Sand has large, loose grains, few nutrients, and does not hold water well which washes out nutrients so that it does not support plant growth. This type of soil does not have well developed horizons.
Loam is a mix of sand, silt, and clay. Most potting soil is loamy as it is rich in humus and holds water better than sand so it remains wet longer during dry periods. Loamy soils have a thick topsoil horizon.
Clay has closely pack particles which means there is very little air space. The particles are extremely fine and powdery. It is rich in nutrients and holds lots of water, but doesn't soak it in quickly.
Humus is the remains of decayed plants and animals, which contains nutrients that plants need to grow. It also helps the soil to retain moisture.
When soil forms, it develops layers, called soil horizons. Scientists use a letter to identify each soil horizon. A vertical section of soil that shows the layers is called a soil profile.
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