Life Lessons from the Compost Heap

You may have noticed this: things change over time. Sometimes change is predictable because the “thing” goes through phases of change that are predefined in advance. Sometimes the change could have been predictable, but your mental model of the world doesn’t include the correct understanding. In that case, your observation of the facts is faulty because most of the time we don’t bother to separate observation from understanding. We “jump to conclusions”.

Long-term readers would know that I enjoy a spot of gardening. My garden has evolved over the years, mostly in response to me trying to figure out what I really want from it. In the early days, I had the great idea of growing lots of vegetables. Then I made a stunning realisation: I don’t actually eat a lot of vegetables.

My food preferences generally lie outside of the vegetable family. One of the places they do lie is in the fruit family. Thus, over the years, I have gradually added more and more fruiting trees and plants to the garden, and I now get tidy yields of apples, pears, citrus, almonds, olives, grapes, and berries.

Back in the early days, when I was still chasing the vegetable dream, I used to do a lot of composting. My skills improved over time, and I remember the proud day when I got my first compost heap above 80 degrees Celsius with the steam gently rippling off the top of the pile and the unique aroma that comes from astronomical numbers of bacteria having a giant party.

Then, something bad happened. The temperature of my compost heaps started to decline. I couldn’t figure out why. As far as I knew, I wasn’t doing anything different from the golden days, but I was struggling even to get a heap above 50 degrees.

Lower temperatures were one problem. A second problem was that my compost heaps were ending up much woodier than they used to be. It took me a while, but I eventually made the realisation that led to the answer. As the fruit trees in my garden had continued to grow, the amount of woody cuttings had also multiplied. I was shredding those cuttings and adding them to the compost heap. After some internet sleuthing, I realised the problem.

Compost heaps are bacteria driven, but bacteria cannot metabolise the lignin in wood. Since lignin makes up about 30% of the mass of wood, I had been adding a great deal of material to the compost heap that couldn’t be digested by bacteria. That’s why the temperature of the heap had declined. It was like I had organised a party but hadn’t purchased enough food and alcohol, so the guests went home early.

It turns out that the lignins in wood can only be digested by fungi. Fungi steer clear of compost heaps when they are hot, and it takes them a long time to digest the lignin even when they do eventually show up. That’s why my compost ended up woody. There would already have been fungi in the heap eating the lignin, but fungi are slow eaters. They’re like the guests at the end of the party who hang around endlessly instead of going home.

These realisations resolved my composting conundrum but now left me with the question of what to do with the woody waste from all my fruit trees, which was now the majority of waste from my garden. That led to the next revelation because it turns out that the bacteria-fungi distinction also relates to the way plants get their nutrition.

It’s only really in the last fifty years or so that we have come to realise that plants (and humans) are in a symbiotic relationship with countless microorganisms. Arguably the most important function of those relationships is digestive. Microorganisms break down certain types of food and make it available to the host entity. In doing so, they help provide nutrition to plants and animals.

It turns out that different plants and animals prefer different types of microorganisms. Vegetable plants rely on bacteria to help with nutrition, while fruit trees interact primarily with fungi. Thus, the reason why compost is beneficial for vegetable plants is because it is full of bacteria. If you add it to the soil, some of those bacteria quickly set up a symbiotic relationship with the growing plant. Meanwhile, trees rely much more on fungi in the soil.

The upshot of all this is that the natural way to make vegetable plants happy is to create a bacteria-rich environment for them in the soil, and the natural way to make trees happy is to do the same with fungi. How do you build up each group? Simple. You just have to feed them what they like to eat. Fungi like to eat lignins. Therefore, leaving wood waste on the soil encourages fungi. This is quite obvious if you think about it because where in nature does woody waste tend to fall except beneath the trees from which it came?

Thus, the minor and seemingly unrelated problem of the falling temperature of my compost heap ended up revealing a whole host of things I never knew about nutrition in general. It’s a nice example of how we come to learn the higher-level abstractions that really govern an activity.

For a beginner gardener, the commandment “thou shalt make compost” is a good one and has no real downsides. Eventually, however, you learn that what you are really doing by composting is feeding plants. Then you can go one step further and realise that you’re feeding the things that feed the plants (microorganisms). At an even more abstract level, that’s all gardening really is. You feed the things you want in the garden (including yourself) and you try not to feed the things you don’t want (“pests” and “weeds”).

I think there’s one other general lesson that my composting story reveals, though, which is that you shouldn’t ignore seemingly small problems. I could have continued to make compost heaps that only reached the mid-50s temperature-wise. The woody result I ended up with would still have been beneficial to the plants in my garden. But the irony was that, at the same time I was making woody compost, I was also buying in mulch to put around my fruit trees. I was feeding lignin to bacteria that didn’t want it and not feeding it to the fungi around the trees that did. In hindsight, the mistake is obvious.

Thus, an understanding of the higher-level principles ended up improving several different areas of my gardening practice. Nowadays, I use the woody waste from the garden as mulch around the fruit trees, which saves me money. Meanwhile, I put non-woody garden waste into a worm farm and use the castings and tea to fertilise the relatively small amount of vegetables I grow. It’s a much more elegant system. I don’t even make hot compost anymore, meaning I have one less physically-demanding chore to worry about. The lesson learned from resolving a seemingly small and isolated problem ended up having an outsized benefit.