Stop adding more compost to tired soil

If your garden is struggling, adding more compost feels like the obvious thing to do. And sometimes it is. Compost can add organic matter, nutrients, improve moisture holding and, depending on the compost, bring useful biology with it as well. But if you’ve already added compost and your plants still aren’t really doing much, another bag might not be the answer.

At some point, you have to stop asking, “What else can I add?” and start asking, “Which part of the soil system isn’t working properly?” That’s a better question, because healthy soil isn’t just about one thing. It’s a combination of biology, chemistry and physics, or structure, all working together.

I was talking this week with Graham Lancaster from EAL at Southern Cross University, and that was one of the things we spoke about. Soil health really does sit across those three areas. Biology matters, but so does the chemistry of the soil, and so does the physical structure that allows air, water and roots to move through it. If one of those parts is badly out of balance, the others usually struggle as well.

I’ve seen that first-hand on farms. You can put good biology into soil, but if the chemistry is badly out, fungi can struggle to establish. You can have the right nutrients sitting there, but if the soil is compacted or poorly structured, roots, air and water still can’t move properly. And you can have decent structure and chemistry, but if the biology has been badly depleted, nutrient cycling and root interactions can still be weak.

That’s why adding more compost isn’t always the answer. You might be adding something useful, but you might also be trying to fix the wrong part of the system. If structure is the main problem, compost may help over time, but it may not be enough on its own. If the chemistry is badly out of balance, you may need to deal with that first. If the biological side is weak, then adding organic material doesn’t automatically mean the right organisms are there and functioning.

This is where a lot of gardeners get stuck. The plants aren’t responding, so they add compost. Nothing much changes, so they add fertiliser. Then some seaweed. Then they water more. Then they change the soil again. Before long, they’ve put quite a lot into the garden, but the plants still aren’t doing what they should.

That doesn’t mean they’ve done anything wrong. Most of the time, they’re doing exactly what they’ve been told to do. The problem is that what you add to the soil and what the soil is capable of doing with it are two different things.

That’s a big part of what I mean when I say, “Your plants aren’t struggling because of what you’re doing. They’re struggling because of what’s missing from your soil.” I don’t necessarily mean one missing ingredient. Sometimes what’s missing is biology. Sometimes it’s the right chemistry. Sometimes it’s structure. And sometimes it’s the way those three things are interacting.

The mistake is assuming that one input is always going to fix all three.

Compost is a good example because we tend to talk about it as though all compost is the same. It isn’t. Some compost can be biologically rich and well made. Some may be more useful as a source of organic matter. Some may carry plenty of nutrients. Some may help structure. The real question is not just, “Did I add compost?” It’s, “What did that compost actually change in the soil?”

Did it improve the structure? Is the soil holding water better? Are roots moving through it more easily? Are nutrients becoming more available? Is there more biological activity? Are the plants actually responding?

Those questions tell you much more than the fact that you added another bag.

And this is also where I think soil biology needs to be kept in perspective. Biology is incredibly important, but it doesn’t work in isolation. Mycorrhizal fungi need a suitable soil environment if they’re going to establish well. Bacteria and fungi need the right conditions to function. Roots need oxygen, water and space. The chemistry needs to support the biology, and the structure needs to give everything somewhere to operate.

That’s why I don’t think the answer is “biology fixes everything.” It doesn’t. But I also don’t think you can ignore biology and expect soil to function properly either.

The better way to think about it is this: chemistry, structure and biology all have to be doing their part.

So if you’ve been adding compost year after year and your plants are thriving, keep doing what’s working. But if you keep adding it and nothing much is changing, I wouldn’t automatically reach for another bag.

I’d start asking which part of the soil system actually needs attention.

Because sometimes your plants aren’t asking for more.

They’re asking for the soil underneath them to work better.

John

Living Earth Biology