It is interesting that the air porosity of the compost they used was higher than that of bark.

This would seem improbable. I wonder what kind of bark they used?

Anyway, here is another link to a different study.

This concludes that while plants can be grown in compost growth rates (and air porosity) improved as the amount of bark in the mix increased. Best growth was found in 75% bark/25% compost (the highest amount of bark tested).

Anyway, you did good talking them out of using the loam in the container. Do they understand the difference between vermiculite and perlite? When you wrote they were considering using either one it sounded to me like they might view them as interchangeable which is a common misconception.

JaG,

It appeared as though they were considering either vermiculite OR perlite. I explained why they are not interchangeable.

Naturally, the 'Composting Council of Canada' is not going to have anything too negative to say about compost - just as the perlite people will have only positive things to say about perlite ..... ;o)

I've mentioned that in the (1:1:1) gritty soil mix, if 2 of the components are the right size, the third component can be rather coarse w/o impacting drainage characteristics significantly. The same holds true with their 1/3 compost and 1/3 loam. You cannot amend that soil with perlite to get aeration to where it should be w/o ending up with something like 60-75% perlite, and the rest compost/loam. I often use the illustration of a tub of pudding (which is pretty much what the 50/50 loam/compost will be before the perlite). How much perlite do you need to add to pudding before it drains well?

If compost was suitable as a significant presence in container soils, nurseries and greenhouses everywhere would use it - it's FREE, so you can't beat the price, yet it is rarely used in their soils.

I think they will make a mistake if they adopt that mix - ESPECIALLY if they use vermiculite as a third ingredient.

Al

Al - Thanks for your input. I had hoped you'd "stop by" and chime in on this one.

JaG - The study you linked to was interesting. If you evaluate it critically, the results were not a whole lot different than the Canadian study. Bottom line, the general trend in both studies (with some exceptions) was for plant growth to decrease as percentage of compost increased.

Yes, that is true.

The part I found difficult to swallow in the first study was the statement that bark had less air porosity than the compost and that growth rates dropped as the amount of bark increased.

I have never seen another study make that conclusion, but I have seen several that reach the opposite conclusion.

I am probably being too nit picky. Sorry ;)

JaG, I too found the air porosity data a bit of a surprise, but in both studies the differences between bark and compost were not profound (often within standard deviations of one another). You may have hit upon the reason in your first post: What was the nature of the pine bark? Perhaps it was too fine to begin with. Indeed, the first study also showed quite a difference in porosity of two different composts as well.

From the first study, I too found the reduction in growth as bark increased relative to peat (in 3 of the 4 replicates) to be a bit surprising. On the other hand, in the second study, as bark : compost went from 1 : 1 up to 3 : 1, growth parameters were statistically equivalent (as in one of the replicates in the first study). Therefore, the effects of increasing percentages of bark are inconsistent across the two studies.

The casual reader of this thread is cautioned not to directly compare the two studies, as the soil mixes measured are quite different in each. However, both studies attempt to answer this basic question: Can compost be effectively used as a soil component in containers? The only similarity between both studies is compost.

Interestingly, both studies come to basically the same conclusions. That is, compost CAN be used effectively as a container soil component, probably best at no more than 25%. AND, that growth parameters fairly consistently diminish as the percentage of compost increases in the medium.

Interestingly, both studies come to basically the same conclusions. That is, compost CAN be used effectively as a container soil component, probably best at no more than 25%. AND, that growth parameters fairly consistently diminish as the percentage of compost increases in the medium.

I would agree with this.