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Dirt is actually not the correct term to describe that complex growing media that plants need to well grow.  Soil is composed of bacteria and microorganisms that help decompose organic matter into nutrients that enrich the soil. As gardeners will tell some soils are better than others for growing various different types of plants. Depending on your conditions your garden soil might need some help.  

Clay soil is known as a heavy soil.  When wet its particles clump together making it impervious to both water and plants. When clay soil becomes dry it stays hard and cracks. Consequently clay soil delights in air and the soil needs to be loosened. Most gardeners will add peat moss or humus to their clay soil in order to prevent the particles from sticking together and will allow water to drain through it. Sandy soils are composed of the largest particles found in soils.

Sandy soil has such excellent drainage that often its problem is an inability to retain moisture for good plant growth. Again, humus or peat moss can remedy this situation. A silt soil is generally a good growing soil since its particles are in sizes between those of clay and sand. A loam soil is usually excellent for plants. It is open, retains moisture well and has a
good structure. Loam is the gardener’s joy. Other types of growing soil can be purchased online or at your local garden center.  

Humus is basically composted topsoil. It is rich with decomposed organic matter and is sold much like regular topsoil. Generally humus can be purchased as peat humus or pine bark humus. The peat humus variety is mostly comprised of decomposed reed-sedge. Pine bark humus comes from decomposed pine bark. Humus is generally used to build up the soil structure for containers, lawns and flowerbeds.

Potting soil is sold for ready use. Also known as houseplant soil it is perfect for indoor plants and for starting plants from seed. Potting soil is a compound of both organic and inorganic materials with such ingredients as compost, peat moss, sand, and vermiculite. 

Top soil is rich with a large quantity of organic composition. Most gardeners turn to packaged topsoil from one time to another to improve the quality of their garden soil. It can be purchased sterile or in its natural form containing microorganisms. Soil less mixes are sold for use in container or tub gardens. These have been sterilized so there is no likelihood of weeds. This growing medium retains water and nutrients extremely well.