Active ingredients

Compost activators are materials intended to make up some deficiency – usually of nitrogen – in the compost heap. Some proprietary compost activators claim to contain micro-organisms, but there should be plenty of these in the compost heap anyway.

Direct contact between your heap and the soil will speed up colonization of the heap by bacteria and soil animals. To be absolutely sure, add a spadeful of soil or compost from an existing heap. Cheapest and best of all activators is human urine, applied directly or diluted and added via a watering can.

Something all activators contain is nitrogen. However, if you follow the high-fibre method, your heap will already have the right carbon:nitrogen (C:N) ratio and no extra nitrogen will be required. If your compost heap does contain too much nitrogen, the excess will simply be broken down and lost as gaseous nitrogen or – worse still – as ammonia, causing a nasty smell.

However, if you are composting low-nitrogen material such as hedge prunings or autumn leaves, extra nitrogen will probably speed things up. We’ll consider the best way to add this when we look at the practicalities of making compost. As we’ve seen, many compost materials that lack nitrogen are also low in calcium, and some proprietary activators do contain garden lime. Again, the high-fibre heap doesn’t require extra calcium, but the breakdown of naturally acidic materials like woody waste, autumn leaves, and (especially) conifer prunings will be accelerated by the addition of lime.

Good activators Ammonium sulphate (cheapest option); comfrey leaves or liquid feed made from comfrey (organic); dried blood (organic); lime (ground limestone); nettle leaves; fresh or pelleted poultry manure (organic); proprietary activator; spadeful of soil or compost; urine.