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By Charles Merfield

Tanalised pine posts are a mainstay of New Zealand agriculture and are also widely used globally in agriculture and for many other purposes, e.g. buildings.  Although tanalised wood has been in use since the 1930’s and has been widely considered to be safe, there is growing concern about leaching from posts into the soil and wider environment, especially where posts are used in high densities, for example, vineyards, and orchards (e.g. see [1, 2]).  The tanalisation preservative is based on copper chromium arsenate (CCA, also called chromated copper arsenate), a chemical combination of three heavy metals (copper, chromium and arsenic) which are inherently toxic, and as chemical elements they cannot be broken down.

There are also issues with disposing of tanalised timber at the end of its life: burning treated wood produces toxic compounds (the heavy metals being chemical elements cannot be destroyed), so treated wood should only be burnt in specialist facilities and the ash contains high levels of the metals so it must be disposed of correctly, typically in landfill.  Unburnt timber should also only be disposed of in properly managed landfill.

In addition, the USA and Canadian government’s organic standards prohibit the use of tanalised posts, and a range of similar preservative treatments, so any organic producers wishing to export to North American markets cannot use treated posts.

Alternatives to tanalised posts are therefore becoming of increasing importance.

In 2010, when the national grape cultivar library vineyard was being established at the BHU (see the 2013 V1 Bulletin article) the USA organic standards had just brought in the tanalised post ban.  As part of the work undertaken by Organics Aotearoa New Zealand (OANZ) identifying alternatives to tanalised posts (including the ‘Over the Fencepost’ booklet) the grape cultivar library vineyard was used as a demonstration site for a number of the non-tanalised post alternatives.

The post collection is a demonstration rather than a trial, so no comparisons are being made among the different posts nor tests of their performance.  Also inclusion of any particular post is not an endorsement of that post by the BHU or FFC nor is the non-inclusion of any post a rejection of that post by the BHU or FFC. However, anyone is welcome to view and assess the posts for themselves during normal working hours.  The vineyard is sited at The BHU Farm, The Hort Research Area, Lincoln University.  Google map. You can also email [email protected] for more information.

If you need specific information on what post treatments and alternatives are acceptable under USA USDA NOP rules, ATTRA have produced a pamphlet “Pressure-Treated Wood: Organic and Natural Alternatives“.

Below is a list of the posts in the vineyard and recent photos that show the posts four years after establishment.

UltraPost (row 1), Industrial Tube Manufacturing Co Ltd

References

1.    Clothier, B.E., Green, S.R., Vogeler, I., Greven, M.M., Agnew, R., van den Dijssel, C.W., Neal, S., Robinson, B.H., and Davidson, P., CCA transport in soil from treated-timber posts: Pattern dynamics from the local to regional scale. Hydrology & Earth System Sciences Discussions, 2006. 3: p. 2037-2061.

2.    Greven, M., Green , S., Robinson , B., Clothier, B., Vogeler , I., Agnew, R., Neal, S., and Sivakumaran, S., The impact of CCA-treated posts in vineyards on soil and ground water. Water Science & Technology, 2007. 56(2): p. 161–168.

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