Specialty Gardening: Container Soils: Water Movement and Retention II, 0 by tapla

tapla wrote:
Horseshoe - For the record, I'm considering your question(s) as a continuation of the discussion about what happens to sawdust during the composting process. You ask "The stabilizing to 7.0 happens only if you add the lime (or other pH altering ingredient), right?" (

No. Actually the (dolomitic) lime serves 2 purposes. It adjusts the initially acidic pH of most peat and bark soils (as it would soils with sawdust in them too, if you were using it) and it also supplies needed Ca and Mg in a favorable ratio. It is not a required additive to stabilize pH, but it is used to bring pH into a more favorable range. Stabilization would happen (gradually/eventually) whether or not you added a liming agent. The organic acids formed as the hydrocarbon chains break down will be broken down or leached from the container. Eventually there will be nothing left of the woody material but small particles of lignin and the pH will be about 7.0.

"Is this why adding lime to a soil/medium takes a while to neutralize (7.0 pH) the soil/medium, due to the fact that the H has to be released first and it takes a period of time?"

No. Adding lime to raise soil pH is chemically complex. As hobby growers, we make our best guess & hope for the best. In applications where it is important to use liming materials to help achieve a target pH, you must take into account several things. The initial pH of the media, its buffering capacity (resistance to change), the target pH, and the effects of irrigation water. Dolomitic lime is slow to react simply because of its low solubility. BTW, the H doesn't come from the dolomitic limestone, CaMg(CO3)2, as you can see by the formula for the compound.

Tigerlily mused: "There is something missing here that definitely pertains to the discussion, esp regarding using some sort of wood chips/mulch and that is time. How long is the plant going to remain in this mixture before either being planted in the ground or having to be moved up to a larger size pot and thus a new mixture? In other words-does it really matter, in terms of wood chips/mulch breaking down/compaction (not PH) if by the time it happens, the plant is already rootbound and/or is going into a new medium?"

We generally don't care what inert soil components are made of as long as their combination as a whole holds air water (and to varying degrees - nutrients) in a favorable arrangement. You'll remember how I stress structural stability in the original post? When the organic component of a soil breaks down quickly, it supports huge populations of microorganisms that out-compete the plant for N. Adding sufficient N is a challenge because the microorganisms are better at absorbing N from the soil solution, so the N simply creates larger and larger populations of microorganisms. It's extremely difficult to the point of being almost impossible to balance N applications in soils made of whitewood chips. You either get in trouble with carryover toxicity when microorganisms die & give up their N which is then added to your recent N applications; or, if you are using the more common container fertilizers (say 20-20-20) you cannot supply enough N to satisfy the plant because the P & K levels go sky high and raise the levels of TDS (total dissolved solids) so high the plant experiences fertilizer burn (plasmolysis).

N immobilization is immediate when using whitewood chips, and lasts throughout the composting process , so the reasoning that the plant will only be in them for a year won't hold up to scrutiny. I'm certain you can do better by changing your soils to either a much higher % or inert/inorganic components or changing the organic component to some type of appropriate size conifer bark.

"Am I right in remembering that you grow bonsai? What about these plants, whose tops are restricted in growth? Does that also restrict the root growth in proportion to the leaf output?"

I am a bonsai practitioner, yes. I think we've left the subject of soils here? Reducing the green portion of plants reduces the amount of photosynthate (food) produced, which limits root growth. The plant will always try to reach a balance between the number of roots and shoots.

If you reduce the canopy only, a tree will focus primarily on replacing the lost foliage. The response varies though, according to when the reduction occurs. In many plants, if you prune in late summer, the plant is not so eager to backbud & grow new leaves as it would have been if the pruning occurred in early spring.

Tree reactions to root pruning only is highly variable by plant & season, so I'm not going to expand. The same is true of simultaneous pruning of roots & canopy - varies by plant & season; but if you have specific questions, I'll answer by D-mail or you can start a thread.

I'm glad to see there's some renewed interest in this thread, it's been pretty quiet for awhile. ;o)

Al