# Pottery Clay - how much to add...and odor?



## Dave P (Dec 13, 2005)

After a number of years away from the hobby, I have been bitten by the bug again and will be setting up a 46 gal. bow front low tech/El Natural aquarium (dirt substrate, no CO2, moderate light).

I am considering a 1" - 1.5" 50/50 mix MGOC and Safe-T-Sorb substrate with a 1" - 1.5" Safe-T-Sorb cap.

I've read some threads over the past couple of months regarding the addition of red pottery clay to the substrate to increase CEC and provide some iron, and have purchased a 5 lb box of Amaco Mexican Self-Hardening Pottery Clay. I'd like to add a small amount of this pottery clay to the substrate.

A couple questions...

When opening the box and plastic bag that the clay is packaged in, I noticed a light odor (perhaps _fragrance_ is a better term). It almost smelled like a floral hand lotion that my wife uses. This has me a little worried. Is this a normal odor for pottery clay? I've seen this specific product cited in several forums, and it sounds like it is safe (the box claims it's non-toxic and the MSDS for the product doesn't seem to raise any alarms)...but the odor is bothering me. I even went back to the craft store where i purchased the clay and slyly opened another box and took a quick whiff (imagine how that looked). Same odor as the box I have.

So, I'm curious, if anyone in this forum has used this product and whether they encountered a similar odor or not. Am I safe to use this product, or should I look for something else, such as Cedar Heights Redart clay?

In regards to quantity, I've seen recommendations varying from 1/4 lb / sq. meter to 1 lb per 20 gallons. Any thoughts on how much I should add based on the substrate mix I mentioned above?


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## Diana K (Dec 20, 2007)

In the mineralized soil threads the suggested amount is 5-10% clay:80-85% coarser soil particles to make up the active planting layer. 
This has nothing to do with the total volume of the tank. 
Here is how I would figure it:

What size is the footprint of the tank? 
Figure 1" deep of planting media, do not include the cap. 
Whatever volume that is, blend a bit more than you think you will need, and make 5-10% of that volume from the clay.

46 gallon bowfront has 3.6 square feet floor area. 
x 1" deep = about 1/3 of a cubic foot. or about 500 cubic inches. 5% of this is 25 cubic inches (make the clay into a block 5" x 5" x 1" thick). 10% is of course twice this much, or a block 5" x 10" x 1" thick. 
Some potting soils are sold by the quart. 1/3 cubic foot is not too far off 7-1/2 quarts. 
STS is sold by the pound (40 lb bags). It is pretty light. The 40 pound bag might be 1 cubic foot. I know it goes really far in filling a tank! Depends on what ratio you want of soil:STS, but I would just get 1 bag, even if you used no soil at all, you would not use up all the STS.


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## Dave P (Dec 13, 2005)

Thank you very much, Diana; these estimates are exactly what I needed.

Over the past few weeks I have procured the Safe-T-Sorb (STS), Miracle Gro Organic Choice potting mix (MGOC), Redart clay, powdered dolomite and potassium chloride (KCl). (I opted against using the Amaco clay, as I believe it has an antimicrobial agent to extend shelf-life. The Redart clay has no such agents/preservatives.)

Now, I'm having second thoughts about the red clay in general.

From what I've read in the forums, a number of folks have used red clay to increase iron in their substrate. In many cases, the clay is being added to mineralized top soil (MTS). The relatively lower percentage of organic matter in MTS (as compared to the MGOC) seemingly limits the solubilization of excess amounts of iron. Potting mixes that are rich in organic matter (particularly peat) presumably liberate greater amounts of organic acids, which in turn can solubilize more iron and other metals.

Diana Walstad discusses the potential danger of metal toxicity in her book, and how it can occur through the mixing of potting mixes and mineral-rich soils, or substances such as laterite.

So, given that this is a low tech, Walstad-type setup (moderate lighting, no C02, no ferts), and given a substrate where the first inch is roughly 50/50 MGOC and STS--supplemented with 5-10% redart clay, light dusting of powdered dolomite and KCl--and topped with an inch of straight STS, am I at risk of liberating toxic quantities of metals from the clay? (I'll include chemical composition of redart clay at the end of this post.)

Factors which lead me to think that I might be ok include:


 Redart clay is not laterite. It has a lower concentration of iron (although it may have higher concentrations of other metals/compounds).

 The STS, with its high CEC, might help absorb any excess metals which might be solubilized by the organic acids in the potting mix.

 The powdered dolomite may help neutralize some of the organic acids in the MGOC.
 Obviously, I'm trying to avoid scenarios which may require me to tear everything down if I'm going to have issues down the road.

Anyone have any experience with the substrate described above? Did it work well, or did you encounter issues? Should I leave the redart clay out of the equation? Should I adjust the ratio of MGOC to STS in the first inch of the substrate?


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## Dave P (Dec 13, 2005)

Here's the chemical composition of redart clay: (from https://digitalfire.com/4sight/material/redart_1191.html)
Oxide Analysis Formula
CaO 0.26% 0.030
MgO 1.59% 0.259
K2O 4.15% 0.290 
Na2O 0.38% 0.040 
P2O5 0.22% 0.010
TiO2 1.09% 0.090 
Al2O3 15.51% 1.000 
SiO2 64.95% 7.107 
Fe2O3 7.05% 0.290 
LOI 4.80


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