Soil is made up of a mixture of different ingredients, both organic and inorganic, air, and water. Inorganic material is composed of rock particles and minerals. Organic material was once living animal and plant matter which has decayed. This usually dark nutrient-rich material is called humus. Humus is essential in the soil for good plant growth. It is rich in nutrients, helps retain water and air, and improves drainage. Soil also contains insects, earthworms, bacteria, and fungi.

Topsoil, or the top layer of soil, is the richest layer of soil. It contains more humus and minerals. Plants grow best in this layer because they easily receive the nutrients they need for good root development and top growth. Plants need at least sixteen elements for growth. They need only small amounts of some of the elements, but nitrogen, phosphorus, and potassium are needed in larger quantities.

Under the topsoil is a layer called subsoil. This layer is more reddish-orange in color and is harder to dig.

Most soils are made up of three ingredients -- sand, silt, and clay. The proportions vary in different soil types. Sandy soil is gritty, drains easily, and is easily plowed. This kind of soil usually needs added humus, water, and nutrients to improve plant growth. Sandy soil is called "light soil."