Compost is the backbone of your soil. It adds nutrients to your soil and creates optimal soil structure to grow healthy plants.

For the science of hot compost to work, it’s a good idea to have two bins which are 1x1x1 metre in dimension. Only if a heap is at least one metre high and wide will the heat build up in the centre to the 70 degrees required to cook your compost. Halfway through the process you turn the decomposing compost into the second bin to aerate it.  BUT, it does also work by creating freestanding heaps of the same dimensions.

To make hot compost you need…

1 nitrogenous material: kitchen scraps; lawn clippings; chicken poo; animal manure; blood and bone; sheep pellets
2 carbonaceous material: dead stuff like trimmings from the perennial garden which are now brown; dead weeds; straw; egg cartons (which have air pockets and therefore allow bacteria and fungi to survive); newspaper (always roll up into balls to contribute to aeration)
3 activators: specific weeds or herbs like borage, stinging nettle, yarrow, tansy, chamomile; coffee grounds; volcanic basalt rock mixed with seaweed and worm castings (commercially known as Nature’s Organic Fertiliser from Environmental Fertilisers); wood ash

The proportion of nitrogen to carbon is around 30 parts carbon to 1 part nitrogen. This is achieved not so much by bulk, but by concentration eg straw is 90 parts of carbon to 1 part nitrogen; egg cartons and newspaper are around 150 parts carbon to 1 part nitrogen; sawdust is 500 parts carbon to 1 part nitrogen.

You’ll need to create a heap for nitrogenous material and a heap for carbonaceous material prior to your making a hot compost. Many people don’t like this part of hot composting because the heaps are perfect places for rodents to nest in. Black bins with a lid on are potentially one way round this, but our recommendation is not to leave a pile or bin for more than a month without using it or turning it.

What we’ve been calling the nitrogenous heap should more accurately be called the biomass heap. We need biomass for bulk. As we said before, unless a hot compost is about 1 metre high and 1 metre wide it won’t reach the required temperature to ‘cook’.

Start your hot compost bin by aerating the soil underneath with a fork.

1 The first layer is carbon – spent weeds and garden trimmings. Then…
2 Grass clippings – make sure they’re sprinkled on rather than laying them down in big clumps, because massed lawn clippings can cause an anaerobic reaction (no air) and create a putrid smell as they rot.
3 A layer of egg cartons.
4 A layer of biomass.
5 Activators in the form of weeds/herbs – borage, stinging nettle, yarrow, tansy and chamomile; also volcanic rock dust, seaweed and blood and bone.
6 Soil from the garden or compost from another compost bin – full of bacteria, fungi and microbes which will help break down the other material.
7 Repeat the process – carbon, biomass, activators – until your heap is 1 metre high.
8 The last thing we add is garden soil or compost from another bin.
9 Then water your heap for about 5 minutes, and cover it with a tarpaulin.

Over the next 48 hours the temperature should be up to 70 degrees. If it’s not, add more water and/or activators. Within a week it’ll have broken down by half, and in two weeks it’ll be down to a third of its original size. At two weeks it’s time to turn the compost into another bin. After three weeks it’ll be ready to use!

CARBON : NITROGEN RATIOS

Production: Ollie Logan, Vince McMillan and James Watson

see the video on Worm farms, Hungry Bins, Bokashi bins and How to make Cold Compost next