You can use bonemeal, but it's very slow to give up its Ca, especially when temperatures, low or high, diminish the micro-organisms necessary to make it available for uptake. I use dolomitic lime with no noticeable effect on media structure. If you're thinking "concrete" you're thinking about to large an amount.

Al

Al,

Thanks! Obviously it will depend on many factors, but is it safe to assume that *small amounts* as you mention would not also alter pH? I have seen a few of your posts regarding pH management and it seems that you do not feel that chasing media pH is a worthwhile endeavor; however, I would like to keep things as they are without changing much if possible. I was all set on using prilled dolomite, but in reading the box all it was about was adjusting pH so I hesitated. I would like to add some calcium, but I need to keep a slightly acidic soil.

Also, how often would you recommend using it? I am guessing a couple tablespoons in #5 pot once yearly would do it (in the years the soil isn't being replaced)? And I suppose I won't need to add epsom salt anymore either.

Because nutrients are available to plants grown in highly organic container soils over an extremely wide pH range, I have never concerned myself with pH in container soils, even though I grow lots of plants that prefer pH south of 7. Those that have attempted it will tell you that pH fluctuates wildly in containers & trying to "adjust" will be a never ending battle that, even if you should win, may not yield noticeable benefit(s). In mineral soils (earth), pH is considerably more important because nutrients are available for plant uptake over a much narrower pH range.

"Prilled" is just a fancy word for pelletized & the product is likely the same as I use - common dolomitic lime. Most container soils wind up in the 5 - 6.5 pH range anyway. If you're concerned about the pH heading north, use an acid-forming fertilizer like Miracid or an azalea or evergreen mix that indicates it is acid-forming. Ca is a very important macro-nutrient and I would worry more about being sure that my plants are getting it and less about container soil pH.

I use lime when I make the soil and again in late summer as a topical application. If it is a planting I will carry over to a second or third year I sprinkle it on & work it into the surface soil (of containers) in early spring and late summer - and you're right about the Epsom salts - generally not needed when using DL.

Al

Thank you again. That was very helpful.

I have read many times about dolomitic lime, and have Ca recommended for plants in containers. Why this emphasis? Is the Ca not part of most fertilizers? Is there a need for additional Ca, and, if so, when: in the fall and the spring? Is the additional Ca needed for root developemnt, or for better winter survival?
Thanks for any info.
PS: I have maples in containers.

Ca is a very important macro-nutrient that you can be nearly certain is deficient in container soils. Ca is required to maintain cell membrane integrity, and without it, cell membranes become "leaky" and lose ions they have already absorbed; plus, uptake of other ions is also reduced because other positively-charged ions (cations) can displace calcium ions from the negatively-charged exchange sites on the cell membrane.

Ca in soils also buffers roots against low pH root injury from hydrogen ions and injury from other cations, like NH4 (ammonium) and Na. Plant roots often chemically rid themselves of H to balance out any excess positively-charged ions they have absorbed. Ca is required for the normal formation of root hairs and for the continued growth of the entire organism.

Ca is considered an immobile nutrient - once assimilated by the plant and used in cells, it is not readily translocated in the plant, unlike the more mobile nutrients N and P that move around in the plant to help balance internal deficiencies. From this, you can see that a continuous supply of Ca is required, unlike the more mobile nutrients mentioned.

When it comes to cold-hardiness and nutrients in general, the essential elements assimilated by plant roots in autumn result in higher concentrations of solutes in cell protoplasm and therefore decrease the nucleation point (temperature at which water inside of cells freezes) which increases the ability of tissues to remain viable after exposure to lower temperatures.

I think that covered the questions?

Al

Thanks a lot, Al, for the info, although I will have to read it a couple of more times.

If I understand correctly Ca is essential in facilitating nutrient uptake. Secondly, you are mentioning low PH root injury. Is this why one uses dolomitic lime in soil mixes with a lot of pine bark?
When you say continuous supply, would watering with tab water - that contains Ca- do the trick? I am asking because I am thinking of collecting rain water (and then the benefit from tab water would not be there anymore). Finally I found a 0-12-12 granular fertilizer that also has Ca under the form of 'lime sulfate'. I am thinking of using it in the fall, and the spring, and as the only fertilzer for very young maples. Does that sound good?
You provide invaluable info.

