Decomposition

Poise and grace may carry us through difficult situations, but sometimes things just need to fall apart. Decomposition is one of those things for which falling apart is a good thing.

Decomposition in a compost pile is a beautiful thing

Decomposition describes the way complex organic structures are broken down into simpler structures. When plants and animals die, all the processes that held them together and kept them functioning stop working and they become organic matter. The study of how things decompose is called taphonomy, from the Greek word for tomb.

Most of the mineral food enjoyed by your plants comes from organic matter created by breaking down dead plants and animals. You probably already knew that, but did you know that the living things that speed decomposition actually reduce the amount of nitrogen in your soil as they feed?

Basically, there are two ways decomposition occurs: with the help of other living things (biotic decomposition) or through physical or chemical processes (abiotic decomposition). Now, you may think that this doesn’t apply to you as a gardener, but what about your compost pile? What is all that mulch actually doing to your soil as it breaks down? It’s really pretty amazing, so read on!

There are three basic stages of decomposition:

1. First, water either evaporates, causing dehydration, or, if there’s enough water in the environment, rot will set in. Rotting is the breakdown of complex structures by bacteria and fungi.
2. Next, local microbes and invertebrates, such as worms, snails, and sowbugs, start chewing on soft plant tissue, breaking it into smaller bits and transforming it into frass. In some cases, it is birds, voles, and even ourselves that breakdown plant material as we eat it.
3. Eventually, chemical changes begin to occur in the plant’s tougher, structural parts as other microbes start feeding.

This is why your compost pile works faster when you chop it up and flip it periodically. The cut edges provide points of entry for all those microscopic workers and the air helps the microbes breath. Warmer temperatures also speed up the process. Like us, those little helpers don’t move nearly as fast in winter! But, as they feed, they take the chemical elements that made up the plant and break them down into nutrients that are available to living plants. 

So what happens when I put straw mulch around my fruit trees?

Air, sunlight, and water will add their two cents to the decomposition process, but straw is pretty tough, so it will take some time to decompose. Local birds may grab a few bits and the sowbugs are sure to start feeding, but it is the microbes who will do the lion’s share of the work. 

The plants and microorganisms that feed on dead or decaying plant matter are called saprophytes. As the saprophytes feed, they poop out simple minerals that plants use and gummy substances that hold bits of soil together into aggregates. These aggregates improve soil structure by creating plenty of macropores and micropores. This means that water and nutrients will move through the soil more easily. These soil aggregates also improve aeration, drainage, water-holding capacity, and water infiltration rates.

As decomposition takes place, carbon dioxide is also released. This CO2 can end up in the atmosphere, or it may combine with water to form carbonic acid. Carbonic acid (H2CO3) works with other chemicals to break down rocks and pebbles in the soil, making those nutrients available to nearby plants.

What about the net loss of nitrogen?

It ends up that there is a delicate balance between the amount of nitrogen available to plants and the needs of the saprophytic bacteria and fungi that make those nutrients available. If you add a bunch of carbon (straw, leaves, shredded paper) to an area, the microbes will begin feeding and breeding like crazy. The only thing is, they also need nitrogen to live, so they will pull it from the soil, creating an initial nitrogen loss. Of course, over time, all of those nutrients will end up back in balance, supporting your garden, but that may take time that your seedlings simply do not have. This balancing act is called the carbon-to-nitrogen ratio. 

Ideally, you will want a 10:1 to 12:1 ratio of carbon to nitrogen. This means adding one pound of nitrogen for every ten pounds of carbon. If you are going to add a large amount of carbon to an area, it is a good idea to also add nitrogen. This keeps all the inputs for your soil microbes in balance as they improve your soil. Personally, I sprinkle blood meal (a good source of nitrogen) over an area, top that with aged compost (a good slow-release of many different nutrients), and then mulch with straw. The mulch keeps down weeds, stabilizes soil temperature, and, combined with the compost and blood meal, provides excellent nutrients and improved soil structure. It’s my way of tucking in my plants for our mild winters or when plants are stressed by too much California sun.

If you have access to a microscope, check out the various stages of decomposition to see what’s really going on down there!