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Compost in containers

Posted by redneck_grower 9 CA (My Page) on
Tue, Apr 7, 09 at 12:11

This is not a new subject. Sorry in advanced if I'm longwinded in this post (and sorry if my link has been discussed before).

However, I bring this subject up again because an acquaintance in another gardeing forum is beginning a community garden that will utilize large containers, and they wish to use some compost in the mix (for economics, primarily).

Their initial plan was to use the following components: 1/3 loam, 1/3 peat or compost, and 1/3 vermiculite or perlite.

I promptly warned them against using the loam, and suggested alternatives to vermiculite (and gave my reasons). I also suggested pine bark, calcined clay, coarse sand, crushed granite, and pumice as other potential constituents. I suggested limiting the compost to 25% or less, but I realized later that I had no actual reason or support to suggest such a thing.

I went back to an old bookmark I had used at other times to demonstrate the nutritional variation and inconsistencies in compost. Better yet, this link highlights a study that tested compost : peat soils and pine bark : peat soils in nursery growing. The study is here: Compost in Containers Study .

Here are some of the conclusions from that study (conclusions enclosed by quotation marks are those of the study authors; those without quotations are my own). I hope this helps shed some light on the issue of compost in containers (it has for me):

Air porosity (AP) of compost was slightly higher than AP for pine bark. AP went down progressively as percentage of peat went up. All mixes tested had adequate AP when compared to published ideals.

Water retention capacity (WRC) was slightly better in compost in comparison to bark. Both compost and bark had WRC levels which were slightly below published optimums. WRC improved as percentage of peat went up. (At the expense of AP, as stated previously).

Shrinkage occurred progressively in all media; degree of shrinkage increased as the percentage of peat increased.

Peat was most prone to shrinkage. Compost was second. Bark was significantly more resistent to shrinkage than compost.

The author's conclude, re: bark/compost shrinkage vs. peat shrinkage: "This reflects the relative stability and compaction resistance of both bark and MSW compost ..... shrinkage was of no practical significance in any of the tested media over a single growing season, although for longer production periods the impact on air porosity and hence root development could be significant...." One very key point bears repeating: this was a "single growing season" study; compaction could (probably would) have a significant impact on aeration/drainage for longer-term plantings.

Electrical conductivity was initially excessive in some of the compost, at reasonable ranges in others. EC went down over time in all media. This could be potentially important in salt-sensitive plant species.

There was a suggestion that, as compost or bark increased in percentage in the mix, plant growth had a tendency to decrease (with one exception, where growth improved as percentege of compost increased). However, the growth differences were not marked if there was AT LEAST 25% peat in the mix. Growth appeared to be significantly hindered if the media was 100% compost or bark.

The authors make the following conclusions: "Media which incorporate MSW compost at rates up to 75% of total volume are highly suitable for culture of woody plants in containers. In some cases (for salt sensitive species such as blueberry, Weigela and Privet), better growth may be obtained by limiting the MSW proportion to 25% in MSW:peat media. MSW compost contributes insignificant quantities of macronutrients to the plants and must be used in combination with controlled release, or liquid fertilization. It may, however, act as an important source of micronutrients.

MSW composts are inherently variable in both their physical and chemical properties. However, the results of this study show that this variability has relatively little impact on plant performance when they are used in combination with peat in container media. This suggests that material obtained from commercial composting facilities can be safely used as a component of a standardized growing medium for the production of woody ornamental nursery plants."

This information gives me some options as I build container soils; I hope it helps you as well.

Follow-Up Postings:

RE: Compost in containers

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.

RE: Compost in containers


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

RE: Compost in containers

  • Posted by tapla z5b-6a MI (My Page) on
    Tue, Apr 7, 09 at 15:51

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.


RE: Compost in containers

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.

RE: Compost in containers

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 ;)

RE: Compost in containers

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.

RE: Compost in containers

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.

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