Last update: April 11, 2017
In some ways, building compost is the most important thing we do at Common Ground Garden (CGG). Plants need food just as we do, and that food has to come from somewhere. Conventional agriculture feeds plants using synthesized chemical nutrients, while organic agriculture may use green manure, livestock manure, and may import compost from various sources. On the other hand, at Common Ground Garden we use the GROW BIOINTENSIVE® (GB) method of growing vegetables, soft fruits, grains and other crops and aim to grow all the compost and nutrients we need by growing a specific combination of crops and recycling them on a “closed-system” basis as much as possible.
This post builds on our previous post about cold composting, and is the first in our new series: The 8 Principles of GB Mini-Farming. Stay tuned for more! Or check out a brief summary of the principles here.
Plants need a variety of nutrients to build their bodies, protect them from diseases and pests, and to flower and fruit. Some of these nutrients come from the atmosphere, such as carbon (C) and oxygen (O), while others come from the soil, like nitrogen (N), phosphorus (P), and potassium (K). When plant material like leafy greens or tomatoes is harvested from the garden, the soil loses some amount of N, P, K, calcium, iron, …and so on. If these nutrients are not replaced by soil amendments and food is continually harvested, then the soil will rapidly become less productive. At CGG, we replace as many of these nutrients as we can by adding crop ‘residues’ (the leftover plant material and kitchen waste from a crop) to our compost piles, which then become amendment for our soil. Of course, not all of the lost nutrients can simply be replaced since we eat a portion of the plant matter we harvest. To maintain healthy levels of certain trace nutrients we must add a carefully calibrated amendment to the soil determined by yearly soil tests with the help of soil expert, John Beeby.
Organic agriculture returns biomass to the soil in the form of compost. This returns a multitude of nutrients and helps to stimulate a healthy ecology in the soil. Organic matter, so important for soil health, is built up over time with the addition of compost.
The combination of genetic modification and petrochemical technology fuels our modern conventional agriculture system. Crops get the bare minimum they need to grow to maturity in the form of synthetic fertilizer, while competition from weeds and attacks from insects are reduced by chemical herbicides and pesticides. The fertilizers supply several necessary plant nutrients but leave out some crucial ones. Many trace elements become depleted as does the diversity of soil micro-organisms, which should keep a host of soilborne diseases in check. Also, one major soil component becomes depleted: organic matter, AKA, soil carbon.
One of the most important reasons to compost is that much of the microbial life in a compost pile works to break down plant material to form humus. Humus is a complex carbon chain molecule that is extremely important to build up in your soil. It can hold 6 times its weight in water, helps hold on to minerals and forms the ideal soil ‘crumb’ structure. This helps to maintain pore space in the soil, allowing air and water to enter it, and resists soil compaction.
The heavy clay soils in our region often suffer from compaction. Have you ever tried to dig a garden bed, and realized that it was like an adobe brick? Building up humus in your soil, and watering using GB techniques, helps to keep this from happening.
Now that you can’t imagine amending your soil with anything but organic compost, what exactly is compost? A compost pile is initially a mix of organic materials (plant material: ‘mature’ “browns” and ‘immature’ “greens”), soil, water, and air. Once it cures it will be a mix of organic materials that have not yet broken down completely (mostly woody chunks), soil particles, organic matter (some of which is in the form of humus), soil life (‘bugs’ and microbes), water, and air.
People often talk about the balance of carbon and nitrogen in the pile. Compost materials have varying carbon to nitrogen ratios, and when you combine all of your materials together into a compost pile, it will have an overall C:N ratio. This can help determine how your pile will behave, because the C:N ratio gives you an idea of how easily accessible the material is to composting microbes. Mature materials have a higher C:N ratio than immature materials. The type of carbon in the material is important: immature materials contain more ‘metabolic carbon’ that breaks down more quickly, while mature materials contain more ‘structural carbon’ that breaks down more slowly. Nitrogen causes your pile to heat up because it stimulates microbes that use it to break down organic materials, and their metabolic processes release heat. Almost all piles will heat up to about 139 F. The difference is how long they remain hot, which is determined by the relative amounts of structural and metabolic carbon in the pile. We distinguish piles by their C:N ratio: ‘hot’ piles are around 30:1 when built; ‘cool’ piles are 45:1; and ‘cold’ piles are 60:1. Hot piles remain at this high temperature for about 3 weeks, cool piles for about 2 weeks, and cold piles for about 1 week.
