The Benefits of Compost

Whatever type of soil you have - from rich loam to sand or clay - adding compost is the single most important thing you can do to improve it and make your plants healthier. In most places, compost can be ordered from and delivered by municipal suppliers for a price, but you can also make your own compost for free, and live more sustainably in the process. By having a composting system you benefit the environment by Compost also benefits your soil and plants in a number of ways.
  1. First, it is a source of slow-release nitrogen, the primary plant nutrient. Synthetic fertilizer, which becomes immediately soluble in water, is much more likely to contaminate runoff and and needs to be reapplied at regular intervals. One application of compost will provide small doses of nitrogen throughout a growing season, and even over several years.
  2. Besides nitrogen, compost is a source of other nutrients essential to plant health (such as phosphorous, potassium, calcium, sulfur, and other micronutrients, depending on the starting material)
  3. It will increase soil water retention. This is especially important in sandy soil which does not hold water well.
  4. It will greatly increase soil nutrient retention (CEC - cation exchange capacity). Compost has the ability to "hold on" to nutrient ions in a form that is immediately usable by plants when they are needed. This is especially important in sandy soil where nutrients are easily leeched out. Increased CEC also results in the soil's ability to resist fluctuations in pH.
  5. In clay soil, compost separates the soil particles and keeps them from sticking together. This increases drainage, add air to the soil, and helps prevent compaction.
  6. As compost breaks down, the humic compounds and microbial byproducts (slimes, gels, gums) help bind the soil into distinct aggregates - giving the soil a crumbly texture (making it "friable"), ideal for plant growth, and making it less prone to erosion.
  7. Compost is rich in beneficial soil microbes. These continue to be active in breaking down the compost material and compete with pathogenic and parasitic soil microbes, helping to maintain plant health.

What is compost & how does it work?

Compost is the product of the breakdown of organic (ie. carbon-based) material in the presence of bacteria. There are a number of complex chemical reactions that happen in the breakdown process, and these will vary with the type of material and microbes present. The basic reactions can be described and understood, though, without too much dificulty.

Source material: known as feedstock, can consist of many materials such as vegetable and food waste, grass clippings, coffee grounds, straw, sawdust, manure, and leaf debris. All of these materials are primarily carbon-based (containing about 50-60% carbon) and also contain some nitrogen (around 1-2% on average). Some materials contain more nitrogen than others. Materials with less nitrogen are said to have a high carbon-to-nitrogen ratio (C:N), and materials with more nitrogen are said to have a low C:N. The importance of this will become apparent.

Microbes, many of which are already present on the feedstock, are able to extract energy from the feedstock by decomposing it. The carbon (along with other molecules like oxygen, hydrogen, nitrogen, and phosphorus) in organic matter is strung together in chains. Sugars, starches, proteins, fats, lignins, and cellulose are all carbon-based chains. Every time one of the carbon-links is broken, energy is released, which sustains the life of the bacteria. Some of the energy is released in the form of heat, causing the compost pile to become warm. Some of the energy is also used to build its body structure. This is done by forming nitrogen (along with C, O, and H) into amino acids which are used to build the structural proteins. Obviously, a certain amount of carbon and nitrogen is needed to make the process work.

The last necessary ingredients to make the process work are oxygen and water. If oxygen is not present in sufficient quantity, then the bacteria can't extract energy and will die. In such cases, other bacteria take over using anaerobic processes, but these are much slower and produce methane as a byproduct - not as desirable for our purposes. Bacteria also need water to function, but too much water will displace air and oxygen and result in a bad-smelling pile. A well-balanced compost mix will produce very little odor. The pile should, therefore, be moist, but not wet.

Some materials - like sugars, starches, and proteins - are easily digested, and quickly broken down. Others like cellulose, lignins, and fats are not easily broken down, and require lots of energy and time to digest. As the bacteria die, the nitrogen stored in their bodies is used by the next generation of bacteria, so that nitrogen is generally conserved while the carbon is broken down and much of it released as carbon dioxide.

One of the keys to efficient composting is having the right balance of carbon and nitrogen. If there is too much nitrogen at the beginning, then the bacteria will convert the excess to ammonia gas which escapes to the atmosphere. If there is too little nitrogen, a more common problem, then the bacteria will try to extract nitrogen wherever they can find it - which means stealing from the surrounding soil if there is any. (For this reason, it is not recommended to add uncomposted organic matter to your soil. It will eventually break down and provide nutrients, but in the short term it will deprive your soil and plants of nitrogen.) If there is still insufficient nitrogen, then the breakdown process will proceed more slowly, using what nitrogen is available. Researchers make a point of monitoring the ratio of carbon to nitrogen and usually strive for a C:N of between 10 and 30. For practical purposes, this means that we should include some low-nitrogen material in our compost and some high-nitorgen material as well. Low-nitrogen materials are generally drier than high nitrogen materials, so a proper balance will also result in a mix that is somewhat moist but not wet.

Low-nitrogen materials (high C:N, aka "dry" or "brown" materials) include

High-nitrogen materials (low C:N, aka "wet" or "green" materials) include

Exact C:N for different types of materials can be found here or here.
More about the biology & chemistry of compost can be found here.
The biology of the soil-food-web is described in detail here.

