magenesium and calcium are abundant in castings. if you use castings 20% of the total mix you should have plenty of both. you can always add gypsum crab oyster etc. and more magnesium sulfate. more dolomite is not the answer either for calcium and magnesium or ph balancing. sort of the whole point of organic growing right? let the microbes do the work.
better to let cc splain it:
Okay - the first goals of wanting Calcium & Magnesium are good we'll stipulate. Whether or not you need a mineral amendment to achieve that will be put aside.
Dolomite Lime is used in 'the real world' when a complete soil analysis has been done and now you have a complete overview of the element levels, CeC, etc. and it's been established that lower levels exist for the long term on the Magnesium percentage. Even then, DL is applied maybe once every 4 or 5 years. The Magnesium in DL arrives as Magnesium Carbonate (MgCO3) but it's a bit more complicated than that.
The reason that it is 'slow acting' is the molecular structure and if you were to hit even WikiPedia and looked at the molecular formula you can easily understand why this material is as slow acting as it is.
If in fact you need a Magnesium jolt then you'd be far better off using a mined mineral compound like Epsom Salts (Magnesium Sulfate) or Sul-Po-Mag (Sulphur, Potassium & Magnesium). The Magnesium in Epsom Salts is in its elemental form like Sul-Po-Mag.
The main straight liming agents, Limestone, Calcite (aka Agricultural Lime), Oyster shell powder and Crab meal are sources for Calcium Carbonate (CaCO3). All are pure Calcium Carbonate with the exception of Limestone which can have a Mg level between 2 - 3% depending on the specific mine, country of origin, etc.
When looking at the numbers on a Calcium Carbonate source you have to multiply the CaCO3 percentage by 0.375% and now you will have the elemental Calcium (Ca++) numbers.
Gypsum (Calcium Sulfate) is the preferred 'liming' agent in the PNW due to the acidic soils we deal with (the west side). That has to do with the adulteration of the clay platelets which no longer carry a pure negative charge (-) on the edges which bind along its edge with the center of adjacent clay particles and now you have clay compaction. All the Rototilling isn't going to change that - ever.
So back to Dolomite Lime and why it's used in commercial potting soils - certainly not used by professional nurseries other than for specific growing schedules like 3 - 5 years in containers. Even then, DL is part of a 'liming mix' that will include Gypsum (Calcium & Sulphur), Limestone or one of the shells meals. Bottom line is the DL is the least expensive because Calcium Carbonate is widely used in animal & human supplements - next time you're in a store selling vitamins and supplements look at the label on the Calcium products - Calcium Carbonate.
Same for livestock and poultry. Calcium is a necessary part of their feed and DL isn't part of that. DL has several industrial, manufacturing, etc. uses - it's not the big deal in agriculture or horticulture like it is in the cannabis hobby gardening paradigm.
All of this assumes of course that the potting soil that you make is deficient in Calcium or Magnesium. It would be highly unlikely that given the compost and EWC you produce that you need additional Magnesium or Calcium. EWC are covered with a slime which is Calcium Carbonate from the worm's digestive tract.
Calcium is not this elusive element that Goober wants you to believe it is
CC
