Plant Doctor Archive

Smelly and gluggy compost heap

Earlier this year I purchased a composting bin and thought this would be a more environmentally friendly way of disposing of garden waste, vegetables, etc (as well as adding to my gardens). However, nine months on my compost is gluggy, smells foul, and looks more like something the cat dragged in.

A compost pile needs four components: carbon, nitrogen, oxygen and moisture. The interaction of these components, plus the heat generated by microbe activity, generates the breaking down of organic matter.

The first two components can be found in organic materials. 'Greens', for example, provide nitrogen — fresh grass clippings, plant trimmings, green leaves, and fruit and veggie scraps. 'Browns' provide carbon — sticks, twigs, sawdust, hay, pine needles and wood ash. Too much nitrogen and your compost pile will turn to mush. Too much carbon and the decaying process will be hindered. A good compost pile requires one part green to about three parts brown.

The next ingredient, oxygen, is absorbed from the air and enables micro-organisms to decompose the material. Micro-organisms require a continuous supply of oxygen, and for a compost heap to break down rapidly, air needs to be able to circulate freely. This is achieved by turning the pile regularly and using compost bins with holes for aeration. If a compost heap begins to smell, there is not enough oxygen in the pile.

Water is another necessary ingredient, but if the compost heap is too wet (or too dry, for that matter) the organic material cannot decompose. Too much water will decrease the amount of oxygen in the pile (and thus make it smelly), so keep the pile covered to avoid excess rainwater getting in. The pile should be damp but not soggy.

Mix in more brown material, and apply some garden lime or dolomite to 'sweeten' your heap. Lime reduces the acidity (and smell) caused by excessive nitrogen-rich materials.

When adding nitrogen-rich material to your heap, mix with dry leaves, straw, sawdust or other 'brown' material — even shredded newspaper if others are not readily at hand.

A clever trick to allow air to the centre of your heap is to add a cylinder of chicken wire, about 15 to 20 cm in diameter, and tall enough so that it extends above the heap. However, if you're using a commercial compost bin with holes, this should not be necessary.

Advice by Dr Dan Blanchon from Unitec's Diploma in Sustainable Horticulture and Bachelor of Resource Management.

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