I use lime in container soils only to be sure it's there & not to buffer pH. I simply do not concern myself with pH in container soils (see above). Soils that use pine bark to replace the major portion of a peat component are going to be higher in pH than soils that are primarily peat. They will likely be in a very favorable pH range of 6-7, but again - pH in container soils is not nearly as important as in 90-95% mineral soils (like in the garden).

To answer your question about the water - I would use the rain water and add a Ca supplement to the soil instead of depending on minerals in the water.

I think the 0-12-12 fertilizer is a good choice for late summer and fall on your maples, but not as an only choice on your young trees (they need N, too). In spring, it depends on what you are looking for in your plants. If you are happy with the size & wish them to slow in development, wait to fertilize with a balanced blend (20-20-20, e.g.) until the first rest period (about when spring foliage is >75%-80% mature) and then about every 2 weeks until late summer. If you are looking for fast growth, begin doses as recommended with same - at onset of growth in spring. Early applications stimulate larger leaves & more branch extension (along with longer internodes). Waiting until the rest period makes for smaller leaves, less branch extension, shorter internodes & better ramification (more twiggy growth). The trees will also use more of the later application of nutrients to build layers of cells in the cambium and roots and to store energy instead of turning it into bio-mass.

Al

I recently had a discussion at the UBC Botanical Garden forum about especially long release fertilization for younger maples, and it was recommneded to stay away from much nitrogen until the maples are about 4-5 years old and ready for a 5 gallon container. Here is the link (but this probably belongs in the maples forum http://www.ubcbotanicalgarden.org/forums/showthread.php?t=11766 ) The idea is to develop the root system, avoid vvulnerabilities, etc. Do you have any thoughts about the relation of age and pot size to fertilizers?
As regards rain water: I intended to start collecting rain water to avoid raising the PH - now you make clear to me that this is not a concern for pots (I also intend to use it for some of the in ground maples). What are the benefits of rain water for container plants? Are there any dangers - I am thinking about how long rain water can stay in a container before it needs to be used.
Thanks.
Al- I hope you are thinking of writing a book so your insight and experience can spread more, btw

Even if I wanted to, I could not quarrel with the advice you received at the UBC Forum. Those helping you obviously know their business when it comes to Acers, and I suspect a good amount of other plant material. I learned quite a bit just reading through the thread.

One thing I tend to forget, is that a young tree to me is up to 20 years old (a bonsai thing) while a young tree to you is likely considerably closer to a cutting - chronologically, maybe in the 3-5 year range. It is always better to allow a plant to get its feet under it before you try to push it, and even then, it's difficult to build a case for pushing. I rarely fertilize plants grown in the garden, and only fertilize container plantings according to signs from the plants that they are lacking something. I would tend to agree that young Acers, grown in a highly organic mix, would not benefit from large doses of N, but they do need some, especially if you are switching to a soil that is only 1/3 organic, but I would still defer to the advice given on the other forum.

The benefit of using rainwater for containers comes in the form of fewer/less dissolved metal salts, lower alkalinity, and lower pH when compared to tap water. If you have an unlimited supply, by all means - use it on the in-ground plants; if not - save it for the containers. I'm not sure how long rain water will "last" in containers - probably a good long while - all summer. I have a friend who puts a couple of goldfish in his rain butts to keep mosquito larvae under control & they live there all summer. I suppose it might depend on what organic things find their way into the containers, but kept clean, I can't imagine a problem. If you're concerned, a little food grade H2O2 now & then should clean up anything anaerobic that might be thinking of causing trouble and will certainly not harm the plants (or the fish) ;o). In fact, they will thank you for the extra O2 that forms from the loosely held O- atom. ;o)

Al

Your thinking as regards potting soils could address some of the issues raised in that thread, especially the number of weakened maples that die a quick death. You also say you need to fertilize more if you use very fast soils (I remember that from the interesting thread on rootpruning maples in this forum). Whitcomb, in his book on plants in containers, addresses a lot of the root issues: I ordered some of his root pruning containers through rootmaker.com - look forward to experimenting with those.
Thanks for the info on rain water - H2O2 might be better for me, if necessary, than gold fish...