Compost in containers
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.