Not all Compost is created Equal

I received some good feedback from my post about compost tea, so I decided to do a similar informational post about compost...

Not all compost is created equal. I find myself, much to my surprise, spending lots of time explaining to people that 'compost' is not a blanket term for a single thing. Some compost is good, and some compost is bad, depending on what you are trying to do with it. Let me start out by defining what I mean by compost.

Since compost is no more than organic matter, and the microbes that devour it, the quality of a compost can be measured through the same lens. As far as inputs go, while the end product will differ based on the inputs, equally important is the way the compost is managed during the break-down process. Those organisms we want in our composts, our soils, and in relation with our plants are almost entirely 'aerobic,' meaning 'oxygen-breathing.' Just like us, beneficial microbes generally need oxygen to survive. What this means for compost is that I will want to keep my pile aerobic for as much of the process as possible. The general threshold for aerobic organisms is about 6% dissolve oxygen (DO), or 6 parts per million (ppm). When our piles drop below 6% DO, which they do as a result of the aerobic organisms using up all the available O2, the 'anaerobic,' of non-oxygen-breathing organisms wake up. These are the microbes that make our piles stinky (which by the way, is no more than our vital plant nutrients like nitrogen being volatilized out of the compost and blowing away in the wind), and they are the organisms that can give us and our plants diseases.

Compost can be made in a variety of ways.
-The most familiar is the thermal compost process, in which we make a pile of organic matter, it heats up due to microbial activity, and the materials break down and are consumed/transformed by the microbes.

Thermal Composting

-The most practiced is probably the cold-pile process, wherein a person continues to add materials (like food scraps, lawn waste, etc) to the top of the compost pile, and then leaves it to it's business. The microbes break down this material without as much ferocity and vigor, so it is a slower process, and generally, the oxygen levels deep in the pile are below our aerobic threshold.

-A third familiar practice is called vermi- or worm-composting. In this practice, a community of red worms (aka Eisenia fetida) are responsible for consuming the material, and transforming it into worm-castings. This method produces a fairly consistent product, mostly aerobic, and is also a cold-process.

Fungal Composting - about halfway done

-One method gaining popularity is a cold-process for creating fungal compost. This method involves assembling a large amount of high-carbon material (woodchips, straw, etc) in a shallow, but broad pile, inoculating it with a compost or humus known to have a high fungal diversity, and covering the pile with a breathable material like cardboard or compost fabric. This method produces a highly fungal compost, is a cold process, and takes as much as 2-3 years to make the final product.

And those are just the most popular ways to make this stuff. Maybe you can start to see how one name just isn't adequate for all these different products...

Now, most of the time, when we talk about compost in agriculture, we are talking about Thermal Compost. The interesting thing about thermal compost is that it is an intensively fast breakdown of the organic material provided, so to keep it aerobic requires a fair amount of monitoring and input. Most commercial compost operations just make big piles and leave them for 30 days until they turn them. This means that in the center of those piles, where air isn't able to easily penetrate, the material is spending a large portion of those thirty days below the aerobic threshold, so the organisms being bred are the anaerobic ones. This can lead to some unhealthy consequences... for example, one of my area's main compost producers actually had one of their piles spontaneously combust last summer. This is what happens when the anaerobic organisms, who can produce byproducts like methane, alcohol, and formaldehyde in their process of decomposition, are left unchecked. Once these systems reach a high enough temperature, those anaerobic byproducts ignite.

I consider it safe to say that I don't want alcohol-producing organisms to be sharing the soil with my food... and as a result, I don't feel okay about putting composts in my garden that come from questionable sources.

So how does one find a genuinely 'good' compost? Here are my 3 rules to live by when it comes to selecting a compost for yourself:

1) Compost should smell good. Healthy soil organisms produce byproducts like water and CO2. Their shit don't stink, and it'll help my plants grow. A good compost smells like rich earth.

2) Compost should be the color of 70% dark chocolate. If you don't know what color that is, go buy a bar and check it out before you eat it. Its not true that darker compost is better compost. High heat, by means of anaerobic decomposition, can actually burn compost, turning it black.

3) Compost producers should know a lot about their compost. A good compost producer will be able to tell you just what goes into their compost, and in what proportions. They also will have data to show you proving, via some third party, that they have a quality product. Chemical testing is good, residue testing is better, and biological testing is best.

Following those rules, while making it a little more challenging to find the right source, has made the difference for my garden, and it can do the same for yours.

Don't be fooled. Not all compost is created equal. Until we catch up with countries like Germany, where there are actually certification bodies for composts, we consumers have to do the research ourselves. The quality of your compost will contribute to the quality of your soil, and none of us want unhealthy soil, so why buy unhealthy compost?

Hope this post has been helpful to anyone looking for information on this topic. Please leave me a comment if you have any further questions or comments!