Why Screen Topsoil?
Topsoil is the top few inches of garden and farm soil. The topsoil layer, known to soil scientists as the "A" Horizon, may be anywhere from an inch to more than a foot deep. This soil contains a lot of organic (vegetative) matter, is where plant roots live and obtain nutrition and is generally darker in color than lower soil layers.
Topsoil is often moved around during construction projects. A common practice is to remove and stockpile the topsoil layer where construction is to take place. When construction is complete, the topsoil is replaced around the structure. This allows the establishment of attractive landscape plantings.
To remove large rocks, half-rotted branches and other chunky material, the topsoil is usually screened before it is returned to the site. Topsoil screeners can remove particles above a selected size, so that anything larger than, say, ½ inch, is removed. Screeners do not usually remove anything less than ¼ inch in size.
The two most commonly used types of topsoil screener are trommels and vibrating screeners.
A trommel is a large rotating drum, the sides of which are made of perforated screens. These screens can be changed to produce material of the desired size. Material is fed into the trommel, often by a conveyor belt system. The trommel rotates, and finished material falls through the holes in the screen onto another conveyor belt that transfers it to a container or simply deposits it in a pile. Reject material (called "overs") tumbles inside the rotating trommel until it falls out the open end of the drum.
There are two types of vibrating screeners: flat and rotating. A flat vibrating screener has a large perforated screen, which is usually at an angle. The screen vibrates rapidly and vigorously back and forth, driven by a motor. Material is dumped on top of the screen plate, and as the screen vibrates, material of the selected size falls through the perforations onto a conveyor belt that carries it to a container or pile. Reject material bounces down on top of the screen plate until it exits at the bottom, where it is collected.
A rotating vibratory screener has circular screen plates that rotate and vibrate. Unsorted material is dumped on top of the screen plates. Selected material falls through the perforations in the screen plates for collection below, while reject material is propelled outward and ejected to the sides, often through a chute or chutes to make collection easier.