Evidently, nobody knows, Juv. I don't either, lol, but just generally, coir is a good material. I see no particular reason why a modest amount-say your 20%-would not work well.

+oM

I haven't tried mixing it with BX, but presently have some heirloom tomato transplants in 32 oz containers, in a mix of 20% coir, the balance is Foxfarm Ocean Forest. I have osmocote plus in there, for crf.

The rationale was the coir rehydrates readily when dry, as opposed to the peat in the Foxfarm. I had it available, so figured I'd use it in case I messed up and let those thirsty tomatoes get too dry before transplant/outside container time. So far, so good.

The coir is bagged Botanicare brand, triple washed, e.c. controlled, pre buffered, so the sodium isn't a problem, and the potassium is supposed to be low, for coir. For tomatoes, a little higher K shouldn't be a problem, if I have it right.

I've used it in gritty style mixes, and it seems to hold up better than peat, rehydrates well enough, but will settle into the pot as it ages. Won't use it that way again.

The pH actually is a bit high for my liking, about 6 - 6.5 in my tests.

Copy/pasted from one of my previous posts:

Sphagnum peat and coir have nearly identical water retention
curves. They both retain about 90-95% of their volume in water at saturation
and release it over approximately the same curve until they both lock water up
so tightly it's unavailable for plant uptake at about 30-33% saturation. Coir
actually has less loft than sphagnum peat, and therefore, less aeration.
Because of this propensity, coir should be used in mixes at lower %s than peat.
Because of the tendency to compact, in the greenhouse industry, coir is
primarily used in containers in sub-irrigation (bottom-watering) situations.
Many sources produce coir that is high in soluble salts, so this can also be an
issue.

Using coir as the primary component of container media
virtually eliminates lime or dolomitic lime as a possible Ca source because of
coir's high pH (6+). Gypsum should be used as a Ca source, which eliminates
coir's low S content. All coir products are very high in K, very low in Ca, and
have a potentially high Mn content, which can interfere with the uptake of Fe.
Several studies have also shown that the significant presence of phenolic allelochemicals in fresh coir can be
very problematic for a high % of plants, causing poor growth and reduced
yields.

I haven't tested coir thoroughly, but I have done some
testing of CHCs (coconut husk chips) with some loose controls in place. After
very thoroughly leaching and rinsing the chips, I made a 5:1:1 soil of pine
bark:peat:perlite (which I know to be very productive) and a 5:1:1 mix of
CHCs:peat:perlite. I planted 6 cuttings of snapdragon and 6 cuttings of Coleus
(each from the same plant to help reduce genetic influences) in containers
(same size/shape) of the different soils. I added dolomitic lime to the bark
soil and gypsum to the CHC soil. After the cuttings struck, I eliminated all
but the three strongest in each of the 4 containers. I watered each container
with a weak solution of MG 12-4-8 with STEM added at each watering, and watered
on an 'as needed basis', not on a schedule. The only difference in the
fertilizer regimen was the fact that I included a small amount of MgSO4 (Epsom
salts) to provide MG (the dolomitic lime in the bark soil contained the MG,
while the gypsum (CaSO4) in the CHC soil did not. This difference was necessary
because or the high pH of CHCs and coir.) for the CHC soil.

The results were startling. In both cases, the cuttings
grown in the CHC's exhibited < 1/2 the biomass at summers end as the plants
in the bark mix.

I just find it very difficult for a solid case to be made
(besides "It works for me") for the use of coir or CHC's. They're
more expensive and more difficult to use effectively. The fact that some
believe peat is in short supply (no where near true, btw) is easily offset by
the effect of the carbon footprint of coir in its trek to the US from Sri Lanka
or other exotic locales.

That's the view from here. YMMV

Coir Study: https://sites.google.com/site/plantandsoildigest/usu-crop-physiology-laboratory/coconut-coir-studies

“Whitcomb-isms”

Dr Carl Whitcomb,
PhD, wrote what is probably the bible on growing plants in containers. Some
"Whitcomb-isms":

<i>"If the root system ain't happy, ain't no part
of the plant happy"

"Roots control the tree, the stems and branches just
<u>think</u> [not my emphasis] they are in charge."

"The more roots to share the load, the faster the dirty
work gets done"

"Roots provide the fuel for the plant engines we call
leaves"

"Each root tip casts a vote to decide what the top will
be allowed to do"

"Top growth gets all the glory, but the roots do all
the dirty work"</i>

He
also notes that "Stress can ALWAYS be measured in the root system before
symptoms appear in the top [of the plant]".

Not to disparage Whitcomb, but that whole screed is just a wee bit off: Foliage-you know, the stuff with chlorophyll in it-does the food-production work for the plant. Yes, the root's delivery of water with certain nutrients contained therein is necessary for this to proceed. Next, auxins-plant growth hormones-are produced in growing tips and sent to roots to "tell" them to commence expansion. Not the other way around, although there are mechanisms working in that directions as well. In any case, it quickly becomes a moot argument: We don't see too many plants thriving with either just top growth or just roots....it's nonsensical. And yes, I will agree that the underground, unseen parts of plants is less-well understood by persons not intimately involved in plants and horticulture. I don't know too many professionals or even hobbyists making that mistake though.

Anyway, I did not know coir had so much K in it. that could be useful in certain circumstances, I should think.

+oM

The point of Whitcomb's I see as arguable is in "Roots provide the fuel for the plant engines we call leaves". If we think of fuel as something that burns, we have to think of carbohydrates, which are produced in green tissues - leaves and stems with the energy provided by the sun. If we think of the fuel as the water and nutritional building blocks plants use for growth and to keep their systems orderly, then the statement makes more sense.

The offering wasn't meant as a lesson in plant physiology; rather, it was offered so others would get the idea that a healthy root system is a prerequisite to a healthy plant.

Al

Once again, great info, Al.

I was attracted to coir initially, on claims that its rewettability was superior to peat, the aeration was very high, it held its structure better than peat over time, was easier on roots than peat, because it didn't change volume much as it dries and rewets, and of course, the claims it is more environmentally responsible than peat.

The environmental claim was most important to me, but appears to verge on "greenwashing". Even if one accepts the idea that coir use means growing coconut plants are absorbing carbon, and peat use means harvesting and using dead plant material, the net beneficial effect seems to disappear when one realizes that pretty much all the carbon sequestered in the coir ends up back in the atmosphere as co2 or methane, anyway.

The detrimental compounds in fresh coir mean the product MUST be properly aged before use. Some companies are conscientious about this, but this is something we consumers have no control over. It also means, coir suitable for use is always partially decomposed.

I suspect CHC's - coconut husk chips - are never really decomposed enough to fully eliminate the detrimental compounds. That may be at the heart of Al's results.

The product I tried was recommended by a researcher at the Chicago Botanic Garden, on the criteria of verified consistent quality. What I have, is a high pith product, much finer than I was expecting.

I've found the rewettability and volume stability to be as advertised. For me, it does seem to help with peat rehydration. Aeration seems to hold up well between repots. However, extended time between repots does see volume decreases, and we all know what that means.

The plants I put in coir mixes are succulents, gesneriads, and now, tomatoes. The coir is not more than 20% of any of my mixes anymore, with the exception of a couple goldfish plants presently, and my initial test plantings. Those initial test plantings were where I noted the eventual volume losses. I am phasing out my use of it, because it's too fine, I don't like the pH, which is too high for my liking, and it's not really environmentally advantageous.

I may use it in 511 mixes for awhile, instead of peat, just to use it up.