Topsoil is the uppermost layer of the earth's crust, and plants do best in soil that provides them with the fewest obstacles. The more cluttered the topsoil, the harder it is for a germinating seed to send down roots and push a new shoot up to the surface. Screening topsoil removes most of the debris and provides a loose, rich environment for plant growth and development.
Soil covers most of the earth's surface and is composed of layers beginning with topsoil, the layer in which all plants grow. Topsoil is darker than succeeding layers because it is rich in decayed plant matter and is the site of fungal, meteorological, bacterial and other biological processes that are constantly at work to break it down. Depending on its location, topsoil can range from several inches to many feet deep, and is an important commodity particularly in areas of high erosion or excavation activity. In its natural state, topsoil may contain significant amounts of debris that has not fully decomposed. Removing this debris through the screening process ensures a cleaner soil suitable for use in gardens and on lawns.
The Screening Process
Industrial topsoil screening is done using large machines that separate the finer soil particles from larger pieces of debris such as twigs, stones and large plant roots. The soil is fed into the machine by a front-end loader and falls onto a screen that vibrates to loosen the materials. The fine soil falls through the screen, and the debris is discarded outside the machine. The machines work in place and leave the screened soil where it is, and the debris is collected and disposed of. Other screening machines collect the processed topsoil and send it to distribution warehouses for bagging or to garden centers and nurseries where it is sold by the truckload.
You can process your own topsoil using a mesh screener, or soil sifter, that works on the same principle as an industiral screener. The soil is shoveled onto the screen and shaken, leaving the unwanted debris on the screen and dropping the fine topsoil into a receptacle or directly onto the ground. Soil sifters or screeners are available commercially or you can build your own using inexpensive lumber and wire mesh screening.
Screened topsoil is ideal for starting a new lawn or garden or improving the soil in any given area. Worked into existing soil, screened topsoil improves its texture, friability and water-retention properties. Adding compost, which is a blend of decomposed vegetation, to the screened topsoil adds valuable nutrients to deficient soil. New topsoil rich in vegetative matter should be worked into the existing base and not simply allowed to sit on top, as this delays the decomposition process that enriches old soil.