# Soil Substrates in Soft Water



## netcode (Aug 14, 2007)

I am having a hard time finding an answer to my question about soil substrates in soft water. 

First, I am assuming that in soft water over time a soil substrate will go acidic. Is this correct? Reading Walstad's book (pg 131) it indicates that at first the hardness will go up. However, over time it looks to go back down, and the graph only shows 16 weeks. To my understanding, the nature of a closed system is that it will continue to become more acidic as time goes on. 

With the above said, I am trying to combat the soil from going acidic, as that creates a multitude of problems (to my understanding, especially with the addition of clay in soil). 

In addition, plants require Ca and Mg, which are pretty much nonexistent in my extremely soft water. I want to feed the plants Ca and Mg via the roots.

So to solve both, I figure a good approach is similar to the MTS method of using dolomite in the substrate. However this appears to be VERY challenging to source. 

I am wondering about other methods? Or maybe my concerns aren't even valid? 

There was a mention of dolomite lime in Walstad's book, however the internet seems to report that dolomite lime is not a good product to use in substrates. AaronT from the MTS method reported it created problems, but never indicated what exactly those problems were.


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## redbee (Nov 27, 2016)

Hey netcode,

I have almost the same questions. 

But I'm thinking... Should we increase KH instead of GH to keep PH more stable?


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## Michael (Jul 20, 2010)

If you can't find dolomite, you can use oyster shell or crushed coral.


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## JSeymour (May 26, 2016)

Oyster shell would be a great soil amendment as it not only has the calcium and magnesium you're looking for, it also has carbonate, which will raise the pH and hardness.


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## netcode (Aug 14, 2007)

JSeymour said:


> Oyster shell would be a great soil amendment as it not only has the calcium and magnesium you're looking for, it also has carbonate, which will raise the pH and hardness.


Have you used oyster shell? If so, what did it buffer your pH at?

The reason I ask, is because with crushed coral, regardless of the amount it totally buffered my pH at 8.2. I had very little control over it and I didn't like that it flat out put my pH at 8.2.

I am more so concerned about the soil going acidic - one of the main concerns is the interaction with clay (as Walstad indicated in her experience with laterite)


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## JSeymour (May 26, 2016)

No, I've never used it with aquarium plants. From a solubility stand point, oyster shell is more stable, being composed of mostly calcite. And here is the selling point, calcite is used in 'neutral water filters' for home well users to combat acidic water as it self stabilizes at 7-7.5 depending on the exact calcite used.


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## JSeymour (May 26, 2016)

I have really hard, alkaline water, so I have the opposite problem.


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## dwalstad (Apr 14, 2006)

If you have soft water, I would add a little crushed oyster shells to the substrate. (I am thinking a spoonful mixed well with each gallon of soil.) It should not hurt and it will slowly introduce calcium and magnesium, two very important plant nutrients, into the system. Otherwise, you are going to have to figure out how to add calcium and magnesium chemicals to the water, and that is not as easy.


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## netcode (Aug 14, 2007)

dwalstad said:


> If you have soft water, I would add a little crushed oyster shells to the substrate. (I am thinking a spoonful mixed well with each gallon of soil.) It should not hurt and it will slowly introduce calcium and magnesium, two very important plant nutrients, into the system. Otherwise, you are going to have to figure out how to add calcium and magnesium chemicals to the water, and that is not as easy.


aren't crushed oyster shells just calcium?

What about dolomitic limestone? To my understanding it has calcium and magnesium


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## dwalstad (Apr 14, 2006)

That's fine, too.


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## JSeymour (May 26, 2016)

netcode said:


> aren't crushed oyster shells just calcium?
> 
> What about dolomitic limestone? To my understanding it has calcium and magnesium


Oyster shell is made mostly of calcite with bands of aragonite. Calcite is different as it has many other ionic compounds: manganese, magnesium, phosphate, etc. It also stabilizes at about 7.5 pH, rather than 8.3 pH with crushed coral. Many nutrients are not biologically available as you get farther from 6.8-7.0, so oyster shell will often be the better choice for many reasons.


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## dwalstad (Apr 14, 2006)

netcode said:


> First, I am assuming that in soft water over time a soil substrate will go acidic. Is this correct?
> 
> Reading Walstad's book (pg 131) it indicates that at first the hardness will go up. However, over time it looks to go back down, and the graph only shows 16 weeks. To my understanding, the nature of a closed system is that it will continue to become more acidic as time goes on.


I have never heard of a soil pH drop-off. All major anaerobic processes (e.g., denitrification, nitrate respiration) consume acid. [As an example, see my book page 65 for the nitrate respiration reaction.] Since a submerged soil is anaerobic, there is little chance of it going acid once it stabilizes past about 8 weeks. All those anaerobic reactions of the soil bacteria are going to keep the soil pH up.

Here is my handy rule for hobbyists. *The aerobic, oxygen-requiring reactions (e.g., nitrification) generate acid. The anaerobic reactions (e.g., denitrification, H2S production) consume acid.*

Many factors affect soil and water pH. See my book, page 5. If you add a little dolomite lime or oyster shells to the soil to provide calcium, magnesium, etc, it should not affect the either water or soil pH that much.


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## NGoyette (Aug 4, 2015)

I to have soft water and when I set up my 30 gal Walstad tank last year I added a several scoops of oyster shells and Turface mixed with my mineralized soil. I must say that I do not have any issues now and my plants and shrimp are healthy.


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## dwalstad (Apr 14, 2006)

NGoyette said:


> I to have soft water and when I set up my 30 gal Walstad tank last year I added a several scoops of oyster shells and Turface mixed with my mineralized soil. I must say that I do not have any issues now and my plants and shrimp are healthy.


That's the way it should be.


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