About Compost and Composting
Compost is the decomposed remnants of organic materials - from either plants or animals like us. Used in both small gardening projects and in big agricultural centers, mixed in with the soil, compost is both porous and absorbent, holding moisture and soluble minerals well, providing the support and nutrients that most plants love. Compost is an improvement over most regular soil structure, increases the amount of organic matter, and provides more nutrients to growing plants than untreated soil.
Compost is a common name for humus, which is the result of the decomposition of organic matter. Compost is used in landscaping, horticulture and agriculture as a soil conditioner and fertilizer. It is also useful for erosion control, land and stream reclamation, wetland construction, and as landfill cover Decomposition is performed primarily by bacteria, although larger creatures such as worms and ants can contribute to the process. Decomposition of organic matter into compost occurs naturally in all but the most hostile environments, such as when the materials are buried in landfills or in extremely arid deserts - both of which prevent the microbes and other decomposer bacteria from thriving.
Composting is the controlled decomposition of organic matter. Instead of just sitting back and waiting for nature to take its slow course, a composter provides an optimal environment in which decomposer bacteria can thrive. To encourage the most active microbes, the compost pile needs the proper mix of the following ingredients:
While decomposition happens even in the absence of some of these ingredients, but not nearly as quickly and not nearly as pleasantly. Like, for example, when you leave a plastic bag of vegetables in your refrigerator, the contents are decomposed by microbes but the fact that there isn't any air in the bag encourages anaerobic microbes - and they are what produce the worst odors.
The goal in a compost pile is to provide a healthy environment, complete with nutrition, for the rapidly decomposing bacteria that the composter wants.
The most rapid composting occurs with the ideal ratio - by dry chemical weight - of carbon to nitrogen, from 25-to-1 to 30-to-1. In other words, the ingredients placed in the pile should contain 30 times as much carbon as nitrogen. For example, grass clippings average about 19-to-1 and dry autumn leaves average about 55-to-1. Mixing equal parts by volume approximates the ideal range. Commercial-grade composting operations pay strict attention to this ratio. For backyard composters, however, the charts of carbon and nitrogen ratios in various ingredients and the calculations required to get the ideal mixture can be intimidating, so many rules of thumb exist to guide composters in approximating this mixture.
High-carbon sources provide the cellulose needed by the composting bacteria for conversion to sugars and heat.
High-nitrogen sources provide the most concentrated protein, which allow the compost bacteria to thrive.
Some ingredients with higher carbon content:
- Dry, straw-type material, such as cereal straws
- Autumn leaves
- Sawdust and wood chips
- Some paper and cardboard (such as corrugated cardboard or newsprint with soy-based inks)
Some ingredients with higher nitrogen content:
- Green plant material (fresh or wilted,) such as crop residues, hay, grass clippings, weeds
- Animal manures (vegetarian, not carnivore)
- Fruit and vegetable trimmings
Poultry manure provides lots of nitrogen but little carbon. Horse manure provides both. Sheep and cattle manure don't drive the compost heap to as high a temperature as poultry or horse manure, so the heap takes longer to produce the finished product.
In an attempt to judge the proper mix of materials, different rules of thumb are available. Some prefer to add one basket full of nitrogen source followed by one basket of carbon source. Mixing the materials as they are added increases the rate of decomposition, but some people prefer to place the materials in alternating layers, approximately 15 cm (6 inches) thick, to help estimate the quantities. Keeping carbon and nitrogen sources separated in the pile can slow down the process but decomposition will occur in any event.
The Compost Pile
An effective compost pile is kept about as damp as a well wrung-out sponge. This provides the moisture that all life needs to survive; in a compost pile, it provides an environment in which microbes can begin to do their work. Bacteria and other microorganisms fall into a variety of groups in terms of what their ideal temperature is and how much heat they generate as they do their work. Mesophilic bacteria enjoy midrange temperatures, from about 20 to 40 °C (70 to 110 °F). As they decompose the organic matter, they generate heat, and the inner part of a compost pile heats up the most.
The heap should be about 3 feet wide, 3 ft tall, and as long as is practicable – the advantage to making the heap at least 1 cubic yard in mass is that it provides suitable insulating mass to allow a good heat build-up as the material decays. The ideal temperature range hovers around 60 °C (140 °F), which kills most pathogens and weed seeds and also provides a suitable environment for thermophilic (heat-loving) bacteria, which are the fastest acting decomposers. The centre of the heap should get quite warm, possibly hot enough to burn a bare hand. If this fails to happen, common reasons include the following:
- The heap is too wet, thus excluding the oxygen required by the compost bacteria
- The heap is too dry, so that the bacteria do not have the moisture needed to survive and reproduce
- There is insufficient protein (nitrogen rich material)
The solution is to add material, if necessary, and/or to turn the pile to aerate it.
Depending on how quickly the compost is required, the heap can be turned one or more times to bring the outer layers to the inside of the heap and vice versa, as well as to aerate the mixture. Adding water at this time keeps the pile as damp as a wrung-out sponge. One guideline is to turn the pile when the high temperature has begun to drop, indicating that the food source for the fastest-acting bacteria (in the center of the pile) has been largely consumed. After the temperature stops rising after the pile has been turned, there is no further advantage in turning the pile. When all the material has become barely recognizable from the original ingredients, turning into dark brown or nearly black crumbly matter, it's ready to use. Some practitioners like to leave the compost to mature further for up to a year as this seems to make the benefits of compost last longer.