Composting has been part of our gardening strategy for almost as long as we have been seriously gardening. Once we started we quickly realized one of our biggest costs to maintain our garden would be soil and organic fertilizers (which for us was primarily animal poop). Since we’ve started we’ve done it continuously for years. If you are thinking about composting yourself or investing in commercial compost this article will help nudge you in the right direction.
Table of Contents
Why is compost important
Composting is the natural process of ‘rotting’ or decomposition of organic matter by microorganisms under controlled conditions. Raw organic materials enhance their suitability for application to the soil as a fertilizing resource, after having undergone composting.
Compost has 2 primary functions:
- It improves the soil structure. This makes the soil easier to work with, improves aeration, improves water retention properties and makes it more resistant to erosion.
- Compost also provides nutrients for plant growth. While compost is not strictly considered a fertilizer its organic acids makes nutrients in the soil more available to plants.
Is compost fertilizer?
The short answer is no, compost is not a fertilizer. The available NPK values in compost are too low to be considered a fertilizer. Compost works differently when compared to the inorganic fertilizers we often see in our garden stores. In addition to the Macro (Nitrogen, Phosphorous and Potassium) and micronutrients released in the composting process, a high-quality compost supports a wide range of microbial life. These microbes are responsible for unlocking the nutrients the plant needs when the plant needs them. This is an important difference between good quality compost and synthetic fertilizers. When using synthetic fertilizers it is easy to over-fertilize or cause nutrient imbalances in your soil because all the nutrients are immediately available. You can (almost) never add too much compost.
Soil fertility refers to the ability of soil to sustain agricultural plant growth. Healthy fertile soil will provide the ideal habitat for plant and soil life, resulting in sustained and consistent, high-quality yields. Soil fertility is a complex process that involves the constant cycling of nutrients between organic and inorganic forms. As plant material and animal wastes are decomposed by micro-organisms, they release inorganic nutrients into the soil solution. Those nutrients may then undergo further transformations which may be aided or enabled by soil micro-organisms.
The importance of organic matter in soil
Organic matter refers to dead plants or animal residue of all kinds in all stages of breakdown and decay.
Organic matter is extremely important to healthy soil. Every living organism in the world is carbon-based. To have a healthy living soil you need to have sufficient carbon (i.e. organic matter) to feed the soil microbial life and support the soil food web.
Soil organic matter is necessary to:
- Feed plants through nutrient exchange and through nutrient release. As the organic matter in soil decomposes, it converts into other forms releasing nutrients into the soil. This constant breakdown of organic matter provides a continual slow-release source of nutrients in plants.
- Add organic acids to the soil. As organic matter in soil decomposes to form humus, the resulting organic acids help dissolve minerals in the soil, making the mineral nutrients available to plants. Organic acids also increase the permeability of plant root membranes and therefore promote the plant roots’ uptake of water and nutrients.
- Support microbial life. Organic matter provides energy for the soil’s microbial life-forms. The microbial population, like any other form of life, needs a source of carbon to grow and replicate. Soils comprising of 4-6% carbon provide an ideal environment for this.
- Maintain soil structure. The microbes that feed on organic matter in the soil bind the soil particles together. Some of the secretions of microorganisms are a bacterial glue (polysaccharides) that hold soil particles, thus improving the soil’s structure. Thus, organic matter is the key to maintaining soil structure, reducing erosion and keeping it in an open porous condition for good water and air penetration.
The Composting Process
Composting can be done in many different ways. Generally, though composting falls under two broad categories aerobic composting (i.e. composting with an abundance of oxygen) and anaerobic composting (composting with limited oxygen). Both methods have their place. Typically for home composting systems where the goal is to retain the maximum amount of nutrients and build healthy soil aerobic composting is the desired method.
For simplicity, we will explain the principle as if we are building a compost pile in our backyard. However, there are many ways you can compost at home. So many, that I believe you can do it even if you had no yard space at all.
The minimum size pile you will need if you are doing a hot, aerobic compost pile is 1 cubic meter. This is important because anything smaller than this won’t heat reach and maintain the required temperature for long enough to truly be considered hot composting.
The Composting Process
Step 1: Selecting a site
When choosing a site for your pile your first choice should be under a shady tree or a covered area. This will allow the pile to be protected so as not to get too much water when it rains or too much sun causes the pile to dry out. If the bottom of the pile is going to be in contact with the ground loosen the 12 inches of where you are going to put your compost pile. This will allow the water to drain off if you accidentally add too much water or if it rains heavily.
The site you choose should be easily accessible to you, a reasonable distance from the house (since pest issues can happen with compost piles) and have access to water.
Step 2: Adding materials to the compost pile
Ideally, you should be aiming for a 30:1 carbon to nitrogen ratio. However, nature doesn’t require us to be perfect, if you just add equal parts of brown and green material then you should be fine. Adjustments can always be made to tweak the recipe as we go along. I’ve put some examples of brown and green material in the table below.
| Green | Brown |
| Vegetable and food scraps | Dried leaves, grass, mulch or hay |
| Fresh grass clippings and yard waste | Cardboard rolls |
| Coffee grounds | Sawdust |
| Teabags | Lint |
| Eggshells | Newspaper (shredded) |
| Vegetable and food scraps | Hair/fur |
| Fresh grass clippings and yard waste | Clean paper |
| Wool/cotton rags | |
| Dried leaves, grass, mulch or hay | |
| Cardboard rolls |
When choosing what to add to your pile, the more diverse the better. Microbe diversity is very important in soil. Many microbes produce antibiotics that help plants resist diseases and healthy plants have fewer insect challenges. Different microbes have different food preferences. One of the best ways to maximize microbe diversity in the compost pile is to build your compost with a large variety of materials.
