9.1: The Dirty Little Secret of Compost

"Producing quality compost is the most important job on the organic farm. A lot of the problems I see on farms I visit could be solved by making better compost." ~ Eliot Coleman

Many organic farmers and gardeners know that compost is a good thing. Many people believe they add lots of nutrients to soils when they spread compost. Not exactly true. We are adding some nutrients to our plants, but very little when compared with commercial, conventional fertilizers or even some organic fertilizers. Compost is NOT a fertilizer. The secret of compost is that it makes soil better, and better able to do its job of providing nutrients.

Compost creates a stable home for nutrients and bacteria and fungi to all live, function, and cycle in the system.

In my opinion, compost should be viewed as a soil amendment rather than as a true fertilizer. We should recognize that compost has a whole different way of supplying nutrients to a plant than simply dumping a pile of elements right at the plant's root. Compost has two mechanisms that are very different from this “dump” method of concentrated fertilizers.

In the first mechanism, consider that compost’s job is to create a habitat for the “good” fungal hyphae. The hyphae work with plant roots to make them hundreds of times more capable of foraging for nutrients by helping them cast a wider net. This goes back to the idea of soil tilth. Compost creates good tilth.

The second way compost performs its magic is to create a great habitat for soil bacteria and other soil fauna (animals). Compost feeds soil livestock! Visualize soil bacteria as mini-pacmen. The bacteria scoot around in the soil and chow down nutrient molecules in compost raw material. The bacteria, which are single cells, break down old plant material and manures, take in those released nutrients, and use them as building blocks to grow. They incorporate the free, unbound nutrients into their “bodies”. Thus the nutrient is said to be “immobilized” since it is bound up in that bacterial cell, or the body of the next biggest organism that feeds on the bacteria. The bacteria do not live long, and when they die, the nutrient, say nitrogen, is available to the plant again. However, the nutrient is available in the sticky mess that is the bacterial cell. The nutrient will not simply wash through the soil as it does in the synthetic method of salt-bound mineral fertilization. So the gummy bodies of the minute soil livestock stabilize the nutrients in the soil.

The secret of compost is that we are amplifying both bacteria and fungi by giving the little darlings what they need to thrive; a small nitrogen source, a carbon source, air, moisture, and a decent pH. Let them do the work of enhancing soil nutrient availability.

So, consider yourself a baker when it comes to composting. You mix ingredients, cook, and create something great. You can vary your ingredients and your bake times and processes, and you will still make something delicious. The “cake” you make is a great soil amendment. Some of you are already pretty experienced at making compost and some of you are just beginning. We all have more to learn. Don’t forget about temperature and the length of time you are incubating the mix of ingredients. Don't forget you need moisture and air. Be wary as you take advice from others about compost. There are many people trying to make a buck off you by getting you to purchase fancy microbial inoculants. Nature provides what you need, don't buy it.

We inoculate our soils with beneficial organisms when we use compost. We are enlivening our soils. This is akin to adding yeast to flour and water. When bread is allowed to rise under warm conditions, we are really growing the yeast that causes the gas that raises the bread. When we compost, we are fostering bacteria and fungi by providing them with a warm, cozy place to grow and rise. When we spread compost or compost tea, we send our homegrown soil livestock out into the field or garden to do more of the work in the soil food web. That is the secret of compost.

The compost recipe can amplify bacteria by adding more available carbon – like molasses, or they favor fungi by adding more protein to the recipe. The pH level also will favor either bacteria or fungi. Your job as the farmer or gardener is to recognize what you are trying to accomplish, and nudge nature toward those goals.

The decomposition of organic materials takes place naturally in forests, fields, and all around us. Composting is the art and science of combining available organic wastes so that they decompose to form a uniform and stable finished product. Composts are excellent organic amendments for soils. Composting reduces the bulkiness of organic materials, stabilizes soluble nutrients, and hastens the formation of humus. Most organic materials can be composted, and the process offers a win-win opportunity: reducing waste and improving soil.

In some ways, composting is microbe farming. If ingredients are combined to provide food (carbon and nitrogen), moisture, oxygen, and shelter in proper proportions, a diverse cohort of organisms will efficiently process the feedstock. These microorganisms perform well at elevated temperatures with plenty of oxygen and moisture. They cover the range of warm (mesophilic) to hot (thermophilic) conditions. Thermophilic temperatures (from 110° up to 160°F) help kill off weed seeds and disease organisms, which sets composting apart from other decomposition processes. At temperatures below 110°F, the more prolific mesophilic organisms take over and the rate of composting again slows down, especially as it drops toward ambient temperatures, a process known as “curing.” At the other extreme, temperatures above 160°F can develop in compost piles; this overheating slows down the composting process by killing off most organisms and by possibly causing extreme drying. High temperatures, in combination with high ambient temperatures and aeration, can also cause spontaneous combustion in barns and at compost facilities. In general, the composting process is slowed by anything that inhibits good aeration or the maintenance of high enough temperatures and sufficient moisture. It has been found that mesophilic temperatures may be more effective at breaking down some pharmaceuticals.

Types of Composting

Some people talk about “low temperature” composting—including “sheet,” worm (vermicomposting) and small-pile composting—and “high temperature” composting. We like to use the term “composting” only when talking about the rapid decomposition that takes place at high temperatures.

Reference

• 13.1: Introduction by Fred Magdoff & Harold van Es is licensed CC BY-NC 4.0. Original source: https://www.sare.org/resources/building-soils-for-better-crops.