5 top tips for the perfect compost – according to science

As a young boy, I had to contend with my grandfather’s compost heap. It was a veritable Vesuvius of foul-smelling, putrescible plant waste, a metre high and hidden behind a privet hedge.

We had placed all the weedy waste in it a year before. As we started the annual spring gardening day, the first area we had to clear was the now weed-covered and unsightly compost heap.

By the time we had cleared the weeds sprouting from it, half the day and most of our energy was gone.

We were doing everything wrong. But it’s not too late for you. You can learn from our mistakes.

Making compost is a cornerstone of sustainable gardening, yet few of us understand the great science behind it.

It all comes down to understanding the requirements of the bacteria and fungi that do most of the decomposing and the processes of cellular respiration. Here’s how to work with them, not against them.

Your compost’s little helpers

For the most part, plant material is broken down by bacteria and fungi, aided by worms, other soil organisms and microbiota.

However, there are different types of bacteria and fungi, and the rates they break down organic matter vary enormously.

Some will completely break down plant material into clean, high-grade compost in just six to eight weeks.

Others, as my grandfather and I saw, could not complete the job in a year or more.

That’s in large part due to the big differences between anaerobic and aerobic respiration.

What’s the difference?

Respiration doesn’t just mean breathing. Biologically, is the metabolic process by which cells break down the energy stored in organic molecules (such as sugar and fats) to release energy.

There are two types of respiration:

• aerobic respiration, which occurs when oxygen is available, and
• anaerobic respiration, which occurs when there is little or no oxygen available.

In our cells and those of larger plants and animals, both forms of respiration can take place.

But in some micro-organisms, only anaerobic respiration is possible.

Anaerobic respiration is an ancient metabolism that evolved early in the development of life on Earth, well before larger multi-cellular organisms existed.

The processes involved in anaerobic respiration are relatively inefficient. Its chemical reactions result in the incomplete breakdown of the food and plant waste; very little energy and heat are produced along the way.

For composting, that’s a problem.

It means the plant material breaks down very slowly. Worse, the temperature is so low that weedy contaminants can survive and germinate.

This explains why my grandfather’s compost heap failed to decompose after a year, grew so many weeds and was a slimy, smelly mess. The conditions inside the heap were anaerobic from the start.

We ended up being very good at spreading weeds around his garden.

Aerobic is better

Aerobic respiration, which evolved when oxygen was more readily available on Earth, consists of many linked chemical reactions that cause plant material to completely break down.

It produces almost 20 times more energy than anaerobic respiration and generates much more heat.

This high level efficiency produces a more rapid metabolism, which quickly breaks down plant material and the heat generated kills most of the weedy contaminants in the plant litter.

This results in lovely, clean compost.

So the key to good composting is to ensure conditions are right for aerobic respiration and for crucial aerobic bacteria and fungi.

It’s vital to provide oxygen.

My top tips are:

1. if you have a compost heap, ensure it is wide, long and low (which ensures a high surface area to volume ratio), and introduce air by dragging a hoe or rake through it
2. if you use a compost tumbler or container, then rotate or stir it often
3. keep the compost moist (but not wet) over the dry summer months
4. keep your compost warm over colder months by ensuring it gets some winter sunlight
5. add some “browns”, such as dry leaves, or shredded cardboard or paper; the carbon-rich browns, added to the high-nitrogen green waste, gives a better carbon to nitrogen ratio and results in better compost.

If your compost is happy, the heat will be high enough to kill most pest eggs and parasites, and may even kill worms.

Don’t add worms to aerobic compost unless you have a worm-friendly composting system; you may end up committing wormicide. Let worms enter the compost naturally.

Rarely, heat from aerobic compost can damage thin-barked trees. So if you’re spreading it around the garden, keep it 20-50mm from the trunks of your trees.

Compost systems and heaps need not be unsightly if you follow the rules for clean and rapid composting.

Aerobic composting is rapid and is neither smelly nor slimy.

The bacteria and fungi that generate your compost efficiently need air, moisture and warmth to be their best selves.

If you resolve to provide the right conditions, you are not only recycling efficiently but getting a product every good gardener wants and needs.