Does Bark Wick?

In my various thoughts about feeding and watering orchids, the question about the delivery process often comes up.

In the articles about Instantaneous Nutrient Capture and Nutrient Uptake Rate, I wrote about roles of various parts of the roots – velamen, on in – at absorbing, capturing, and transferring water and nutrients to the plant. An aspect that had not been directly addressed, however, was how those stored in the potting medium are transferred to the roots.

Think of this scenario, using a bare-root plant as an example: if we water it with a “rain head” type device, the water droplets coming into contact with the roots are captured and absorbed, but the others pass right by and drop to the ground, being of no value to the plant. Shifting that scenario to include a potting medium, we now have moist medium to contend with, but the question remains as to whether a solution absorbed by the media particles ever reaches-, or in other words, is “wicked” to the roots.

As we have shown, several types of inorganic media, like lava rock, perlite (sponge rocka), pumice, and LECA (lightweight expanded clay aggregate), as well as sphagnum moss do a reasonable job of wicking solutions to the roots, but how about “bark”-based ones? With the help of a few other growers, I assembled a small array of such products; fir bark, “Kiwi Bark”, Orchiata, and some coconut husk chips (CHC).

The bark-based products are close enough in size to lend themselves to a reasonable comparison, while the CHC is considerably finer, but it won’t hurt to look at that, as well. Here’s how they were avaluated:

• Each product was placed in a clear plastic container while being jostled and “banged” on a coutertop to densify them, as one might do when potting up a plant.
• Product was continued to be added until the 500 ml volume was filled before capping the containers.
• Each was weighed to estimate a bulk density.

• 50 ml of water was injected into each container via a hole drilled in the side, two inches above the bottom.
• The holes were taped-over to seal them and the bottles allowed to sit at room temperature.

With the containers sealed, there is no chance for evaporation, so only absorption and wicking come into play.

After 4 hours, none of the samples showed any indication of wicking watrer upwards in the column:

In this case, the red lines indicate the original water level and the yellow lines indicate the level 4 hours later. Darkening of the sample color indicates wetting and absorption. From left to right:

• CHC has absorbed the most water, as indicated by the reduction if the liquid level in the container.
• Fir Bark has absorbed a small volume.
• Kiwi bark shows very little reduction in the liquid level.
• Orchiata shows a similar absorption volume to that of the fir bark.