I found no info on the web about the
pH of MG w/MC except one assertion
that it was "too acidic" for some crop,
with no measurements.
Anyway, it is peat-based, plus perlite,
coir, some dilute fertilizer, and compost.
Compost tends to be close enough to neutral
(bacteria and worms buffer the acidity
or alkalinity of the feed materials) not to
require pH adjustment on its own, fertilizer
at those levels is a very mild acidifier, perlite
is neutral, coir is close enough to neutral
not to need any pH raising for vegetables,
and peat moss is acidic, with wide variation
depending on the source, from around
pH 3.5 to 5.5. So the peat is the key factor,
and it is probably 50-60% of the container
mix. It could vary from "way too acidic for
tomatoes" to "more acidic than we would
like for efficient nutrient utilization."
According to
http://edis.ifas.ufl.edu/CN002
dolomite lime takes from several weeks
to several months to break down,
depending on particle size. Lilly Miller
Soil Sweet looks to be about 99%
particles finer than salt (I have a
half a bag of it here) but coarser
than talc, so it is probably in the
range of 6-8 weeks breakdown time
before the nutrients become available
to the roots.
If we added different amounts of that kind
of dolomite to some set amount of MG w/MC,
kept them moist for 8 weeks, put them in
containers that we could fill up with water and
soak overnight, and then tested them all with
litmus paper (coarse grained test, but it is very
cheap and will show neutral by not changing color),
in 2 months we would know about how much dolomite
to add to a given volume of
this year's MG w/MC
container mix to keep the pH close to neutral during
the bulk of the growing season. (I am assuming that
the manufacturer probably gets the peat moss in train
car loads at a time, so a given pallet of bags at a vendor
probably has little variation in pH from one bag to the
next, while there might be much greater variation
in pH seen in bags of the same product bought over a
period of months or years.)
Not much immediate help there for the question
of how much dolomite to add to adjust the pH
of your container mix to an ideal value (requires
another sets of tests and an 8-week wait), but that
does answer the calcium availability from dolomite
question, if we take the document at the URL above
at face value: within 8 weeks, the calcium from a fine
grade of dolomite should be mostly available to the plant.
We could ask Scotts (manufacturer of Miracle-Gro
products), but the pH of a peat-based product may
be a moving target even for them, depending on
where exactly the peat moss in a particular bag
of container mix was harvested.
Anyway, I would not back off from using a whole cup of
dolomite for one side of your containers. The native
container mix is likely to be quite acidic, with a low
enough pH to hamper nutrient uptake.
The dolomite/gypsum+epsom_salt test may not be needed
to test calcium availability from commonly available
commercial grades of bagged dolomite like Soil Sweet,
but you could still do it to see how much the native pH
of the container mix affects growth and productivity.
(We would expect the side with dolomite to do better
than the side with gypsum because of the upward
pH adjustment on the dolomite side.) If you are taking
calcium availability from both dolomite and gypsum as
a given, then you no longer need to use a fertilizer without
calcium in it to avoid skewing the results, as long as both
sides have the same amount of fertilizer.
(Kind of long winded, but this is science, even if not very
precise science. There are a lot of details to consider.)