Several months ago, I found the website http://gardenpool.org/ and it has me thinking. The way they integrated all those systems into such a small space to get such a productive system is truly inspiring. I had long planned on having a small tank in the corner of my future greenhouse to store water and help work as a thermal mass to mitigate temperature swings. I really liked their idea with the tilapia, but didn't think I had enough space to have my own.
This morning I thought of a new way to do it, and not only does it allow me to have tilapia, but it may actually give me more space to grow my plants and even solve some of the problems I have been struggling with.
The original plan was to pour a concrete slab and seal it. Then I would build cinderblock walls. The problem with this is it doesn't (in my opinion) adequately protect the subsoil from the constant moisture of the floor of my greenhouse. In my area, expansive clays are the norm, and introducing water to expansive clays under your foundation is a recipe for disaster.
Instead of that I would have a contractor come build a swimming pool with some modifications. This would keep the water in very effectively. There would be a drain with a closed valve at the bottom of the "pool" and another with an open valve at two feet up. Also, unlike most pools, the corners would all be square. Then I would build interior walls out of cinderblock with mortar missing from the vertical joints in the bottom course. That would allow water to flow freely from one side of the wall to the other. The interior of the walls, where the plants would eventually go, would be filled with clean sand up to the two foot mark where the water level would be. I would make my growing soil above that. Too much organic soil beneath the water line would cause anaerobic conditions that are not only not healthy, but might actually harm the fish as the water continually flushes from the beds into the pond.
Then I would build a grate two feet up over the path. It would be able to be lifted in sections for maintenance or harvesting of fish. The section under the grate would then be filled with water as a habitat for tilapia. The fish would provide me with a constant source of fresh fish that I could harvest whenever I wanted.
There are several nice things about this. The only space I lose was designated for growing ground cover and mushroom logs. The ground cover, if necessary, could be grown elsewhere. The mushroom logs could, likewise, be grown elsewhere. It frees up the tank I was going to put in the corner for more planting area. It also eliminates most of the rainwater storage tanks I was going to locate elsewhere on the property. It provides a much greater thermal mass, allowing the greenhouse to avoid the huge temperature swings most greenhouses have. It won't get so hot in the summer and it won't get so cold in the winter.
Watering the greenhouse becomes a bit easier. Most of the water would come from greywater that has been filtered through a wetland system elsewhere in the greenhouse and then through the soil. I could also put a pump on a solar panel that would run all day, pumping water out of the pond and cycling it through the soil via a trench irrigation system. This would allow the fish waste to be removed from the fish and used for the benefit of the plants. Also, because the water would be in the lower sand level in the garden, the perennial plants, and even some of the annuals, could put their roots down into the water and get water directly. In effect, it would create a water table about 3' below the surface of the garden.
As I said, I just had this idea this morning, so there are lots of ideas to work out. My wife, ever the devil's advocate, has already provided an extensive list. Will there be mosquitoes? Will it get too hot in the summer and then heat up our attached house, keeping us from being able to cool it adequately? The list goes on. I also have plenty of my own questions and a good start would be to find a good book on how to raise tilapia. Then I need to think on it for a while and make sure I can't come up with any reasons why it won't work.
