Bark

Since many years, bark is an additive for potting soils. The bark of the Pinus sylvestris is used as it is plenty available, but the disadvantage of this bark is the stability: as it is a rather weak bark, it breaks down rather fast, something we want to avoid in substrates. During the decomposing of the bark, two things happen simultaneously:

• The structure will be finer and therefore the potting soil will lose air porosity;
• The material will stay wet for too long and the bacteria will use the nitrogen so N will be fixated in the bacteria.

This can eventually hurt the culture, as there is often a lack of Nitrogen found.

For these reasons a bark from the Pinus maritima is used. This bark, with a more red colour, is much stronger than other types of bark. Therefore the decomposition is slower and the retraction of the Nitrate is reduced.
The origin of the Pinus maritime is mainly the south west of Europe. The Landes de Gascogne (just below Bordeaux, France) is famous for the culture of this tree. You can also find this tree in Portugal and Spain (and smaller amounts in Italy and Croatia).
Finally the bark was a rest product, used for the ceramic industry (heating), but in the last decades this bark became more popular for orchid growers, tree nursery and also for mulching gardens and others.

Now that there are more possibilities (more fractions are available) the bark is used in many types of potting soil, also because the demand for peatfree potting soils is growing and people are looking for alternatives.
As some years ago problems occurred with bark from Portugal (they found the Bursaphelenchus xylophilus) the restrictions for exporting have increased.

The sanitation of Bark is very important.

At the moment the obligation is to heat the wood up to 60oC in a special chamber and although the rule was for some time for the bark the same temperature should be reached by self-heating (like compost factories do), nowadays the bark is also steamed. After this procedure, no infection was found and therefore the bark is safe to use. The reasons to use bark in potting soils can vary. When Bark is used in potting soil mixes for Orchids, this makes sense because an Orchid is a Epiphyte which means the roots need “humid air” as they usually grow against trees etc. Therefore bark is often used in mixes for Orchids, sometimes mixed with course coco chips or Sphagnum moss. Another reason can be the demand for “peat-free potting soil”. Then alternatives like coco, compost and also bark are rather popular.

This is just a brief note.

If you want to know more about the origin, the fabrication, the use, the advantages and disadvantages of bark for you: don’t hesitate to write us a mail at info@substrate-consulting.nl