Composting Systems

1. The Compost Pile or Heap

This system is especially used by large-scale composters with large quantities of different materials on hand. The materials are mixed in a way to give a good balance of C:N, and then left to sit in a pile.

Turning the pile
The pile needs to be turned or remixed from time-to-time to give it sufficient oxygen and eliminate excess moisture. Generally, more frequent turning will result in faster breakdown (several months with regular turning). If the pile is NEVER turned it will still breakdown, but the breakdown time will be higher (1 to 2 years). More detailed information on turning times can be found here.

Size matters
the ideally sized pile will be between 3x3x3 ft and 6x6x6 ft.

Take the temperature
Temperature reading can tell you how well your compost is breaking down. High temperatures indicated a high level of activity and are desirable for their ability to kill pathogens and weed seeds, but temps over 160 deg F will also kill the good bacteria. 150 deg F is an ideal temperature to aim for, but lower temps are also acceptable.

2. Compost Bins

Obviously more material can be stored in a square bin or pit than in an open pile, and several bins can be placed side-to-side to save more space. Otherwise, the process is the same as a compost pile described above, including turning. Bins can be easily and cheaply built, while more fashionable bins are also available.

Building the Bin
Easy-to-build methods include using wire mesh or a snow fence. Sink 1 or more wooden stakes or poles in the ground and wrap the mesh or fence to form a cylinder or a cube, depending on your preference. A sturdier unit of multiple bins, which will facilitate turning, can be built of wooden posts and wire mesh. This takes more effort and money to build, but is still relatively easy and inexpensive. Specifications and details on building different types of bins can be found here.

Another method of composting with bins that works well for individual households is to build the mix in alternating "wet" and "dry" layers. Typically, this means that vegetable and food scraps and grass clippings are added to the pile until they form a solid layer, then a layer of leaves, hay, newspaper or other low-nitrogen material is added on top of that. This creates the right balance of nitrogen and carbon and moisture in the bin. Some people also add a 3rd layer of old manure or throw in a handful of fertilizer to give an extra boost of nitrogen. This is not really necessary unless you have an abundance of dry materials at hand.

3. Pits and Trenches

A pit or trench can also substitute for a bin. When the trench is filled with feedstock it can be covered with 6" of soil, and will decompose by anaerobic means after about 1 year. After that the compost can be dug up or you can plant right over the trench. Worms will also burrow in and help the process. Pits can be turned or left unturned for a longer decay.

4. Barrel composters / Compost Tumblers

Compost tumblers are a great solution for the home composter. Both wet and dry materials should be added in the proper ratios as described above, but turning the mix and keeping it oxygenated is made vastly simpler by the spinning design. Every time you add some new feedstock to the barrel, you can give it a quick spin to mix the contents and provide fresh oxygen. A load of compost should be ready in 2-3 months. I always recommend having 2 or 3 barrels so that once one barrel becomes full, it can be given time to decompose fully as the next barrel is being loaded up. It may also be advantageous to keep a bin to store large quantities of fall leaves, if you live in a heavily treed area. These can be added to the barrel throughout the year to balance out the wetter ingredients, and should be used up or mostly decomposed by the time fall rolls around again.

5. Worm Composting

Like bacteria, worms are excellent at turning organic waste into high-quality organic fertilizer with all the same benefits as compost. Worms also require similar conditions as bacteria - a balance of high- and low-nitrogen foodstock, moist but not wet conditions, and a supply of oxygen. If you use a compost trench or pit (described above) or if your compost pile sits on the ground, then it is likely that earthworms are already at work in helping decompose your wastes. If you don't have space for an outdoor compost pile, or you just don't want to walk that far, you can make a worm compost bin which you can keep right in your house - or outside if you perfer. Here are several links to great sites which will teach you all about worm composting.

Related Concepts

Composting Greenhouse

Consider that two of the products of composting - heat and carbon-dioxide - are needed by plants, especially during cooler times of the year. This has led to the building of greenhouses which are heated by compost and atmospherically enhanced by carbon-dioxide. Read more about the concept here.

Compost Tea

Soaking compost in water for a few hours to several weeks will cause the microbes in the compost to be transfered to the water and multiply. Billions (actually up to 1011) of microbes can be found in each drop of compost tea. Varying the "brewing" time will favor either fungi, bacteria, or protozoa - all of which have their uses. Using a bubbler to add air will also favor aerobic bacteria over the anaerobic type. Unchlorinated water is always used, as tap water will kill the microbes, and starting with a sterile container and pumps is a must.

When sprayed onto the soil, these microbes will colonize the soil and compete for resources with the pathogenic bacteria and fungi, thereby reducing their numbers and influence. The microbes also have anti-biotic effects, and also serve the important function of making soil nutrients and water more available to plants. Compost tea is sometimes sprayed on the leaves and stems of plants in an attempt to reduce disease, but microbes left on the plant surface are especially succeptable to being washed off by rain and to death from sunlight - especially UV radiation. All-in-all the research is incomplete, and seems to suggest that compost tea is effective to some degree (sometimes very effective) with specific pathogens on specific crops, and not effective at all in others. We support the use of compost tea as a means of strengthening and diversifying the soil-food-web and encourage you to experiment, but warn that it is not a magic bullet.

Soil Food Web
More on Compost Tea Research

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