Compost is the foundation to a thriving garden. The trouble is, few of us have the perfect conditions to make ideal compost every time. We’ll look at some common compost problems, and easy ways to solve them.

The Perfect Compost

Perfect compost has a fine, crumbly texture and pleasant, earthy smell like a forest floor. The original ingredients used to make the compost will no longer be visible, having been transformed into dark-looking organic matter with an even consistency. Mature compost is gardeners’ gold. Use it to mulch around plants, make potting soils or to dig into soil to improve its nutrient content and moisture-holding capacity.

For instructions on how to make good compost by adding the right balance of ingredients, read our article How to Compost – Easy Steps to Success.

Compost Problems

Few gardeners get composting right every time. Common problems include smelly compost bins, slimy ingredients that have become excessively wet, or compost that has simply stopped rotting down before it's ready.

Problem 1: Too Wet

The most common problem is excess moisture, which causes foul odours, flies, and the production of substances harmful to your plants. Adding too much fresh material, instead of a balanced mix of fresh and dry materials, is the usual culprit.

Fresh materials such as vegetable peelings and grass clippings have a high water content, which makes them heavy. If too much is added to your compost heap at once it can become compacted, excluding air or filling air spaces with water. These oxygen-starved ‘anaerobic’ conditions enable harmful microbes to thrive – the same microbes responsible for creating the unpleasant smells that arise from such putrid conditions. Fresh materials are mostly 'greens' which have a high nitrogen content, so mixing in more carbon-rich 'browns' will help solve the problem.

If your compost heap is too wet, dig it out completely, then turn the ingredients to incorporate more air before restacking. Add dry materials into the mix to get a balance of greens and browns, improve drainage and prevent the compost from clogging up again.

Ingredients such as shredded prunings, sawdust, straw and cardboard torn into smaller pieces will create channels within the compost that allow air to percolate and excess moisture to drain away. Scrunched up newspaper makes a good short-term option if you haven’t got enough of these dry ingredients to hand.

Problem 2: Grass Clippings

Grass clippings are often generated in large batches. Don’t be tempted to add thick layers to the compost bin just to get rid of them or they could create a soggy mat. Instead, sprinkle grass clippings in thin layers and balance them with dryer ingredients. If you have too many clippings lay them as a mulch around fruit trees and bushes where they will slow down weed growth and lock in soil moisture.

Be aware that enclosed plastic compost bins let less air in than open heaps. They require extra care to ensure a balance of dry to fresh materials. Never stamp or force materials down in order to fit more in, or you run the risk of over-compacting your compost ingredients and artificially stimulating anaerobic conditions.

Problem 3: Too Acid

Compost is naturally slightly acidic but sometimes an abundance of some 'wetter' ingredients can upset the balance. This can cause the compost heap to become a bit smelly and slow to decompose.

Acidic ingredients such as citrus fruit can also contribute to an excessively acidic compost bin. Counteract the acidity by sprinkling handfuls of ground lime or wood ash into the mix, plus plenty of 'browns' if the bin is wet and other fresh, green material to kick-start the composting process again.

Problem 4: Too Dry

If your compost bin is too dry it will stop decomposing as the bacteria and fungi responsible for the composting process won’t be able to work effectively. Re-wet the heap by watering it - ideally with rainwater, but if you don’t have any stored rainwater ordinary water will do. Apply it evenly using a watering can fitted with a rose, mixing the materials at the same time if you can.

Bins with too many dry materials can be given a boost by adding lots of fresh material to balance out the ingredients. Dig out the compost bin, add your fresh materials then refill the bin. Or, if you have two bins side by side, mix the extra materials and water as you turn the materials from one bin into the other.

The exception to the rule is leafmould, which is a form of compost made entirely from fallen leaves. Leafmould naturally takes up to three years to fully mature before it’s ready to use, by which time the leaves should no longer be visible.

Using Compost

Mature compost can be sieved into sturdy plastic bags or garbage cans for storage. Any lumps or part-rotted materials left behind in the sieve may be thrown back into an active compost bin to continue decomposing, helping to transfer beneficial microbes into the next batch.

To make a general-purpose potting soil suitable for growing most vegetables, mix two parts compost to one part sieved garden soil and, for added drainage, one part vermiculite. For a potting soil suitable for containers and window boxes combine two parts soil to one part compost and one part sharp sand. Garden compost is likely to contain weed seeds, so to avoid a flush of weed seedlings, cap your home-made mix with a layer of sterile potting soil.

Compost is an excellent soil amendment. Apply it directly to beds and borders. Fork it in or leave it on the soil surface as a mulch. This valuable organic matter will work gradually to improve your soil’s nutrient content and overall structure, resulting in healthier plants and better crops for you.