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Discussion Starter #1
I have heard of people having issues with substrate depletion in soil planted aquariums. Diana writes about using food to replenish lost nutrients. Has anyone here experienced the depletion of nutrients in the substrate?
 

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I have a 50G that's been running for over 3 years and everything is still growing well. I planned on taking the tank down about a half a year ago but it just keeps running. I don't have any fish in it anymore, but it keeps producing plants (2 kinds of crypts, dwarf sag & hygro) to sell and trade so it stays running.​
 

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How do you fertilize the tank?
Well, with no fish in it anymore, it's not getting ammonia/nitrates from food and waste and I've only been topping off the water not even doing changes so there really hasn't been anything added that could be considered fertilizer (natural or synthetic) in many months.

The plants are getting all their nutrients from the substrate/soil I assume, the water column must be pretty much devoid of nutrient as a large part of the tank (20-30%) is filled with H.difformis which AFAIK is a bit of a nutrient sponge. The only thing I've done recently (two months+ ago) was add a 2-3 ml of Seachem Flourish (traces) and some Seachem Equilibrium, maybe a 1/2 tsp.

This tank is a testament to the amount of "neglect" an El Natural, Walstad, Soil based (call it what you want) type of tank can endure. I almost feel bad, but it's become a bit of a science experiment, waiting to see when things will stop growing. That said, it really has slowed it's growth, but it's still steadily producing a good crop of plantlets and cuttings every 2-3 months.

Just like nature, it seems to do just fine when left alone.
 

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Discussion Starter #6 (Edited)
I was wondering if you were still using fish food. It's decay should provide all of the nutrients you need. Especially of you have some emergent plants to gather CO2 from the air.

I am guessing one would have to consider what is leaving the system to analyze depletion. In your system the only thing that is leaving is CO2 from the surface and your cuttings. Since the CO2 will create an equilibrium with the atmosphere, CO2 can travel both ways.

Your cuttings would have all 17 required elements in them. Since you are not adding any other elements you will eventually deplete your substrate. I have heard this process takes about 2 years.
 

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Just curious as to why there is a concern about soil substrate depletion? I would think that any substrate would deplete over time. Do other substrates release nutrients much slower? If too slow, what good is that to the plants?
 

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I believe Diana writes in her book that soil contains years (decades?) worth of most nutrients plants need. I also believe that she writes of having a tank set up for many many years without any soil depletion. All she does is feed a little extra for the plants. My longest NPT I had for 3 - 4 years with no problems.

-ricardo
 

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I doubt fish food alone could replenish the substrate. May be you could try using substrate fert? They all work pretty well for root feeders.
 

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Discussion Starter #10
The only minerals can leave the system is in trimmings, right? Perhaps if the trimmings can be added back to the tank. If they were dried and ground and buried in the substrate they would be slowly available. I am just thinking aloud here...
 

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You don't have to dry and ground your trimmings to add the nutrients back to the soil, just let any dead leaves to decompose and form mulm over the substrate. The idea in the Walstad method is to allow this mulm to accumulate on the substrate. The mulm comes from extra food, fish waste and dead and decaying leaves.

While it is correct that changing the water will remove nutrients from the water column, that's irrelevant because in a well-running El Natural tank, there is no need to change the water except for once every month or two. Also, it won't remove the the nutrients from the substrate unless you vacuum it which you shouldn't do too often.

-ricardo
 
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