University of Maryland Extension

Compost

(PDF) Backyard Composting 

compostWhat is compost? 

Compost is a dark, crumbly, earthy-smelling material produced by the natural decomposition of leaves, grass clippings, and many other organic materials.

Compost “happens” without human intervention, since microbes and soil animals are on the job 24 hours a day, decomposing plant and animal remains. Composting allows you to expedite this natural process to produce a regular supply of compost (a.k.a. “black gold”) for your landscape. Finished compost contains an assortment of major and minor elements and other nutrients necessary for plant growth and also improves soil structure.

Why should I compost?

There are some very compelling reasons to compost:    

  • It reduces the amount of material going to landfills. Municipal waste is composed of 13% yard wastes, 12% food waste, and 34% paper, most of which can be composted (U.S. EPA, Office of Solid Waste 2005).
  • Compost is a valuable and free soil amendment that saves gardeners the money used to buy alternatives, such as peat moss, fertilizer, or vermiculite. Compost improves soil tilth, aeration, and water-holding capacity and contains a wide range of plant nutrients. All soils benefit from regular additions of compost.
  • Compost suppresses some soil-borne diseases. Populations of some microbes in compost may out-compete pathogens for food and habitat while others attack or repel plant pathogens.
  • It’s good for the environment, fun, educational, and an activity the whole family can help with.

How is Compost Made?

Bacteria, fungi, and other microbes are the key players in composting. These organisms “feed” on organic matter and use the carbon and nitrogen it contains to grow and reproduce. The heat generated by your compost pile is a result of microbial activity. Microbes are active in small numbers at temperatures just above freezing and are most efficient at 130º–140º F. They are assisted by many larger organisms like earthworms, slugs, snails, millipedes, sow bugs, and various insect larvae that feed on plant and animal matter in soil. These same organisms are responsible for the decay of both forest floor litter and the corn stubble in a farmer’s field.

Composting microbes use carbon for energy and nitrogen for growth (protein synthesis). When you mix various forms of organic material in your compost bin, it is important to achieve a proper balance of carbon to nitrogen (C:N ratio). The proportion can vary; the microbes will function well at C:N ratios from 25:1 to 40:1. A mixture of materials containing 30 parts of carbon to 1 part of nitrogen is considered ideal. Most organic materials do not fit the 30:1 ratio exactly, so different materials are mixed together. With the proper mix, microbes and other digesters will quickly start working to make compost for you. Finished compost has a C/N ratio of 20 - 25:1.

Make a Compost Bin

Instructions for a Wire Compost Bin (PDF)wire composting bin

What can I put in my compost?

Organic matter for composting comes in two broad forms:

  • Green materials, such as fresh grass clippings and weeds, kitchen waste*, and manure. These are high in nitrogen and moisture, but low in carbon. They are also high in readily digested sugars and starches.
  • Brown materials, including mature woody materials such as fallen leaves, straw, wood chips, and sawdust. These are high in carbon (mostly cellulose and lignin), low in moisture, and slow to break down.

*Be aware of local ordinances! For example, in Baltimore County you cannot compost food wastes above the ground (except in worm bins).

A blending of equal parts (by volume or weight) of green and brown materials gives the decomposers an ideal diet.

Composting Tips

The composting process is a mixture of science and art to which you can add techniques that work for you. No two compost piles are exactly the same and there is no single “correct” set of composting techniques that gardeners should follow. Select your methods based on your circumstances. That said, some tips may help you compost more effectively: 

  • Mix materials thoroughly; it’s usually not helpful to layer materials.
  • To speed up the process add an extra nitrogen (e.g., cottonseed meal, blood meal) source at each turning.
  • Keep your compost pile moist (but not soggy) for efficient decomposition. Excess moisture causes anaerobic decomposition and offensive odors. During dry weather it may be necessary to add water at weekly intervals.
  • Do not add branches and other woody materials unless they are chipped into small pieces.
  • In dry weather, cover the pile to prevent excess moisture loss and to aid decomposition. A tarp or other cover also protects the pile from becoming too wet during periods of heavy rainfall and helps prevent nutrient leaching.
  • Turn or mix the pile regularly. If fall-gathered leaves make up the bulk of the pile, turn the pile in mid-November before freezing occurs. Do not turn the pile in winter because this allows too much heat to escape and slows decomposition.
  • Whenever kitchen scraps are collected or composted it is helpful to mix in a dry, high-carbon material, such as leaves, sawdust, or shredded paper.

Frequently Asked Questions about Composting

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