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Old 01-27-2010, 01:11 PM   #165 (permalink)
Diana K
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Default Re: How-To: Mineralized Soil Substrate, by Aaron Talbot

Here is a way to test any soil you are thinking of using. You need about a handful to a double handful. It should be dry to start with.

Put a piece of masking tape on a jar so you can write on it. The jar should be straight sided. The tape will go from top to bottom. The jar might have a volume of half a liter to a liter.

Break up any clods and put the soil sample in the jar. It might half fill the jar, maybe 2/3. Grinding it with a mortar and pestle is best. Pounding it with a hammer might be a low-tech way of making sure all the pieces are broken up. Tamp it in a bit to get rid of larger air masses. (Tap the jar on the table) Mark on the tape where the top of the soil is.

Now add water and a little bit of soap. Pretty close to fill the jar, maybe 3/4 full. Certainly more water than there is soil. Dish washing soap, hand washing soap... does not matter much. Just a drop. Powdered detergent is fine, too, a few granules.

Secure the lid and shake...and shake...and shake... (This is why a jar much larger than a liter is not so good) If you find you did not add enough water add more. When you are sure all the soil is really well wet:

Set the jar down and time it. Be ready to mark the tape:

In less than 30 seconds sand falls out. At 30 seconds mark the tape where the top of the soil seems to be.
Between 1-2 minutes silt falls out. Silt is a soil particle size that is almost too small to see, but is not clay. Mark the tape where the top of the soil seems to be at 2 minutes.
The colored water that remains has clay particles suspended in it.
Anything you see floating is organic matter. It will not have gotten wet enough yet to mix with the water or settle out.
Over a period of 24 hours the larger clay particles will settle out.
If the water is still cloudy it has coloidal clay. This is clay particles that are so small that Brownian motion keeps them suspended.

Interpreting the results:
If the water is still cloudy after 24 hours forget it. This soil MIGHT be OK after mineralizing, but maybe not. Over the years in the garden microorganisms will bind clay together so it is better soil for the plants and all the life in the soil, but I for one do not want to wait that long to set up a tank.
If there is a lot of organic matter floating, and you have to pay for this soil, then think of the waste. You will need to remove most of it. Floating it off will work as described above. Each time you cover the soil with water, skim the surface.
If this is free dirt you have shoveled up from your back yard this is not much of a $ problem, just start with a bigger batch. You might even screen it before starting the mineralizing to get rid of the larger pieces.

Now look at the amounts of sand, silt and clay that have settled out in the jar.
The sizes of these particles are very carefully described in soil classification charts, but I have found that the stuff that settles out as sand and silt make better soil for an aquarium. If there is too much clay (more than 5%) then it is too much clay.
Somewhere around 70-80% sand, and the rest mostly silt is really good. Clay has the highest cationic exchange capacity, but it does not take much to handle the needs of an aquarium. (Or a garden)

Note that I am NOT saying to go buy a bag of sand. I am saying the soil scientists' description of soil uses the word 'sand' to describe the particle size that works well in an aquarium. In a mixed soil sample the 'sand' will be all the sizes from coarse enough to feel gritty to an almost smooth feeling particle size.
Silt particle sizes will feel smooth, but not gooy or sticky, and clay will feel smooth and gooy. A blend of sizes mostly in the 'sand' size range, with a reasonable amount of silt and a little clay is great.

Note also that this does not say anything about what nutrients are present in the soil. Some clay is high in iron. Some soils are high in Ca and Mg.
You can use your aquarium test kit to test the water that has cleared in the jar, but better to avoid the soap by repeating the shaking of a separate sample of soil and water in a jar, but do not add soap. If your GH test shows it is high in minerals you might not want to add so much (or any) dolomite. If the iron test is too low, then add some laterite.
There are also garden soil tests that you can use at this point.

I see several people have asked about variations in the wet/dry cycle, and storing the soil in containers.
The timing of the cycle is based on the growth of certain microorganisms. In cooler weather they grow slower, and using the length of time it takes for the soil to dry as a timing method is pretty good.
These microorganisms need oxygen, so spreading out the soil is important. It will not mineralize very well in an open container like a bucket. I suspect that doing this in the shade would be better in really warm weather.
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