If you are curious about the balance of C:N in your pile, you can estimate the C:N ratio, using 5-Gal. buckets as measures, from the following table:
|Guidelines for building hot, cool, and cold compost piles|
|Desired ratio of carbon to nitrogen in built compost pile||Volumetric proportions
for building compost pile (for example, 5-gallon buckets)
Proportion of cured compost compared to built compost pile
|Mature material||Immature material||Soil||Range||Average|
|This is a modified version of Table 1 on p. 18 in Ecology Action’s Booklet #32: GROW BIOINTENSIVE Composting and Growing Compost Materials.
Note: The range in column E may depend on the type of immature crop used, since different immature crops can contain significantly different percentages of dry matter at their harvest point. For example, alfalfa contains over twice the dry matter as comfrey at the harvested point.
Healthy compost has a high diversity of microbes, and fewer microbes can survive at high temperatures. So, cooler compost will likely have a healthier micro-ecology when finished. Human and pet waste should not be added to compost, so there is little need to worry about sterilizing the compost of pet or human pathogens (see Future Fertility for more on this).
Also, the heat in a compost pile is the result of chemical bonds breaking as the material is digested by the microbes. So, the longer a pile remains hot, the more biomass you lose from the pile in the form of gases, like CO2, that escape into the atmosphere. Hot piles will almost always result in less cured compost than cool or cold piles. This also means that more carbon is sequestered in cold piles, helping to reduce the global greenhouse effect!
Having too much easily decomposable C in the form of immature materials and kitchen waste can result in a pile that breaks down too rapidly. This is common in home compost (it can end up smelling like a garbage can!). Compost that has a bad smell is unbalanced, and/or has probably been constructed incorrectly. Maintaining too much moisture in your compost pile can result in anaerobic conditions (too little oxygen) that produce a bad odor.
At Common Ground Garden, we aim to have the C:N ratio in our pile at around 45:1. At this ratio, piles take around 3.5 months to cure when built during the main growing season (depending on the weather), but may take longer when built in winter. Compost cures more quickly in warm and humid conditions than when dry or cold. The process can be accelerated by maintaining a good moisture level. Piles built in a sunny spot usually need to be watered lightly daily to ensure an even moisture. Prepare a small opening in your pile with a hand fork, and then insert your hand to determine if the moisture level inside is right. At CGG, we only need to water our piles a couple of times per week to maintain moisture, since we build them in the shade (see How to Grow More Vegetables about pile placement).
Some people like to build piles with a C to N ratio of 30:1 and turn them often so they maintain a high temperature and finish more quickly. We turn our piles only once at most, at around the 3-week point, after the temp of the compost pile has peaked and then fallen significantly. This helps to even out the pile’s decomposition and its moisture, as you bring undecomposed material to the center of the pile and add water as needed when you turn the pile. Read on for our process. Our piles take longer, but we are patient farmers!
Colder compost piles can result in more cured compost than hotter piles. Since a primary aim of the GB method is to increase soil carbon and organic matter, we want to maximize the amount of finished compost we obtain. Assuming that a hot pile produces about 25% cured compost per unit of built material, a cold pile can produce about 40% more cured volume than the hot pile, that is, about 35%cured compost per unit of built material. To reduce the temperature of the pile, it is built as a free-standing cube, and mostly left uncovered (we cover the piles during rain so they do not get too wet).
The minimum pile size is a 3ftx3ftx3ft cube (1 cubic yard). A 4-ft-cubed pile (2.37 cubic yards; more than twice a 3-ft pile!) is even better. This is because smaller piles tend to dry out too fast and will not have enough insulation to maintain a sufficient temperature to stay properly active in cold weather.
When adding layers as materials become available, stop adding when you achieve the desired dimension, and start a new pile. When the pile has matured into finished compost, it will have shrunk: a hot pile will shrink to approximately 25% to 33% of its original volume, a cool pile to approximately 30% to 40%, and a cold pile to approximately 35% to 45% (see table above).
You could build a compost pile with only your hands, but several tools make the job much easier. We use a compost thermometer to get an idea of how well we are balancing our immature and mature materials. Temperatures much higher than 130 F likely mean that we should have reduced the amount of immature material in the pile. Buckets are useful for collecting (seedless) weeds, kitchen waste and other materials for spreading on the pile. A spade is handy for spreading a layer of soil over the pile. Lastly, a spading fork helps to structure the pile, by pulling out the sides and aerating the interior.
