Topsoil is the first few inches of soil of the earth's surface. True topsoil, formed over time from the natural processes of weathering and decomposition, may be 8 inches or more in depth, though this depth is unlikely on developed lands. Mulch is a layer of organic material applied to the surface of a garden or other landscaping area. Topsoil and mulch are two separate elements that work together to aid in the growth of plant life.
Topsoil is made up of decomposed mineral and plant matter, air and water, sand and clay particles, and microscopic life forms. Without the life forms, from enzymes to worms, topsoil would fail to form. Mulch is any organic matter that breaks down easily. Mulch may be pine needles, straw, leaves and grass clippings. Other materials, such as newspaper and wood chips can also be used as mulch.
Topsoil is loose and easily shifted, making it vulnerable to movement by both human and natural forces. It is dark in color and holds together when wet, and drains easily. This characteristic allows plant roots to penetrate the topsoil and draw nutrients up through the roots. Topsoil is crumbly and soft to the touch when dry. Mulch is applied to the surface of soil to aid in retaining moisture without obstructing airflow into the soil. Mulch decomposes, adding its own nutrients to the soil.
Topsoil is a part of garden soil. It's the layer in which seeds germinate and seedlings form their first essential roots. Without a healthy layer of topsoil, plants may fail to thrive. The layers of soil beneath the top soil may become compacted and fail to attract microscopic life. This results in "dead" soil. Mulch acts as a protective layer, guarding the topsoil against erosive winds and other disturbances. Mulch also preserves needed moisture in the topsoil and layers of garden soil. This aids in plant growth as well as water conservation.
The top layer of garden soil isn't automatically topsoil. In desert regions, for example, little if any topsoil is available due to lack of substantial plant life and an abundance of rock and sand. Topsoil may also be removed during housing and commercial developments. In gardening and landscaping projects, bagged topsoil is used and incorporated into the existing top layer of soil to encourage new topsoil formation.
Though true topsoil takes many years to form, adding organic compost to garden soil and consistently using layers of mulch may result in a top layer of soil similar in content and value to that of naturally occurring topsoil. To form topsoil, the soil must have access to mineral and plant materials, water, air through aeration via elemental and animal intervention, microscopic life forms and living plant life occupying the surface.