MISSISSIPPI STATE -- One gardener's trash can become his treasure in a matter of weeks.

The piles of weeds, clippings and leaves often discarded can instead be naturally recycled. A well-tended compost pile quickly changes mounds of organic matter into rich soil additives.

Dr. David Nagel, Mississippi State University extension horticulturist, said compost is the result of microorganisms processing organic waste.

"Compost is unrecognizable organic matter," Nagel said. "The freshly decomposed matter can be added to soil where it is a great slow-release nutrient source."

Composting is an aerobic decomposition process. Under proper conditions, compost piles do not smell and can reach 150 degrees inside from the heat given off by the microorganisms.

Nagel said there are four elements to a working compost pile. First, don't add fat of any sort, which includes meat scraps, egg yolks and vegetable oil.

"The fungi and microorganisms that decompose the pile cannot handle fat," Nagel said. In addition, the fat can smell and attract insect and animal pests.

Layer compost piles with soil. Nagel recommended alternating three to eight inches of plant material with two to three inches of soil. The soil adds the needed microorganisms to the pile. In place of soil, starter microorganisms can be bought and added to piles, but using soil is free.

The third key to a successful compost pile is to keep the organic matter small.

"Anything bigger around that your thumb is too big," Nagel said. "Keep the clippings small and run over leaves with the mower so they will break down faster."

Lastly, keep compost piles moist and moved. The pile should be damp, neither bone dry nor soggy. Turn the pile so it can receive the oxygen needed to decompose. The pile preferably should be turned every eight weeks, but at least every three months, Nagel said.

Another important element of thriving compost piles is proper nitrogen content, which microorganisms need to complete their life cycles. Grass clippings are an excellent source of nitrogen. Without grass clippings, add 1/2 cup of ammonium nitrate to each eight bushels of leaves to meet the nitrogen needs. Cottonseed meal or blood meal also works well.

There is a simple test of when a compost pile is done.

"Stick your hand into the pile," Nagel said. "If the center is cool, it's either time to use or turn the compost. If the middle of the pile is totally broken down, it's time to use it."

A properly maintained compost pile containing leaves, grass clippings and garden trimmings, turned once, can be composted in 10 to 12 weeks. A pile containing small limbs and dead leaves may take six months.

Norman Winter, extension horticulturist at the Central Mississippi Research and Extension Center, said compost can be used as a mulch or built into the soil.

"When compost is mixed into tight clay soils, it loosens them, lets oxygen get to the roots and allows water penetration," Winter said. "In sandy soil, it helps hold in water and nutrients."

If used as a mulch, two to three inches added to the top of a garden helps keep the soil cool, holds moisture and acts as a weed deterrent.

"Don't ever bag up leaves and straw and throw them away," Winter said. "You can never have too much compost."