Composting Woodchip

The use of  composted wood chip and sawdust as a bulking agent in food composters is quite common and wood chip may be used as a source of carbon in conventional compost bins as well as being composted after use as chicken or pet bedding.

Fresh wood chip produced by landscape gardeners from woodland maintenance and tree surgery may also be available to allotment societies to make paths and any surplus could be used as mulch or to make compost.

 In addition to chippings from trees, branches (including leaves) wood chip is a term used to describe a range of wood products including sawmill residues and sawdust.

 However, concerns are often raised by home composters as to whether woodchip or wood-based compost is safe to use because of the risk of nitrogen depletion

It is true that wood chip consists mainly of carbon compounds (Browns) and the lignin and cellulose may take considerable  timeto be brocken down  by the composting fungi and bacteria. Wood chip only contains a small proportion of nitrogen and if fresh wood chip were added directly to the soil,  the composting microbes would take up nitrogen from the soil depriving growing plants of soil nitrogen resulting in the risk of nitrogen deficiency.

However, the answer is simple do not add fresh wood chip compost it first. It can then be used as soil improver or mulch. It is reported in some sources that nitrogen depletion is less significant where the wood chip is used as surface mulch.

If the wood from which the wood chip was made included leaves, it will contain more nitrogen but should still be composted before use.

The initial C: N ratio of a wood chip heap may be in the region of 150:1 but as composting progresses this falls to about 40:1 as the decomposition releases carbon dioxide to the atmosphere from the breakdown of carbon, so just leaving it in a heap has a positive effect. (Compost C: N Ratio  ).

Webber and Gee describe a technique for the composting of large quantities of wood chip that may be appropriate for community composting sites (https://www.trees.org.uk/Trees.org.uk/files/a0/a0b752e9-19e2-4f02-8530-dbb592074388.pdf)

 Under this technique, the wood chip is arranged in windrows up to 2m wide and up to 1.5m high; this size allows air to percolate through the heap, the top of the windrow cupped to help retain water. The wood chip is moistened by adding 30litres of water per cubic meter of woodchip to give a moisture content of 50-70% (Compost Moisture)

  The rate at which the chip decomposes can be increased significantly by the addition of a nitrogen source. If large quantities of wood chip are being composted this could be by the addition of Ammonium Nitrate, Ammonium sulphate or Sodium nitrate when building the windrows rather than adding “Greens” which we would recommend if home composting or using conventional bins on an allotment.

 If using windrows the temperature should be monitored (temperature   ) and the pile turned when the temperature declines below 50-55°C. Turning may be necessary at about two weekly intervals and should continue until turning does not result in an increase in temperature. Once the active stage is completed, it has been suggested that the immature compost is left for between 3 and 12 months to mature. However, to be on the safe side it might be better to leave it for one to two years and then leach it well before incorporating into the soil.

 One of the advantages of wood chips is that in smaller quantities it will compost aerobically on their own without the need for turning or aerating, providing the moisture level kept at a sufficient level throughout the pile.  This can be difficult as while the freshly chipped branches start with an adequate moisture content this will be reduced during the initial heating process.  Hosing the pile often results in the water running off and soaking the surrounding ground.

 In fact for wood chip to be composted effectively, and in a reasonable time in a home compost bin, it needs  to be mixed with a good supply of “Green” material, e.g. lawn cuttings, kitchen waste, chicken manure should result in the material   reaching the immature compost stage within a year.  Some composters overcome this problem by using a hot composting system, layering  the woodchip with freshly cut grass clippings or manure  to help retain moisture and provide a source of nitrogen rich Greens and turning it regularly during the initial stages. I would certainly recommend this variation in composting woodchip in a New Zealand or a plastic bin at home or on the allotment. It should then be allowed to mature after which it is dug into the soil.

 

 

Home Composting wood chip in a dedicated bin

If there is sufficient woodchip available to warrant the use of a dedicated “woodchip” compost bin, I would suggest using a double width pallet bin made using six large pallets where the rear, and front, of the bin are made from two pallets. In the six-pallet model, the sides consist of a single pallet each side but it is better to use eight pallets with sides, the front and back consisting of two pallets. In the case of wood chip, composting size does matte.

As a alterative to pallets a wire netting bin, similar to those used to make leafmould could be used again or even an old builders bag as used to deliver sand (these are non-returnable so you will be doing neigboroughs a favour by taking them ).

 

The woodchip will need to be soaked regularly so it is best to use a concrete or slab base or alternatively a thick plastic sheet so that the water can be drained by a gutter in the ground into a storage container sunk also sunk into the ground enabling it to be reused.

If making a pile solely of wood chips do so in layers, soaking each layer as it is added. When almost at the top of the pallet bin added wet autumn leaves from one of your “leaf mould” sacks.  The leaves should be soaked with water once added. Cover with a tarpaulin to help retain the moisture.  The bin will need soaking as the wood chip decomposes to maintain the moisture level.  The moisture level can be monitored by inserting the probe from a moisture meter through sides of the pallet bin.

Some report that the wood chip will decompose in a few months and while this may be true in the centre of the heap, it might be better to allow up to three years depending on the conditions.  

Composting sawdust:Sawdust and Manure mix

Sawdust being high in carbon will benefit from the addition of a source of nitrogen during composting such as grass clippings. An alternative on the allotment or vegetable garden would be farmyard or stable manure.

 A composting mix using pig manure and sawdust  using  manure: sawdust ratio of 4:1 was shown to be effective based by   Troy, S.M., Nolan, T., Kwapinski, W., Leahy, J.J., Healy, M.G., Lawlor, P. (2012) 'Effect of sawdust addition on composting of separated raw and anaerobically digested pig manure'. Journal Of Environmental Management, 111 :70-77.

There is no reason why a horse or cattle manure could not be used instead of pig manure.

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Composting Wood Chip and Sawdust from toxic trees and shrubs

More serious concerns about the use of wood chip are raised as to in respect to making compost using shavings, sawdust and shreddings from poisonous or toxic trees. The concern   being related to the effects on the person handling the raw material and whether the product will poison the plants or be taken up by them and affect those eating produce grown in the composted soil.

 Before looking at information on composting sawdust from toxic trees, it would be useful to note that is generally accepted that treated wood and sawdust from such wood is best not added to the compost bin as they contain metals. Similarly, painted wood is best avoided. 

 

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