Fast Hot Composting System
At Strawberry Fields Eco-Lodge (SFEL) we use a fast hot composting system that can deliver well decomposed compost within 3 weeks. It was developed based on the technique we we’re taught by Dan Palmer when he co-facilitated two PDCs with us in 2008 along with Rosemary Morrow.
Hot composting is an aerobic process of fast oxidation which breaks raw organic materials into humus at temperatures of up to 80°C within three weeks. It is performed by a particular type of bacteria, that you can recognise as a white crust which starts to appear on the materials within the steaming interior heap once you really have the process working. I am not really up on the exact biological details of the bacteria, whether it is just one species or there are a range of species which can do the job, but once you have it working you have to maintain it, a bit like a culture of yoghurt, to get the best results. Like any living organism the bacteria has an ecological niche, that is to say a specific range of conditions in which it can live and within which it can thrive, so we have to maintain those as best we can if we want the organism to do this job of producing compost for us as best it can.
The bacteria works best at high temperatures, with good availability of oxygen and a decent level of moisture. If the heap is water-logged however then there will not be enough oxygen available and it will go anaerobic and hence rancid. In order to ensure adequate oxygen supply we should keep the heap small enough that the surface area to volume ratio is adequate for air to reach all parts of the heap as required by the bacteria. On the other hand it should be large enough that it will not lose heat too quickly and hence fail to reach the 80°C the bacteria needs to work at full whack. The ideal compromise between these two is a size of one cubic meter: a meter wide, a meter high, a meter deep. Dan originally showed us how to do it using a heap above the ground, but we found that in our dryland situation this tends to dry out too fast. So we have modified this by digging a series of pits which are 50cm x 1m x 1m so that the heap is half under-ground. The pits are behind a terrace wall and under a tree which means they are sheltered somewhat from the wind and the sun, so they don’t dry out. UV light from the sun also kills bacteria, so we want to keep the sun-light off.
The process of composting is as follows: Three components go into the mix – kitchen wastes, crop trash/dry grass (we use these first to mulch our paths, so that they are crushed to a ragged pulp, which is then collected and replaced with fresh mulch) and animal manure (which we get from the local villages where they are throwing the stuff away). The materials go into the first heap in layers of 3:2:1 ratio. First of all a lattice of sticks is laid down on the floor of the pit, which lets air circulate under the heap. Then three layers go on top of the sticks: 15cm of crop trash, then 10cm of kitchen waste and finally 5cm of animal manure. Those three layers are repeated twice more to make the heap 1m high. The whole thing is lightly hosed, continuously, as it is laid down so that it is wet-through but not soggy and it should not be sitting in a pool of water. Lastly, holes are punched from the top of the heap to the bottom with a long sharp stick, which allows the hot air to rise from the bottom of the heap to the top sucking in fresh air from below and from the sides. Then the heap is covered with a layer of dry (unchopped) grass and then card-board (old boxes) or news paper, which keep the wind and the sun off while insulating it against water and heat-loss. The reason for the layering is that the bacterial likes a specific ratio of carbon to nitrogen and the layers of dry grass, wet organic material and manure are correct for this.
To make sure the bacteria are present we stick in a bit of material from an older heap in the middle. And within 12 hours the heap should have heated up to full temperature if all conditions have been arranged properly. The heap is allowed to sit and digest for 3-5 days. After this it will be turned. We turn it because we want uniformly decomposed seed-free compost. The material in the middle gets the most heat and will break down the fastest, and the seeds of the weeds or whatever is in the kitchen waste will be killed by the heat at the centre, but not at the edges. So each time we turn the heap we put the edges to the middle as well as the top to the bottom and vice versa. We have seven pits in our system, so by the time the compost comes out of the seventh pit it is three weeks old and it is already a lovely rich dark light and fluffy humus. We mix it with silt for potting, for filling tubes in our nursery and put it down on our vegetable beds. What fantastic stuff fresh compost is!