How Much Does a Compost Pile Shrink?

Composting is an easy way to be kind to the environment.
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Organic materials naturally decompose and turn into compost. Homeowners can take advantage of that fact to improve their gardens. Composting materials such as plant trimmings, shredded paper, grass clippings and kitchen scraps minimizes the amount of landfill-bound waste a household produces. Composting reduces the volume of those waste materials, and in the process produces a rich garden additive that's useful throughout the home landscape.

Shrinkage Process

Compost materials shrink as enzymes and insects digest the organic matter; heat generated by the composting process breaks down the materials' cellular structure (see References 1).

Types of Materials

The U.S. Department of Agriculture composting tip sheet recommends adding almost any available organic material to your compost pile, including leaves, woody stems and brush, kitchen vegetable scraps and egg shells, grass clippings and shredded paper. Livestock manure is safe, but dog and cat litter is not. Don't add diseased plant material, either. For best composting results, you need a mix of materials high in nitrogen, such as grass clippings or livestock manure, and materials high in carbon, such as dried leaves and twigs. Small, soft materials decompose faster than large or woody materials, and the outside temperature affects the composting rate too. (See References 2, pages 1-2)

The materials you use in your pile will affect the amount of shrinkage. For example, in an old publication on composting, William Connellan advised that a compost heap of manure and sod would shrink by about 1/3, while one of manure and "muck" will shrink by 20 percent, and one of muck will shrink by only 10 percent (see References 4, pages 57-60). A typical home compost pile composed of larger pieces of a greater variety of materials will shrink more than these more homogeneous piles.

Time to Breakdown

Several easy steps speed up the composting process and the pile shrinkage. Turning the pile frequently, as often as every two days, adds necessary oxygen (see References 2, page 3). Shredding compost materials before adding them to the pile helps them decompose quickly. If you don't have a shredder, run over your materials with a lawn mower to chop them. Keep the pile moist but not soggy for the most efficient insect and microbe activity. (See References 3)

Ready to Use

Compost that's ready to use is dark, with a crumbly texture and an earthy smell (see References 1). According to the Louisiana Department of Environmental Quality, your home compost pile will shrink to approximately half its original volume by the time the decomposition process is finished (see References 3).

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