# The PRIMARY purpose of sand/gravel cap in Walstad Method



## tacp18 (Jan 16, 2019)

Just want to verify that the PRIMARY use of the gravel/sand cap, in a Walstad setup, is to hold dirt down right? I get that it does plenty of other things (with bacteria and mulm), but I'm concerned about the PRIMARY use of it.

For instance: If there was a dirt that I didn't have to be concerned about getting into the water column, would the cap be necessary?


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## Michael (Jul 20, 2010)

As far as I know, yes. I have unintentionally created some capless soil tanks (don't ask how!) and they were fine except for the cloudiness if disturbed.


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## dwalstad (Apr 14, 2006)

Some mineral soils don't cause that much cloudiness. If you're careful with one of these, you don't need a cap. I had one tank with just my clay soil substrate and it was fine. Sometimes, I used a mechanical filter (50 micron pore size) to filter out clay particles. Ideally, you don't want "dirt" to collect on the plant leaves. Remember that clay soils will improve as they age and collect bacterial biofilms (my book, pp. 69-71).


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## JoanToBa (Aug 23, 2018)

Well, in a wild pond you will usually find that, when slightly disturbed, the soil very quickly clouds the water. So in a natural setting, the cap is inexistent, but because we don't have much of a steady hand, it's better to have a nice cap to be able to siphon it and plant things easily.


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## DutchMuch (Apr 12, 2017)

here in hartwell GA, in the local lake (one of the top 10 clearest lakes in the US) has in most areas, a Extremely hard unpenetrable layer of clay as the substrate. Surprisingly many plants grow here so its up for pickins'!!! I imagine that creating the hardest layer of clay possible in a tank as the only substrate would be a pretty interesting experiment.


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## zolteeC (Dec 26, 2017)

tacp18 said:


> Just want to verify that the PRIMARY use of the gravel/sand cap, in a Walstad setup, is to hold dirt down right? I get that it does plenty of other things (with bacteria and mulm), but I'm concerned about the PRIMARY use of it.
> 
> For instance: If there was a dirt that I didn't have to be concerned about getting into the water column, would the cap be necessary?


I guess if you have fish you definitely want to "cap it". Not only for aesthetic reasons, but the fish cause some turbulence that would disturb the substrate.

On the other hand, I have a nano tank with shrimp. It started as a "standard Walstad" tank, but over time I'd say new soil layer developed over the gravel. I do not see any negative effect with this, I don't clean it up. My shrimp does not stir it up.


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