This diagram shows the structure of each layer of a GB Cold compost pile 60:1 C/N ratio as it is built up from the ground:
|TOP (more layers are built on top as needed, and a full 5-Gal. bucket of soil is used to “cap” the top of the pile.)
|##################||Thin layer of Soil — one quarter of a 5-Gal. bucket|
|++++++++++++++++++||Immature metabolic carbon material: more-easily decomposable C — 0.5, 5-Gal. bucket|
|&&&&&&&&&&&&&&||Mature structural carbon material: less-easily decomposable C — 2.5, 5-Gal. buckets|
|%%%%%%%%%%%%||A 4” thick layer of thick woody material as the base|
|BOTTOM (ground level; loosen soil about 12″ deep)|
As each layer of the pile is built, it should be watered evenly, so it will be much easier to maintain the moisture content in the pile. Once the pile is built, the moisture level in the pile can be checked by hand, and it is watered additionally if it begins to dry out. Including a soil layer will hold both moisture and nutrients in the pile. It also jump-starts the composting process by adding microbes. After adding a series of layers to the pile (mature, immature, and soil) up to the desired height, the last soil layer can be doubled to “cap” the pile and protect the organic materials from volatilizing (breaking down and escaping as gas) in the sun.
Woody and stalky materials (stems and branches), that make up much of our mature material at CGG, should be placed on the pile in rows in an alternating pattern so that successive mature layers are oriented at 90 degrees from each other (e.g., one layer runs right to left, the next runs back to front, and the next runs right to left again). This helps with the structure of the pile and keeps it standing more stably as a cube.
After the temperature of the pile has peaked and dropped, you may choose to turn the pile. (See above for the advantages of turning; see Blog #1 for the disadvantages.) To turn a compost pile:
After 3 to 4 months, even if the pile has been turned, the inside of the cured compost pile will be broken down, and the outside may only be partially so. A compost sifter (like a giant flour sifter) is often used to sort the cured compost from the material that has not yet broken down. Whether sifted or unsifted, the cured compost can be used immediately or dried out and stored in the shade until it is needed. We add the uncured parts to a new compost pile in the mature material layer.
…As much as you can produce! At CGG we optimally amend our 100-square-foot beds with at least 2x 5-Gal. buckets of compost, and preferably 3x 5-Gal. buckets, if we produce a sufficient amount, before planting most crops. Amending before an Autumn compost crop is unnecessary, but helpful if you have sufficient cured compost to utilize. The great thing about cured compost is that, the more you amend the soil with your compost over time, the better your yields and plant health will be— and the more compost you will be able to produce!*
You can get started by building a simple compost pile in your garden today, and our composting resources can help!
* Caveat: It is actually possible to over-amend your soil with cured compost and nutrients in organic fertilizer form. Ecology Action, our parent organization, has found that when 6x 5-Gal. buckets of cured compost are spread per 100 sqft bed (rather than the recommended 3), there is relatively little additional build-up of soil organic matter by the next year. This is similar to adding a lot of wood to your fireplace all at once, rather than spacing it out over an evening. You get a hot fire immediately, but it doesn’t last very long.
To determine what your soil actually needs, consider visiting GrowYourSoil.org to find out about getting a soil test. Their inexpensive Basic and Trace Minerals Test will help you balance your soil and will save money, since you’ll only be adding nutrients to the soil that are actually needed. Some people over-fertilize and, in doing so, may throw the soil out of balance!
For more details, see Compost Sections in:
The Farmer’s Mini-Handbook, Margo Royer-Miller, Ecology Action, 2010. Free, downloadable at: http://www.growbiointensive.org/Self_Teaching.html
(A good hot composting resource.)
The Sustainable Vegetable Garden, John Jeavons and Carol Cox, Ten Speed Press, 1999. Available from Ecology Action’s non-profit Mail Order Service, www.bountifulgardens.org
(A good hot composting resource.)
How to Grow More Vegetables—and Fruits, Nuts, Berries, Grains and Other Crops Than You Ever Thought Possible On Less Land Than You Can Imagine, John Jeavons, Ten Speed Press, 2012. Available from Ecology Action’s non-profit Mail Order Service, www.bountifulgardens.org.
(A good hot and cold composting resource.)