Do add
- Brown materials such as leaves,
- cardboard, shredded (non-glossy) paper
- Uncooked food and food waste.
- If adding eggshells get them as small as possible so that they break down more easily.
- You can add molasses or waste from a herbivorous (e.g. goat, sheep or cow manure) animal to boost the microbial population quickly. However, I have found that adding animal waste of this kind can encourage an explosion in the unwanted insect population (roaches)
Don’t add
- meats, oil, grease or cooked food
- Plants infected with disease or have severely been attacked by insects.
- Poisonous plants that can harm soil life or plants that take extremely long to break down
- Weeds
- Cat or dog poop or any poop from any meat eating animal. These can contain pathogens that are not always killed off with the heat in home compost piles
Can I add soil to my pile?
Yes, you can add soil to your pile. If you have decent quality soil in your yard adding it to your pile can be very beneficial. Soil can contain a good starter supply of microorganisms. It also contains some bacteria that help stabilize nitrogen in a pile. If your pile starts to stink turning it or adding some soil can solve your problem very quickly.
Microorganisms found in soil can bind nitrogen surpluses reducing leaching from your pile. These surpluses will be released gradually in your garden as your plants need them. By adding soil to your compost you can allow your native microbes to flourish.
Step 3: Watering the pile
Water so that it is evenly moist. The moisture level in the pile should be as wet as a well-wrung sponge. Water is needed for the microbes. Too little water results in decreased biological activity and too much can fill the air spaces and cause the pile to go anaerobic.
Step 4: Monitoring your pile
The breakdown of organic waste begins as soon as the waste is generated. Once optimal physical conditions are established microbes colonize the organic material and initiate the composting process. As the material breaks down it goes through 2 phases. The active phase and the curing phase as shown below.
Active phase
As the material in the pile breaks down the temperature will begin to rise. Regularly check the internal temperature of the pile using a compost thermometer. Once the pile goes above 140°F for a few days (say 3 to 6 days) you can turn the pile. At Over 160°F thermophilic microbial populations are killed. If you see your internal pile temperature reaching or exceeding 160 °F turn the pile immediately.
Turning the pile adds fresh oxygen to the system, this is important to maintain an aerobic environment. Although the temperature will drop after turning (due to the heat escaping from the centre of the pile) it will rise again very quickly as the aerobic bacteria’s oxygen supplies have been rejuvenated and they will multiply rapidly in an oxygen-rich environment.
Curing phase
In the curing phase, organic materials continue to decompose, but more slowly and at a lower temperature. Potentially toxic organic acids and resistant compounds are also stabilized during curing.
Do not rush or dismiss the curing phase. If you add compost to your plants without letting it cure sufficiently any volatile organic acids (VOAs) that may have been produced during the thermophilic phase will not have a chance to stabilize. This presence of VOAs will either inhibit plant growth at best or, at worst, kill your plant.
Once complete the final pile will be about 1/3 of its original size. The result of the composting process is the conversion of organic matter to the biologically stable humus.
How To Tell When Your Compost Is Ready?
Compost is ready when it is a dark rich material that crumbles in your hand. You shouldn’t be able to discern the original material and it should have a very earthy smell to it. The length of time to complete the hot composting process can vary. Generally, it takes between 3 to 5 months to convert our organic material to finished compost.
Humus
As your compost pile passes its thermophilic stage, microbial activity slows and the pile begins to cool. Most of the remaining organic matter is in the form of humus compounds. Humus included the living and dead bodies of microbial life. As humus is formed, nitrogen becomes part of its structure. Humus stabilizes nitrogen in the soil. Humus also acts as the site for nutrient absorption and exchange for plants in the soil.
As plant roots grow through the soil searching for nutrients they also ‘feed’ the humus. Plant roots are surrounded by a halo of hydrogen ions (these ions are by-products of the plant roots’ respiration). Think of the plant roots as bartering with the humus to exchange some of its positively charged hydrogen ions for some of its positively charged nutrient ions stuck on the surface of the humus. This is essentially how the plant chooses which nutrients it needs for its internal uses.
Adding compost to the garden
What happens after you add compost to your (garden) soil?
Once you’ve properly managed your composting process, most of the initial nitrogen that was added to the compost pile will remain in the final product. The microorganisms in the soil will continue to feed on the humus after the finished compost is spread on the soil. This process allows the nutrients bound in the humus to be released in forms that the roots of the plants can use.
When should I add compost and how much?
A little goes a long way when applying compost. In most situations adding about half an inch of compost to your soil every six months should be sufficient. However, If you are using containers you can add it when you see your soil levels in the container become depleted. Compost can also be used as part of a soil mix when starting or transplanting new plants.
How to add compost to soil?
When adding compost to your garden you can simply apply it to the top of your soil. There is no need to till it in. The nutrients will be relocated naturally through larger soil organisms like earthworms and insects and when water flows down into the soil after watering or when it rains.
After application give your soil a good water (with rainwater preferably) and cover your soil with an appropriate mulch to protect it from the elements.
Final Thoughts
Adding compost to your soil is extremely beneficial. Organic matter improves the physio-chemical and biological properties of the soil. As a result of these improvements, the soil:
- becomes more resistant to stresses such as drought, diseases and toxicity;
- helps the crop in improved uptake of plant nutrients; and
- processes an active nutrient cycling capacity because of vigorous microbial activity.
Compost contains some of the macro and micronutrients but is especially important for the addition of trace elements back into the soil. Making your compost yourself has the added benefit of saving you money while reducing the amount of waste that goes to the landfill.
Good compost creates healthy conditions for microbial life and organisms that live in the soil such as earthworms and beneficial fungi that keep unwanted nematodes and pests in check. Healthy soil produces healthy plants.
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