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A Quick Guide to Breeding Compost Bacteria

A pile of sweet-smelling, rich, crumbly brown compost is a real treasure for a gardener. Not just because it is so wonderful to have, but also because it costs so much to get! For pile of ready to use compost I budget about four hours to accumulate all the ingredients. One hour to construct the pile, and one hour each time to turn it – a total of 8 hours labour. If I paid myself wages, that pile of dirt would be worth several hundred dollars.

Compost making is so labour intensive that sheet mulching, chooks, and earthworms are better strategies for the bulk of the soil building you need to do. But compost making is worth the effort to have the result for giving seedlings a good start, and as a side benefit you can use it to clear and prepare new beds.

The trick to making good compost is to be aware that it’s a living culture, like yoghurt. What you are actually doing is breeding up a particular species of bacteria – one that’s very good at turning organic matter into giant molecules of humic acid, without losing any elements along the way. Once you get the idea that you are dealing with living creatures, it all begins to make sense.

Lots of species of creature will get into a pile of dead stuff, but most of them are not good at turning it into compost. You are aiming to give a competitive advantage to the species that do the best job of adding value to the material you spend so much effort collecting.

The kind of bacteria you want need air, moisture, lots of carbon-rich food, not too much nitrogen-rich food, micro-nutrients, an acid environment, and heat. If you set up the right conditions, you’ll get the right creature.

  • The moisture comes from wetting down the pile as you make and turn it. It should be damp like a squeezed out kitchen sponge.  If it dries right out, they will die.
  • The air comes from using some ingredients that are bulky and have air pockets, rather than those that will matt down. Turning the pile also helps mix in air. The bacteria will drown though if the pile is so wet there is a shortage of air.
  • The carbon rich food comes from plants, anything except the fruit or growing tips. Dryish or woody plant material like hay or raked leaves or practically anything you can put through a mulcher. This sort of material should make up about two-thirds of your ingredients.  Too little of it and your pile is likely to be occupied by micro-organisms that are not good compost makers, and will instead release lots of those elements you collected into the air (with accompanying smells).
  • The nitrogen comes from green, growing plant material, like lawn clippings or comfrey, or from legumes, or from kitchen scraps, or from animal manure. This kind of material should make up about one third of your ingredients. Even if you have lots of high nitrogen green material, a little animal manure is better than none, since it also has starter cultures of bacteria, and enzymes to get them going.   Too little of this material and your creatures will breed too slowly to hold the territory.  The carbon and the nitrogen rich ingredients need to be mixed up or layered. Your micro-organisms will breed out from the edge where they can get both.
  • The micro-nutrients come from mixing in as big a variety of materials as possible, including seaweed, waterweeds, herbs, and weeds.
  • The acid environment happens of its own accord, so long as you don’t add lime, dolomite or wood ash to the pile until the compost bacteria have done their bit.
  • The heat is generated by the compost micro-organisms themselves, but if your pile is too small, or not a nice compact dome shape, it won’t keep the heat in enough, and the right kind of bacteria won’t be able to live in it. You need at leas two box trailer loads of material, all at once, to make good compost, and its more fail-safe in summer when heat conservation is easier for the creatures to do. If you can’t get this amount of material together at once, maybe worm breeding is a better idea?

That’s the recipe. I’d love to hear how you get on, and your tips and tricks.