GardenLine | Yard & Garden | Does Your Compost Smell?

Bonnie Willie

With growing concerns over the fate of our fragile and ever- changing environment, more and more people are using composting as a means of reducing the amount of waste that enters our landfills. This not only reduces the amount of waste disposed of by households but also returns nutrients to the soil while improving the water holding capacity in sandy soils and improving drainage in hard, compacted, clay textured soils. But this process is not quite a simple as throwing this and that into a pile, and ignoring the material for a few months and then dumping it in flowerbeds and gardens in the hopes that the material will enhance plant growth.

Some consideration should be given as to what materials are put into the compost, but more importantly, the quantity of each type of material in relation to other compost components require more than just a passing thought. Microorganisms that break down the organic matter in compost, require the correct proportion of the elements carbon and nitrogen in order to live and function properly.

The carbon is required for energy and the nitrogen for forming protein. The correct proportion of carbon to nitrogen is referred to as the C/N ratio. The ideal C/N ratio for compostable material should be approximately 25-30:1. If the C/N ratio is too high in carbon (ie. greater than 30:1), the decomposition of organic matter will be slow and the nitrogen content depleted. If the C/N ratio is too low (less than 25:1) the nitrogen will volatilize (escape) into the air and leach into the soil and water, causing an unpleasant odour while creating a source of air pollution.

In general, materials that are high in carbon are usually brown or yellow in colour, dry and bulky. Materials higher in nitrogen are often green, moist, and tend to be more "sloppy" in texture.

Ingredients high in carbon are best mixed, or placed in alternating layers with materials high in nitrogen. Because sawdust, paper and pine needles are relatively high in carbon, their use should be limited to small quantities, unless they are mixed with large amounts of alfalfa hay or other materials relatively high in nitrogen.

If the proper ratio of carbon to nitrogen is established, the compost pile should yield compost with a final C/N ratio of about 10:1.

This is a list of some common compostable materials and their C/N ratios. This information, along with other helpful hints can be found in Rodale's All-New Encyclopedia of Organic Gardening.

References:

Rodale's All-New Encyclopedia of Organic Gardening, 1992, Edited by Fern Marshall Bradley and Barbara W. Ellis.

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Sustainable horticultural information, offered free of charge to the public with the support of the University of Saskatchewan Extension Division, the Department of Plant Sciences and the Provincial Government.