# Soil Depletion?



## Dr.Awkward (Jun 6, 2010)

Hi. I've had a 12-gallon tank set up using the Walstad method for just over two years. I followed the instructions in the book exactly and had wonderful success with the tank until the last few months. Now, the plants aren't growing as fast as before, I'm having trouble keeping floating plants healthy and algae has become an increasing problem.

The tank has been moved twice to different cities during the months when the trouble started. I originally thought the difficulties were coming from the change in city water but now I'm pretty sure that's not it because the water where I am now is almost identical to the water at the original source.

I'm caring for the tank just as before, feeding the same with basically the same stock, adding water at the same frequency and amount.

Is is possible my soil layer has been depleted? If not, do you have any ideas why I'm running into this trouble? Thanks.


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## Zapins (Jul 28, 2004)

Soil does eventually become depleted. My soil tanks have lasted anywhere from 1.5 - 4 years so you might be in that stage. It wouldn't surprise me since you have a 12 gallon tank and probably didn't put more then 0.5 - 1 " of soil in the bottom. Some of the larger tanks I have that lasted 4 years had a 2 inch thick layer.


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## Dr.Awkward (Jun 6, 2010)

Thanks Zapins. I was pretty sure that was it but really wanted a second opinion before tearing the whole thing down and starting over. I wondered if the small tank size would enter into it.


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## Michael (Jul 20, 2010)

You don't necessarily need to tear it down and start over. You could increase the number of fish and feed them generously. This puts nutrients back into the system. Although the nutrients in the soil may have been used up, the soil still has the capacity (CEC) to sequester nutrients and hold them until plant roots can use them.

If you can't increase fish/feeding, you can dose fertilizers in the water column, as you would in a non-soil tank. The same thing will happen to those nutrients. And you can use plant tabs or other substrate fertilizers to increase nutrients in the depleted soil.


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## xxbenjamminxx (Oct 18, 2011)

I agree with Michael. If your not ready to tear it down and start over I would first try some root tabs and increase dosing to see how that goes. If one can grow the same plants in an inert substrate with these things it will def do it with soil as well.


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## activesize (Jun 26, 2011)

Your problems could be due almost entirely to all the variables involved in moving the tank twice to different cities. Assuming you didn't break the tank down completely before each move, then transporting an active NPT with a small amount of water in it would be absolutely expected to cause an algal bloom since the soil, cap, and plantings would all be disturbed and thereby introducing all kinds of soil nutrients, dead leaves, mulm, etc. into the remaining water. Since your rooted plants are growing slower this could indicate that the soil and cap have been flushed of nutrients to a degree, probably from all the sloshing around and water removal caused by your two moves. Furthermore, floating plants could have a tough time getting reestablished with such massive water changes as involved in transporting and setting up at two different locations. The algae wouldn't help either, attacking leaves and also helping to keep the water column sufficiently low in nutrients to interfere with the ability of your floating plants to thrive in your new water. 

Consider just stepping up your maintenance schedule. This will allow the problem to correct itself naturally in perhaps a few months.


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## Dr.Awkward (Jun 6, 2010)

Hmm. I last moved the tank five months ago. It's so small that I was able to move it keeping all the plants in place with about 1/3 of the original water. I did probably half a dozen 1/2-inch gravel siphons during the weeks right after I set it back up to clean up all the stirred up mulm and have been keeping a close eye out for accumulating organics. I haven't done any massive water changes for five months now. It seems like the tank should have stabilized by now.

I'll try the root tabs but I really don't want to have to fertilize the tank - that's why I switched from high tech in the first place.

I don't think I can really add more stock as the tank kinda of does that all by itself . It's currently stocked with least killifish and dwarf crayfish.


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## Zapins (Jul 28, 2004)

Well, another method you can use to enrich the soil is to take soil add water and freeze it in thin sheets. Then you can insert small frozen chips of soil into the patches of plants that arent growing well.


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## activesize (Jun 26, 2011)

I don't see how it's possible to even keep much of the soil and cap separated when it's sloshing around in 1/3 of a tank of water during a move to another city- twice. A lot of the soil would have had to make it's way over the cap. If you've been siphoning the bottom that much then I think there is your answer. The volume of soil remaining and it's composition is probably considerably reduced and simplified now from what is was before the moves. If that's the case then maybe you can add more soil but since it's such a small tank then it's probably simplest to just remove everything and lay down the bottom again with a fresh soil layer.


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## Dr.Awkward (Jun 6, 2010)

No, none of the soil moved. I have a substantial gravel cap and the tank is very heavily planted. The substrate is basically a big root ball which is why I can only go down 1/2 inch with the siphon. I couldn't hit the soil later without doing root damage. Maybe I'm having problems because the plants are root-bound.


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## Tex Gal (Nov 1, 2007)

Sounds like you just answered your own question. if the plants are root bound then they have eaten up all the soil in your tank. You could thin it out by cutting off the tops just below the gravel cap. Those plants will die (unless they are crypts or plants of that type) and will rot under there and replenish your soil. OR you could rescape, do root tabs etc.


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## activesize (Jun 26, 2011)

Dr.Awkward said:


> No, none of the soil moved. I have a substantial gravel cap and the tank is very heavily planted. The substrate is basically a big root ball which is why I can only go down 1/2 inch with the siphon. I couldn't hit the soil later without doing root damage. Maybe I'm having problems because the plants are root-bound.


Perhaps your plants are root bound but I've never heard of that before in an NPT. How thick is your gravel cap? You mentioned that you like to siphon into the gravel itself. It seems that you would like to siphon more than 1/2" into the gravel if you could. Are you sure that you're not sucking up soil when you do this? I know that mulm looks very black in appearance when you suck up a thick layer from the surface and the siphon water becomes black. If you push the tube further in then you could easily and quickly suck up a lot of soil without realizing it.


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## Dr.Awkward (Jun 6, 2010)

I have an inch of soil capped with 1.5-2 inches of 3-5mm red flint gravel. All of the plants in the tank are crypts. You can't see the soil level but the entire gravel layer is visible from the side of the tank. The top half inch looks clean but everything below is full of mulm and roots. The mulm gets darker and more compacted the lower you go.

If I have to pull out a plant, the soil, mulm and roots that come up with it are blue-black from the anaerobic pockets. The roots are always healthy on the inside, just blue on the outside. The mulm on the surface and first 1/2 inch is brown - that's what gets sucked up by hovering the siphon during routine maintenance. I don't really push the tube in, just sort poke it around the top to kick up a little gravel to get the big chunks off the surface. It didn't used to accumulate like that before the soil became compacted with mulm. The snails did a good job of working it in.


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## activesize (Jun 26, 2011)

Perhaps your problems are basically stemming from the thickness of your gravel cap. That is, the weight of 1.5-2" of gravel combined now with a build up of mulm finding it's way down into the gravel is compacting the soil and contributing to a less than ideal anaerobic condition within the soil. Also, I expect that your relatively large cap volume is a two edged sword leading to excessive root growth now. Of course, root growth is very desirable, but the larger the volume in which the roots have a ready and clear path in the medium in which to propagate, then so they will do. 

So, if this is the case then It follows that permanently removing a half inch to an inch of gravel and trimming the exposed roots will start to correct the situation. Circulation may also have to be addressed.


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