Step 2. Preparing the substrate

Substrate preparation for my dirted 90 gallon

Let me start by saying that dirt in an aquarium has worried me in the past!! I have bad memories of attempting a dirted aquarium in my 24" back in the 90's. For me, that was an experiment that ended in an algal mess that smelt like badly decomposing mud, to which I concluded that dirt was not a good idea.

So what's changed?..... the internet and Diana Walstad!

Dirted substrates are a serious option for the planted aquarium enthusiast. Particularly those seeking a low-tech, low-cost option. Essentially, soil in the substrate and fishfood provides plants with the nutrients they require and decomposition of organic material in the substrate provides a source of CO2. As CO2 and nutrients are not in abundance, lighting needs to controlled, otherwise algae will take-over. There are plenty of examples of awesome dirted tanks around the internet. The impression I got looking into it was that they are tricky to establish but once they're running they are low maintenance and can look great!

Soils

Soils ain't soils.... or so they say. Soil differs in it's composition and consists of clay, organic matter, potential harmful chemicals and pesticides etc. So there is no single recipe for the perfect dirted substrate. Diana Walstad recommend's Miracle Grow, but I couldn't find that here in Melbourne, so I resorted to a packet of Organic Topsoil from Bunnings mixed in with a small portion of organic topsoil I had delivered for my garden beds a few months ago which has been sitting around getting rained on in a pile. I mixed this in as the packet mix organic topsoil from Bunnings looked to me to be almost entirely organic (lots of tiny bits of wood/bark) and not a lot of dirt.

I loosely followed this article on how to mineralise the soil prior to putting it into the aquarium. To be honest it didn't get as much love and affection as what's recommended. Here's what I did:
  • I mixed garden topsoil with the packet organic mix in a burrow
  • I flooded the burrow and scooped out any large floating bits of organic material (repeated this a couple of times)
  • I flooded the burrow again and left in the sun for a few days to dry out a bit.
  • I spread the soil in a 1 inch layer over a tarp
  • I soaked it with a hose and let the sun dry it out (about 5 cycles over 3 weeks)
I tried putting the resulting soil mix through a kitchen strainer, but this would have taken me days so I prompty gave it a miss and just picked out large particles by hand.

Ferts

I've got very soft tapwater here, so I'll add a dusting of dolomite lime and muriate of potash just before adding the soil underlay to the tank. 

Capping

A dirted substrate needs to be capped, typically with coarse sand or gravel. I decided on pool filter sand, which in Melbourne is available at Clark Rubber and most pool shops (not Bunnings!). It's a coarse river washed sand which IMO has a nice natural look to it. I would have preferred something slightly coarser, but this stuff isn't that fine that it get's blown around by the filter. I also like the look of a black sand, but couldn't find anything easily available. The pool filter sand is cheap as chips but takes alot of rinsing (even though it says it's pre-washed).

Putting it all together



The general rule here is that your soil underlay should be no more than 1.5" deep and your capping no more than a couple of inches on top of that. With this in mind I put a half inch layer of PFS below the soil underlay to achieve some terracing. A credit card works well to ensure that the PFS fills in all visible edges of the aquarium so the soil remains unseen.

Added 25/3 - I've realised that I should have wet the soil as part of this step prior to adding the capping. It should be the appearance of thick mud, whereas mine is dry. I suspect that this may cause me some bubbling and air pockets as a result. 



I'll add a few more pics of the substrate preparation at some stage.

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Appreciate your feedback! I'd love to get some insight from other dirt enthusiasts out there.