Aquaponics is typically operated with the plants growing in some sort of media, such as gravel or clay pellets. Most books and websites actively discourage using soil. Even coconut coir is generally advised against. There are really three reasons that I have read for this: 1) The breakdown of organic matter produces tannins and humic acids that will color the water and restrict your ability to observe your fish, 2) The small particles of soil, or the small particles produced by the breakdown of organic matter will clog your plumbing, and 3) Decomposing matter produces acids that will lower the pH in your system.
I can tell you right now that all three of these are completely true. But they still aren’t enough to keep me from building a soil-based system. I mean, when the only piece that draws your interest is removed, what is really left? Besides, how big of a drawback are these really?
The first one, the color of the water, is probably the biggest drawback. Since I have been using soil in my system, my water has usually been somewhere between the color and turbidity of weak tea and very strong tea. After adding fresh compost, sometimes it approaches weak coffee. Visibility through the water ranges from 6” to 3’ or more. Yes, it varies. I suspect that has something to do with the filtering ability of the various organisms feeding from the system, but I don’t really know. But the fish don’t seem to mind it at all. Sometimes a fresh addition of compost or a new bed will make them slow down or stop feeding for a few days to a week, but they always come back and deaths are rare. As for the other problem, that of not being able to observe my fish, I guess it hasn’t really been a problem. I put lots of cleaner organisms in the tank, like snails and crawdads, so if fish die, they don’t hang around for long and rot.
The second concern, that of soil particles clogging the system, is a bit more of a problem. It was certainly the first major challenge I had to solve. When I first hooked up my bulkhead fittings on my trays, I used filters that fit the bulkhead fittings and are readily available in hydroponics stores. The problem is, these are meant to go with large, chunky media, and the fine soil particles flow right through them and clog up everything pretty quickly. I tried solving this by tying pieces of an old t-shirt over the filter, but that clogged even more quickly. In the end, I lined the entire bed with landscape fabric. It is fine enough to hold the particles at bay, porous enough to let the water flow straight through, and tough enough to last a while.
The third concern, the pH lowering effect, isn’t actually a concern for me. Where I live, in Mesa, AZ, the tap water right out of the tap measures a pH of 8.0. I briefly had a system with just media, and the evaporation and refilling concentrated the minerals in the water, bringing it up to 8.3 over a few weeks. When I first added my soil bed, the pH dropped to 7.0 in just a few days. Since then, it has risen a little and has settled at 7.6 for months with no additions of chemicals to lower it. The plants and fish both seem to be pretty happy at this level, so I just leave it.
What about filtering? After all, the purpose of the media in aquaponics is to give a matrix that ultimately filters solids and impurities (like nitrogen) and solid waste from the water and converts it into a form that is usable for the plants. So, how does soil measure up in this aspect? Well, an aquaponic filter operates on the basis of its “biofilter.” In other words, how much surface area is covered with the bacteria that absorbs and processes the waste? If you dig into a bed of aquaponic media that has been operating for more than a few months and pick out a piece of gravel from deep in the bed, it will have a slime on it. That’s the biofilter. How well does soil act as a biofilter? Soil is a natural biofilter, ripe with all kinds of bacteria, fungus, and invertebrates that process the waste quite efficiently.
There are a couple of ratios that are important in aquaponics. In an aquarium, the general rule of thumb is one inch of fish per gallon of water. In aquaponics, it is one pound of fish per two gallons of water, a higher stocking density made possible by the superior filtering of the water. As of this moment, I probably have about 60 fish averaging 5” or so in my 250 gallon tank. So I am over aquarium densities by a little bit, but a bit shy of aquaponic densities. The second ration is bed to tank. Usually you want to start with a 1:1 ratio, eventually moving to 2:1 to ensure that you get enough bed area to effectively filter the water. I have about a 1:2 ratio, with only about half of the bed area I am going to need.
To date, I haven’t had a water test that returned any nitrogen at all since the tank was finished cycling. In fact, it is just recently beginning to produce enough nitrogen that my plants are showing vigorous growth. Even then, in my large bed, the plants closer to the input of water are growing faster than those near the drain, a sure sign that the nitrogen isn’t making it far enough.
But what about filtering the solids? That is one of the big problems with aquaponics. The fish produce solid waste as well and that can build up over time and cause problems. That is usually dealt with in one of two ways. The first is that the pore space between the media is big enough to handle the solid waste. Worms are put into the system to break down the solids into a more usable form, but eventually it will build up and need to be cleaned. The second is some sort of filtration system that separates out the solids and removes them from the system. This is usually costly and requires more maintenance.
In my system, I just have big enough pipes that the solids pass straight through. The soil catches them at the end. If I go out at night, there is usually a huge knot of earthworms feasting on the pile of vegetative leftovers and fish waste at the end of the pipe. They just get incorporated right into the soil.
So I can say that I have been using soil in my aquaponic with great success. I’ll talk about soil composition in my next post.