The efficient compost heap

One of the best soil conditioners is garden compost, and it's free. Added to your plot it makes clay soils more workable and in sandy soils helps retain moisture and nutrients.

Autumn is a time when compost heaps start to bulge as gardeners tidy their plots. Compost heaps generally come in two types - those that work efficiently and quickly turn vegetation into compost, and those that take an age. Understanding how the composting process works is the key factor in making your heap efficient.

The most important part of the composting

process you will never see, the countless billions of microorganisms working on your

behalf. Look after these and your stack of plant material and kitchen waste will quickly transform into well-rotted compost, a perfect soil conditioner.

It is essential to provide microbes with the right conditions to carry out their work. Firstly, like us, they need a balanced diet. The balance in their case is the proportion of foods high in carbon to those rich in nitrogen. Carbon (C) provides microbes with energy while nitrogen (N) is used for biological processes. Theoretically, the magic ratio to aim for your compost heap is 30 parts carbon to 1-part nitrogen (C:N = 30:1). If the C:N ratio is too high (i.e. too much C) decomposition becomes terribly slow. However, if the C:N ratio is too low, (i.e. too much N) the nitrogen will vaporize as ammonia gas and your heap will start to have an unpleasant smell.

But don’t worry, calculating the exact ratio of a heap is practically impossible. You would need to be a mathematician and know the exact C:N ratio and weight of each item added to the heap, and if we were to do that nobody would ever make compost! Fortunately, common sense saves us with this guideline: ‘mix the heap to mix different compostable items’. This is possible by knowing which commonly composted materials are considered high in carbon or nitrogen. Horse manure and fruit waste are naturally around the ideal C:N ratio, but you wont be that fortunate with most of the items added to your heap.

Compostables increasingly high in nitrogen: coffee grounds > cow manure > poultry manure > vegetable waste > grass clippings

Compostables increasingly high in carbon: leaves > pine needles > straw > newspaper > bark > sawdust > paper

The second important factor is temperature. Heat is generated as micro-organisms decompose plant material. 43-60°C is required for the best composting rate to be achieved. Between these temperatures, weed seeds and most diseases cannot survive. It is important that piles are turned frequently to ensure that all parts are exposed to these high temperatures. Thirdly, turning the heap allows air to enter, which the microbes need to survive.

Finally, keep the compost moist, especially during summer because decomposition can use up to two-thirds of the original water content.

By broadly following these guidelines, your heap will become efficient and provide your garden with wonderful free compost.