Know Your Compost

This week I delivered the last of three gardening sessions organised by Bandstand Beds on Clapham Common. The session was on Soil and Compost, a topic I am particularly passionate about.

With this post I’d like to share some tips and tricks for your own compost experiments at home. These points come out of my own experience of composting, teaching on the topic and hearing common questions from learners. A lot of it is about getting to know what you’re doing and why you are doing it.

1. Get to know your soil

We can all make compost. Everything composts eventually. And while we do it we can of course learn what works and what doesn’t.

In the long term, it is however also important to understand what soil really needs. And it doesn’t need to be super complicated.

Soil composition

Healthy soil is usually dark in colour, with good smell and high in organic matter. It consists of about 45 percent mineral content, 5 percent organic matter (dead and living plant and animal material), 25 percent water and 25 percent air. Healthy soil drains well and is also able to retain water and nutrients well; this needs a good balance of sand, silt and clay, organic matter and ample soil life. In this case water and air can take turns in filling the pores in the soil, water flowing through and pushing out stale air while pulling in fresh air.

Heavy clay soils or generally compacted soils can get clogged up, water doesn’t drain and life in the soil drowns. Sandy soils don’t keep much water in the soil at all, and any rainfall washes out nutrients as it flows through.

What soil have you got? You get a sense of that by getting to know your soil, listening to it, feeling it or better, doing some simple soil tests (see jar test for soil texture).

2. Team up with microorganisms

In a handful of healthy soil are more organisms than humans on the planet. That is amazing. The organisms are important. 98 percent of the nitrogen (one of the most important plant nutrients) in the soil is locked up in organic matter. Microorganisms break down organic material, mineralising nitrogen and other plant nutrients and make them available to plants. 



In fact, they are the labour force that goes to work in our compost piles. The better we design their living space, the more efficiently they work.

And yes, they need, water, air and organic matter…

Life in the soil will contribute to a good soil structure by roaming the undergrounds, creating pores for water and air to get into and by creating aggregates out of mineral particles. Aggregates are the clumps of soil we find. The particles are held together by chemical reactions between the minerals and water as well as by the film that microorganisms produce around mineral particles.

Soil life also controls disease and pest by keeping a healthy balance in the soil.



3. Know your greens and browns and mix them well



All organic matter is made up of substantial amounts of carbon and lesser amounts of nitrogen. This is the carbon:nitrogen ratio (C:N). When making a pile it is important to know which materials are high in carbon (often called browns) and which have a higher nitrogen concentration (often called greens). See a list below. Research has shown that the optimal ratio for organisms to thrive on is 25 or 30:1.

When making your pile you want to layer it well with a mixture of both for optimal performance. An example would be a layer of straw, then a layer of weeds such as nettles and other garden greens, then a layer of woodchip followed up with some manure; on top of that a layer of autumn leaves and then a layer of foodscraps; you bring in a layer of straw again and so on. If you only have straw and manure you can still make a compost pile, however a diversity of material is recommended to attract a diversity of bacteria and speed up the process.

These layers shouldn’t be more than 10 cm thick. If you mix this well, your pile will get hot within a few days.

Photo by Ollie Smallwood

Photo by Ollie Smallwood

4. Keep your pile wet enough

What I often observe, also in the UK, is that our piles don’t have enough moisture. The ideal moisture content for a compost pile is 60 percent.

You can test that quite easily by taking a handful of the compost material and squeezing it. If you find water running down your arm, it is too wet, if there is no sign of water at all, it is too dry and if you have a drop of water forming on the palm of your hand, staying there, it is just right.

My tip is to build or turn your pile when it rains. If it gets too wet you can cover it afterwards.

5. Spend time with your pile

The more you turn your pile, the quicker it will go. Turning the pile will mix the material and bring air into the pile. If you don’t turn it, it can be helpful to stick a few branches in as you build it; you can move these around from time to time to get air in.

Spending time with your pile and turning it regularly, also allows you to learn what works and what doesn’t and become a composting specialist.

If your pile gets too hot and stinky, often with a white fungal film covering a lot of the organic matter, it has too much nitrogen. In that case you add carbon material to your mile as you turn it. If your pile doesn’t get hot and the organic matter is not breaking down, you need to add nitrogen (or moisture).

6. Find solutions that suit you and your situation

You might not have time, space or enough material to make turn compost piles.

If you mainly have food waste, have little space or are living in a flat, a wormery might be the right low-maintenance option for you. A blog post on how I built my worm compost will follow soon.

You can also make liquid fertilisers or compost teas, which I recommend in any way use during the growing season.

The black bins that London councils often recommend are great for food waste composting too. Usually the worms get in and help the process. When it smells and there are flies, add carbon. I personally don’t like using them if they are for general garden waste only, as you can’t turn the waste that is inside. If you do use it for general garden waste, keep the C:N ratio in mind and make sure it is wet enough (these bins have a lid, which can have the material dry out).

7. Look after your soil

Make sure you look after your soil. Don’t walk on the patches you want to use for growing food, it will compact your soil. Mulch the gardens well, it will keep moisture in the ground and add organic material. Make sure you have enough minerals in your soil; it can help to add rock dust from time to time.