roots could lack oxygen. Silt particles are between 0,05mm and 0,002mm in diameter.
Soil needs to be a mixture of all three types of particles. It must also contain humus. Humus is what is left of the organic material, such as dead leaves, which is broken down by decomposer organisms. When humus is mixed with clay, soil crumbs are formed. Soil crumbs are rich in minerals and they improve the drainage of the soil. Soil crumbs also allow more air to circulate through the soil.
A soil which contains a mixture of sand, silt, clay and humus is called loam. Loam is a fertile soil, providing plants roots with water, air and minerals.
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