What Is Compost and Why Does It Matter?

Compost is decomposed organic matter — a dark, crumbly, earthy-smelling material that acts as a soil conditioner, slow-release fertiliser, and habitat for beneficial soil organisms. Adding compost to your garden improves soil structure, drainage, moisture retention, and long-term fertility. It's the single best thing you can do for your growing space.

The Science Behind Composting

Composting relies on microorganisms (bacteria, fungi, and actinomycetes) breaking down organic material. These microbes need four things to thrive:

  • Carbon-rich "browns" — cardboard, paper, straw, woody prunings, dried leaves.
  • Nitrogen-rich "greens" — fresh grass clippings, vegetable peelings, coffee grounds, fresh plant material.
  • Moisture — the heap should feel like a wrung-out sponge.
  • Air — oxygen is essential for aerobic decomposition.

The key is balance. A rough ratio of 2–3 parts browns to 1 part greens by volume works well for most home composters.

What Can (and Can't) Go In

Good Compost Materials

  • Fruit and vegetable peelings
  • Tea bags and coffee grounds
  • Grass clippings (in thin layers)
  • Fallen leaves
  • Cardboard and newspaper (torn up)
  • Eggshells
  • Annual weeds (before they set seed)
  • Soft prunings and plant stems

What to Avoid

  • Cooked food, meat, fish, and dairy (attract pests and create odour)
  • Diseased plant material (diseases can survive composting)
  • Perennial weed roots (e.g. bindweed, couch grass) — these can regrow
  • Dog or cat faeces
  • Glossy magazines or heavily inked paper

Setting Up Your Compost Bin

You don't need an elaborate setup. A simple wooden pallet bin, a plastic compost tumbler, or even a heap in the corner of the garden will work. Aim for a minimum size of around 1 cubic metre — smaller heaps struggle to retain enough heat to decompose efficiently.

Site your bin on bare soil rather than paving, so worms and beneficial organisms can move freely in and out from below.

Building and Managing the Heap

  1. Start with a layer of coarse material at the base (wood chip, twigs) to aid drainage and airflow.
  2. Add materials in alternating layers of greens and browns.
  3. Keep the heap consistently moist — cover it in dry weather to retain moisture, and uncover it in heavy rain to prevent waterlogging.
  4. Turn the heap every 4–6 weeks to introduce air and speed up decomposition. Move material from the outer edges to the centre.
  5. Avoid adding large quantities of any single material at once — thick layers of grass clippings, for example, will mat together and become slimy.

How Long Does It Take?

A well-managed heap that's regularly turned can produce usable compost in as little as 3–6 months during warmer months. A less actively managed heap may take 9–12 months or longer. Cold-process composting (leaving it undisturbed) is slower but requires almost no effort.

How to Know When It's Ready

Finished compost should be:

  • Dark brown to black in colour
  • Crumbly and loose in texture
  • Sweet and earthy in smell — not unpleasant
  • Free of recognisable original materials (apart from some woody bits)

Unfinished compost can be left to continue decomposing, or used as a mulch on the surface where it will break down further over time.

Using Your Compost

Dig finished compost into vegetable beds in autumn or spring, spread it as a mulch around fruit trees and shrubs, or mix it into potting compost for containers. Your plants — and the soil ecosystem beneath them — will thank you for it.