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Compost is the decomposed remnants of organic materials (those with plant and animal origins). Compost is used in gardening and agriculture, mixed in with the soil. It improves soil structure, increases the amount of organic matter, and provides nutrients.
Compost is a common name for humus, which is the result of the decomposition of organic matter. Decomposition is performed primarily by microbes, although larger creatures such as worms and ants contribute to the process. Decomposition occurs naturally in all but the most hostile environments, such as buried in landfills or in extremely arid deserts, which prevent the microbes and other decomposers from thriving.
Composting is the controlled decomposition of organic matter. Rather than allowing nature to take its slow course, a composter provides an optimal environment in which decomposers can thrive. To encourage the most active microbes, the compost pile needs the proper mix of the following ingredients:
Decomposition happens even in the absence of some of these ingredients, but not nearly as quickly and not nearly as pleasantly (for example, the plastic bag of vegetables in your refrigerator is decomposed by microbes, but the absence of air encourages anaerobic microbes that produce disagreeable odors).
All guidelines for building compost piles have the goal of creating the proper environment for a decomposing ecosystem. The ecosystem in a compost pile is a microcosm of larger ecosystems. The correct environment must be maintained for a healthy and vigorous community of decomposers. In addition to the decomposers that work directly on the organic content of the pile, compost piles provide habitat for those that prey upon direct decomposers. Their waste also becomes part of the process.
The most effective decomposers are bacteria and other microorganisms. Also important are fungi, molds, protozoa, and actinomycetes--which is something between a fungus and a mold and is often seen as white filaments in decomposing organic matter. At a macroscopic level, earthworms, ants, snails, slugs, millipedes, sow bugs, springtails, and others work on consuming and breaking down the organic matter. Centipedes and other predators feed upon these decomposers.
The goal in a compost pile is to provide a healthy environment--and nutrition--for the rapid decomposers, the bacteria.
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.
There are two primary methods of aerobic composting:
- Active (or hot) composting, which allows the most effective decomposing bacteria to thrive, kills most pathogens and seeds, and rapidly produces usable compost
- Passive (or cold) composting, which lets nature take its course in a more leisurely manner and leaves many pathogens and seeds dormant in the pile
Most commercial and industrial composting operations use active composting techniques. This ensures a higher quality product and produces results in the shortest time (SEE compost windrow turner).
Home composters use a range of techniques varying from extremely passive composting (throw everything in a pile in a corner and leave it alone for a year or two) to extremely active (monitoring the temperature, turning the pile regularly, and adjusting the ingredients over time) and combinations of both.
Some composters use mineral powders to absorb smells, although a well-maintained pile seldom has bad odors.
Microbes and heating the 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 1 m (3 ft) wide, 1 m (3 ft) tall, and as long as is practicable – the advantage to making the heap at least 1 m³ (1 yd³) 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 recognisable 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.
Some like to put special materials and activators into their compost. A light dusting of agricultural lime (not on the animal manure layers) can curb excessive acidity that can slow down the fermentation. Seaweed meal can provide a ready source of trace elements. Finely pulverised rock dust can also provide needed minerals, but watch out for rock dust that consists mostly of clay.
The animal manure part of compost source materials can be collected by composting toilets (in this case, human feces). However, such compost is usually not used as a fertilizer for plants that are directly edible (e.g., salad crops) but should instead be used on trees, bush fruits or else on the ornamental garden .
- Container composting
- German mound
- Leaf mold
- High fibre composting
- Worm compost
- Spent mushroom compost
- Sheet composting
- Windrow composting
- Compost Guide
- Home Composting
- Master Composter
- Cornell Composting--Science and Engineering
- How to Begin a Compost Pile
- The Humanure Handbook
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