A few MTS Questions for the experts...

Update: I had originally bought a bag of Scott's Topsoil. It was pure junk... Although it "looked" like topsoil originally, after 1 soak/dry cycle followed by sifting through a 1/4" and then 1/8" screen I discarded 2/3s of it. The 2/3rd I discarded was peat, wood chips, and wood fibers. What I was left with I looked at under a microscope, and it appeared to be 95% organic matter (smaller fibers that made it through the screen).

So, I found a new dirt supply: MOLEHILLS! I got 16.7 lbs of molehill dirt near the parking lot at work, and after the first soak and dry session, it looks GREAT! I only lost about 3% during the screening session. Today, I purchased my Dolomite powder and Potassium Chloride powder. I STILL can't seem to locally obtain REAL clay. (People seem so confused when you say that polymer clay isn't real clay...)

EI dosing just refers to dosing in quantities large enough that none of the nutrlents is ever going to limit the plant growth rate, then doing regular large water changes to make sure none of the nutrient levels ever get too high. You can reduce the amounts dosed if you wish, and reduce them until you notice adverse effects on the plants. Then increase it a bit, and you have the optimum dosing level for your tank. The closer you are to that optimum level, the less often you need to do big water changes, and the smaller the water changes can be. It is by far the easiest and cheapest way to fertilize a tank, in my opinion. It is anything but complicated, and it doesn't involve doing any testing at all, and no calculations of dosage amounts.

When you have a nutrient rich substrate, as MTS is, the plants can get what they need from that substrate, but you can't be sure that all of the nutrients needed are in MTS. Plus, if the plants use only the substrate for nutrients, the substrate can become depleted of some of the nutrients before you are ready to replace the substrate. So, dosing the water plus using the nutrient rich substrate is about the best possible way to provide the plants with food.

Soils that are fine enough to be classified as clay will usually be OK in the aquarium in the mix you are working on.
Here is how to tell if they are safe:
If they came from an area away from any possible pesticide/herbicide use, and away from the sides of the road where petroleum or other pollutants might have contaminated it. If the weeds (not crops) are thriving, then the soil is probably pretty good.

Here is how to tell if the particles are fine enough:
Get some wet, and roll it in your hands like you are trying to roll it into a worm. If your worm falls apart even while it is still between your hands, or if it never forms a worm, this is sand.
If the worm is OK as long as it is between your hand, but falls apart when you roll it longer so it sticks out beyond your hand this is silt. Bring some home for a further test.
If you can roll a worm that keeps on growing out of your hands, this is clay. The longer the worm the higher the clay content. Bring some home for another test.

Here is a second test:
In the palm of your hand sandy soils will feel gritty.
Soils high in silt will feel slick.
Clay soils will also feel slick, but there will be a silky, smooth feeling that is not the same as silt.

What to do with a sample you bring home:
Put some soil in a straight sided jar. (If there are clods break them up pretty well) Put some tape on the side of the jar, and mark how high the soil is on the jar.
Add water and a drop of dish soap. Shake. Shake. Shake. Shake. Shake some more.

Set the jar down and watch the time.
Mark on the tape how high the soil is at 30 seconds.
Mark on the tape how high the soil is at 2 minutes.

In 30 seconds all the sand falls to the bottom of the jar. For this purpose a little sand does not hurt, and if there is a lot then skip buying any soil at all. This might be just what you want, pre-mixed!
In 2 minutes all the silt falls to the bottom of the jar. Some silt is OK, too, even if you are really looking for clay. Like the sand, if there is a lot of silt, then this would be a good aquarium soil, and never mind buying anything.
Anything remaining is clay. The larger clay particles may settle out after 24 hours. This is the best. If the jar still has cloudy water after that the particles might be too fine, and would make a mess in the tank. Might be OK to use even this really fine clay in the very small amount that is in the mineralized soil recipe.
Anything that floats is organic matter (leaves, grass, roots, sticks...)

If you find a sample that is pretty close to pure clay, then use it to mix with whatever other material you have bought. A little sifting if there is a lot of organic matter, or go get some more, but scrape away the upper few inches before taking the material you want.

If you find a mix that is up to 60% sand, around 20-50% silt and perhaps 10% max of clay, then forget buying anything, you have found some pretty good soil right there!
This sort of mix will not make a very good worm when you roll it in your hand, but it will sort of take a shape, probably better than a sand castle. (depends on the exact ratio of sand silt and clay)
When you rub it in your hand it will have some grittiness to it, but also some smoothness.