By Iain Tolhurst

‘Tolly’ of Tolhurst Organic Produce is an Organic Consultant and Horticultural Adviser; he writes from his Berkshire farm which produces 120 tonnes of stockfree vegetables per year for a local box scheme. (www.tolhurstorganic.co.uk).

Woodchips don’t have a good reputation as a source of fertility within organic circles. I had a negative experience with them myself back in the early 80s when we were in Cornwall growing strawberries. I came across a lovely big pile of woodchips at a local joinery workshop, mostly pine, great smell. Knowing that strawberries benefited from a dose of pine needles (a little knowledge can be a very dangerous thing) I assumed that pine wood ought to do roughly the same sort of job and keep the weeds down as well; we had always used straw under the fruit to keep it clean. The woodchips were much easier and quicker to spread, looked nice too and did a brilliant job of keeping the weeds down; the resultant crop was looking very good. That was until we tasted them – they had a flavour like pine scented Jeyes fluid. The resins had got into the ripening fruit and they were unmarketable. Fortunately only a section of crop was treated in this way so we managed to remain sort of solvent, but the worst was yet to come.

The crop was turned in, having got to three years old, along with all the woodchips. Stupid thing to do! I had an idea that high carbon materials were not too good for nutrient availability, but something drove me to get the whole lot out of sight, so an expedient method seemed to be incorporation, followed by a green manure of mustard and rye sown in August. The green manure germinated normally but by the end of autumn it was clear that not all was well. The crop was yellow and stunted; the effect was in strips coinciding exactly with where the woodchips had been placed, with lush green strips in between that had previously been pathways. This effect was clearly the locking up of N and it continued for a further two seasons. Clover eventually put things right but it caused a disruption in the rotation; not that we had a particularly clever rotation in those days.

A lesson learned

Cupressus Leylandii: it might need lopping but the loppings can be put to good use. From commons.wikimedia.org

It was a lesson that taught me a great deal but it also made me wary of woodchips as a source of organic material. I kept clear of it for several decades after that, with the exception of surface mulching perennial crops, for which it is perfect. Allowing the earthworms to process it slowly avoids any issues with N lock-up. Over the past few years I have got back into woodchips as a raw ingredient for compost. As with many things in life this just sort of happened without too much design, when the material became available in reasonable quantities from a tree surgeon friend. You know how it goes: evening party, a few drinks and things, and you get unusually co-operative and friendly. ‘Hey Tolly I’m having to pay to tip my woodchips at the local dump, how about me putting them in your field when I pass by?’ So I reply ‘Yes of course, no problem at all’ not really thinking it through and assuming that the beer is talking and it is unlikely to happen. Next morning there it was – a huge pile of wood chips steaming away in the entrance to my field. My mate Ian works locally felling and tidying people’s trees, mostly ornamental stuff with plenty of Cupressus Leylandii. Now I know why people grow it around their gardens. It’s not to screen out the neighbours – it’s for making compost! The material is dumped around 2 cubic metres at a time and comes with either a mix of species or just one type. The chips are usually quite small, long and narrow, with the largest at around 50mm. During the summer it tends to be around 50% green and 50% brown and during the winter this changes to predominantly brown, apart from the Leylandii. If it has any green in it at all it heats up rapidly, especially so with the Leylandii.

I leave the material for several weeks just as it has been dumped, in a long windrow. After around 6-8 weeks it is turned using a small digger with an extended bucket that speeds the job up. I can shift 50 cubic meters in a couple of hours. The material is always hot, although there will be areas that are very dry, especially summer material which heats up rapidly and tends to dry out. With mixing, the wet is turned into the dry material. In general it is too dry compared with traditional compost materials; this I initially thought was going to be a problem so I tried setting up an irrigation system over the heap. It was ineffective because the water tended to run right through the material and out the bottom, so I gave up.

Completing the process

During the summer I turn twice, and the same again during the winter. A total of 3-4 turns is enough to produce a very well broken down and friable material. The problem of dryness seems to disappear after the 3rd turning, the material becoming damp enough to break down. We also mix in other organic wastes, mostly crop trimmings from the packing, and clearouts from tunnels; this is loaded onto the top of the windrow and gets mixed in with the next turning. Temperatures can get too high, up to 70C or more, which has a suppressing effect on the micro-fauna and causes the heap to cool rapidly. The more frequent the turnings the faster the process would be and it ought to be possible to produce compost within 3 months; this however is a lot more work and I am quite happy to wait a year for completion. Once the windrow is started the process is a continuous one, with new material being added to the end of the heap and mature compost taken away. Heaps are usually moved sideways with turning, allowing easier access for the digger and more effective mixing, producing a more homogenous material.

At the end of the process little of the original wood is evident and the material is reduced by around 50% from the original; it is very similar to woodland humus. I have never seen any weeds growing within it, though mushrooms are common. The material weighs around 500kg /cubic metre. It is very easy to work with and for the smaller user would be very easy to turn and move by hand. Ideally the composting site would be on a concrete area, but we don’t have such a thing so it is on soil. I have never seen any sign of leachate from the heap so am not worried about pollution problems, and even in a wetter climate I think that is unlikely due to the very absorbent nature of the wood. As the wood breaks down it absorbs any available moisture. If a section of the heap is very dry I spread it out half a metre deep for a few weeks during the winter and let the rain soak it, before adding it back to the windrow. The windrow is 3 metres wide, 2 metres high and as long as you like, allowing plenty of air into the heap. I have never found a need to cover it as it generally needs the moisture from rain to aid the process.

Versatility and value

One of Tolly’s veg boxes

During the growing season 50 cubic metres per ha is spread, always onto green manure crops, which are the main fertility part of the rotation. This allows for a final breakdown of material on the surface, and incorporation by worms. It disappears rapidly into the soil and is much loved by soil fauna. It also gets used in polytunnels at a higher rate of around 200 cubic metres/ha, being spread during a vacant period every two years. We are now also using it as the main ingredient in substrate mixes. For this purpose it is loaded into bulk bags and allowed a final decomposition for a further 12 months to avoid any problems of nitrogen lock up. The results look promising.

The use of wood waste in this way has distinct advantages:

– It’s free

– It’s a local material

– It’s traceable

– It’s free of contamination since amenity trees don’t get sprayed

– It’s a good source of organic material

– It’s easy to handle either by machine or hand tools

N.B. You are supposed to have a licence to receive waste materials onto the farm. Contact your local authority for details.

This article appeared in Growing Green International magazine Num 28 (Winter 2011/12), p36. It first appeared in the Organic Grower magazine issue 13 (Winter 2010) – see www.organicgrowersalliance.co.uk. The process described here is different from and additional to that provided in this article on chipped branch wood www.stockfreeorganic.net/chipped-branch-wood