Recipe for Hot Compost

Why compost?

Healthy soil depends on having soil microflora and microfauna, known collectively as the ‘biomass’ For healthy plants, the biomass below the soil must exceed the weight of living things above the soil. Humus (the end result of composting):

  • sustains the biomass
  • improves the water-holding capacity of soils
  • insulates the soil
  • improves the structure of the soil which in turn increases the depth of oxygenation & capacity for plant roots to forage more efficiently.

Organic Gardening, by Peter Bennet, National Book Distributors 1992

Why hot compost?

Hot composting is a structured way of composting that provides the right conditions for thermophilic (heat loving) micro-organisms. It takes a bit more preparation than other methods but produces beautiful compost in a fraction of the time. It also destroys weed seeds & diseases.

Principles of good composting

  • Keep it moist, but not wet
  • Keep it insulated, especially against wind
  • Keep it aerated to maximise aerobic bacteria
  • Keep the ratio of carbon:nitrogen at about 25:1 (but don’t get hung up on this)
  • Variety is best – the greater the variety in your source materials, the greater the variety of nutrients in your final compost

Ingredients

The two most important ingredients for creating the right conditions for a hot compost heap are nitrogen and carbon. You’ll need a large volume of material that contains a high carbon content (e.g. autumn leaves, straw etc) with a smaller but essential volume of material that contains a high nitrogen content (e.g. animal manures, blood and bone). Apart from carbon and nitrogen, a healthy heap needs water, a variety of trace elements and some healthy garden soil to inoculate the heap with a broad spectrum of soil micro-organisms.

Here are some suggestions for ingredients:

  • Carbon-rich materials
  • autumn leaves – broad flat leaves should be shredded. We leave ours with the chooks for a few months
  • straw, old grass clippings, mulched garden waste
  • include some coarse materials (eg shredded sticks) to keep the heap open/aerated
  • Nitrogen rich materials
  • manures of any kind
  • urine
  • blood and bone
  • Mixed Carbon/Nitrogen materials
  • green grass clippings, vegetable/kitchen wastes (variety is best)
  • Potash, rock powders etc (but avoid use lime or dolomite as you will lose nitrogen as ammonia gas during the composting process)
  • Water
  • Air

Hardware

  • A ‘container’ between 1 cubic metre(1m x 1m x 1m) and 2.25 cubic metres(1.5m x 1.5m x 1.5m). We use straw bales because they have ideal insulating properties. You could use any form of sheeting.
  • Perforated agricultural pipe (agi pipe): this allows air to circulate through the heap.
  • Tomato stakes: we use these to make chimneys with the agi pipes (for aeration)
  • A cover – again we use straw, but you could use an old blanket, hessian or under felting.

Tools

Fork (a ‘mulching fork’ is ideal), rake, shovel, hose with spray gun, wheelbarrow.

Method

  1. Prepare good drainage at the base of your heap, by either roughly digging the soil and laying down about15 cm of coarse material, such as sticks, OR by laying a couple of lengths of agi pipe across the base, & protruding from the heap.
  2. Make a few ‘chimneys’ exceeding the height of the heap. We use agi pipe threaded over garden stakes. Tip: cover the open end of the pipe to avoid filling it up as you make the heap.
  3. Make your first layer of about 15 cm of plant material (anything from the list of carbon-rich materials). It should be moist, but not wet. If the material has become really dry, then rake it through as you wet it to avoid dry pockets.
  4. Make your second layer of about 5 cm of manure (or as little as 1cm if using a stronger manure such as pigeon).
  5. Make your third layer a thin layer of soil and some blood and bone.
  6. Repeat the three layers until the heap reaches the desired height. Water between layers if the materials are too dry, using a rake to ensure that the layer is wetted evenly. Avoid water-logging the heap.
  7. Aim to lay some more agi pipe horizontally about every 40-50 cm.
  8. Finish with a layer of soil. Cover to keep the heap well insulated.
  9. The heap will reach a thermophilic stage in a few days, reaching a temperature of between 66 and 72. This temperature will be maintained for several days, and then it will slowly cool over a matter of weeks. This temperature will knock out all pathogens and weed seeds.
  10. The compost will be ready to use in about 2-3 months, but this can be accelerated if you want to turn it regularly.