Since many years, bark has been additive for potting soils. 
In the past the Pinus sylvestris was used as it is plenty available. The disadvantage of this bark is the stability: as it is a rather weak bark, it breaks down rather fast and this is something we don’t like in potting soils. During the decomposing of the bark, two things happen simultaneously:

  1. The structure will be finer and therefore the potting soil will lose air porosity and will stay wet for too long;
  2. The bacteria will use the nitrogen so N will be fixated in the bacteria, which eventually can 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.
As some years ago problems occurred with bark from Portugal (they found the Bursaphelenchus xylophilus) the restrictions for exporting have increased. 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.

If you have any questions feel free to send an e-mail.

Pinus sylvestris
Pinus maritime