Miracle Gro Organic Choice Potting Mix
(MGOC) is often recommended, and by Diana Walstad herself in her on-line article about small aquaria for shrimp at http://www.atlasbooks.com/marktplc/00388Shrimp.pdf
. Unlike many other “potting soils”, MGOC has a clear list of ingredients and a nutritional analysis on the bag:
50-55% composted bark
Sphagnum peat moss
Pasteurized poultry litter
“organic wetting agent” (whatever that is)
total nitrogen 0.10%
available phosphate (P2O2) 0.05%
soluble potash (K2O) 0.05%
". . .feeds up to 2 months. . ."
This tells us several important things. First, this product is 100% organic matter. Remember, natural soils are almost never 100% organic matter—less than 20% is more common. So this soil is going to undergo a lot of decomposition in the aquarium.
Second, it has chicken manure in it, which makes it much more fertile (higher nutrient level) than most other potting mixes. But, these nutrients come from slow-release organic sources, not synthetic inorganic chemicals. Not that there is necessarily anything wrong with synthetic fertilizers, but they behave differently when submerged than organic ones do. This higher fertility is expressed in the analysis (which shows much more nutrients than typical for potting mixes or natural soils) and the last statement, “feeds up to 2 months”.
What does MGOC look like when you open the bag? It is dark brown, with a mix of fine particles and some pretty big chunks of not-yet fully decomposed bark. These chunks are a source of concern, for several reasons discussed below. I did a quick test with a ¼” soil sieve, and about 20 to 25% of MGOC will not go through the sieve, even after rubbing it hard with a gloved hand.
MGOC has three major advantages:
1. It is a nationally available product
2. The ingredients and analysis are clearly listed on the bag
3. It is relatively consistent no matter where you buy it. (Many other products vary greatly from one region to another.)
Used straight from the bag, MGOC has four major disadvantages:
1. The big pieces and many of the smaller pieces float, which can make a big mess if your cap is not heavy enough, or if you change your mind and move a plant.
2. The high nutrient content usually causes an ammonia spike in the first month following tank set-up.
3. The partially decomposed bark releases a lot of tannins into the water. This is not usually harmful to fish or plants, but the tea-colored water may look bad to you.
4. Because it is 100% organic matter, if the soil and/or cap is too deep, the soil layer may become very anaerobic. This is bad for many reasons.
Fortunately, there are easy ways to deal with all of these disadvantages.
The simplest ways are a thin soil layer, patience, and water changes. This soil is hot stuff, you do not need much! How deep a soil layer to use depends on size of tank and types of plants, but I would never use more than 1.5”. For beginners and small tanks, 1” or less is plenty. Patience is necessary to allow the biological filter to develop properly and absorb the ammonia. Water changes help with that, and also remove the tannins. Eventually the big pieces become saturated with water and no longer float.
The more sophisticated ways to deal with these problems involve processing MGOC in some way before you use it. A quick and effective process is “soak and drain”. Put the soil in a big bucket, cover it several inches with water, and stir well. Let it sit over night, then carefully pour off the floaters and the brown water. Fill, stir, and let sit over night again. Repeat the soaking and draining cycle until you see no floaters and the water is reasonably clear, or until you can’t wait any longer, LOL. Seriously, three complete cycles is usually enough to make a big difference. This method will result in a loss of total volume of soil of 25-30%, so start with more than you need for the tank.
Another way to process MGOC is to mineralize it. This process is described fully in several great threads in the library forum. Mineralization greatly speeds decay of organic matter into a very stable form called humus. Humus does not release ammonia into the water, and is unlikely to become anaerobic.
And there is one last tip for using MGOC or any other highly organic soil: mix it with an inorganic substrate that has a high cation exchange capacity (CEC). Examples are laterite, Flourite, Turface, plain cat litter (no perfume, antimicrobials, or clumping agents), and Safe-T-Sorb. Remember, natural soil is almost never pure organic matter. Mixing the organic matter with inorganic high CEC substances means that the ammonia and other nutrients produced are held in the substrate where plant roots can use them, but where they will not harm fish. And by reducing the percentage of organic matter, you reduce the likelihood of the soil becoming anaerobic. I like a 50/50 mix. Phil Edwards first gave me this advice, and it has worked well for me.
Let's hear from eveyone else! What kinds of soil have you used in your Walstad tanks, and how well did they work?