The challenge:
I am attracted to the concept of hydrponic gardening from its simplicity of purpose (isolating plant growth down to its barest elements) and its potential for significantly reducing plant nutrient requirements. Unfortunately, hydroponic systems often have a problem with the balance of individual nutrients and/or the pH getting off target over time. Some gardeners approach this by simply disposing of the nutrient solution without recovering it at all. Others approach the problem with limited reuse for a couple weeks, followed by disposal and complete replacement of the solution. Some commercial growers use ion-selective electrodes to measure the quantities of individual nutrients, but these cost hundreds of dollars each, and integrating them with an automatic replacement mechanism adds more complexity still -- pricing this approach out of the range of most home-growers. Disposed-of nutrient solution imposes environmental consequences, in the form of encouraging algal blooms if it makes it into waterways.
Concept:
To counter this, I came up with a process, and a method of testing the process, to provide automatic nutrient balancing for a continuous flow hydroponic system (such as NFT). Powders of the nutrients, such as potassium nitrate, calcium nitrate, monocalcium phosphate, magnesium sulfate, copper sulfate, manganese sulfate, ferrous sulfate, sodium molibdate, boric acid, and zinc sulfate, and optionally other trace elements, buffers, and disinfectants are stored in individual waterproof containers with a layer of water over them. This water will stabilize at the solution's saturation point; any new water that gets added will become saturated with this nutrient. The containers are kept at a level just below where they would overflow into a common "refill" container.
Solution for the NFT system is pumped from a low base reservoir through a filter up to the top, where it flows down past the roots back into the base container. A float in the base container opens two valves when the level falls too low: one for freshwater into the refill container, and the other offering a diversion route for the nutrient solution feeding back from the NFT system. The diversion lets it empty into the far end of each of the nutrient concentrates. The rate at which it flows into each is controlled by user-adjustable taps, as is the rate at which freshwater flows into the refill container. The addition of nutrient solution into the concentrates causes overflow of concentrate into the refill container at a level proportional to how fast liquid is flowing into it. When the refill container gets high enough, a siphon fills, emptying its contents into the base reservoir. The float raises, shutting off the valves and thus shutting off the filling of the refill container.
One potential issue that comes to mind is whether the concentrates of the various nutrients will truly remain concentrates of *that* nutrient over time. After all, the concentrate that leaves into the refill container is being replaced with not pure water, but water with a mix of various nutrients in it. Will the nutrients that are already in solution in that replacement remain in solution and thus leave the tank harmlessly on every cycle, or will less soluble ones be deposited and replaced by a more soluble powder in a given container, hence gradually converting the concentrate to a different type of concentrate and leading for the potential for nutrient swings down the road? I set out to find out. If the results prove to be favorable, and no big flaws are seen by those with experience, I intend to eventually try setting up such a system.
Testing Process:
I took two clear 2-cup measuring cups. To one I added 1/2 cup of white sugar, then filled up to the 2/3 mark with water. To the other I added 1/4 cup of salt and then filled up to the 1/3 mark with water. Sugar and salt were selected because of their ready availability and ease of distinction by taste. I then stirred the two containers until their respective liquids reached saturation. It took notably more sugar to reach saturation than salt (** After the fact, I looked up their saturation points, and this makes sense; salt's is 35.9g/100ml, while sucrose's is 211.5g/100ml). The undissolved sugar and salt remained at the bottom of their respective containers to keep them saturated.
I then repeatedly filled a third "mixture" container with 1/2 cup of water and a teaspoon of water from each of the sugar and salt concentrate solutions, taking care not to skim any undissolved solute from the bottom. After a quick swish of the mixture container to mix it up, I then added one teaspoon of it into each of the sugar and salt concentrate solutions (note: the taste of the mixture cup was of both sugar and salt, although the salt was somewhat stronger). The mixture cup was dumped and then refilled with a new 1/2 cup of water, a new teaspoon of salt and sugar concentrates, and so forth, starting the process over. Approximately every third cycle, I gave the sugar and salt concentrates a good stir to ensure that the liquid in them remained saturated, and let the undissolved solute settle.
Results:
I went through approximately 16 cups of fresh water before the sugar container contained only the tiniest amount of visible sugar remaining (the salt container still had a measurable amount left). I then did taste tests
Sugar concentrate container "At least" 20:1 sugar:salt by taste, in comparison to manually titrated sugar/salt solutions. Could be much higher. I'm not really sure I could taste the salt at all.
Salt concentrate container: could not detect sugar taste at all
Conclusion:
The approximate 32 water replacements amount to 2/3rds of a cup for each concentrate container, meaning that as a net result, the initial water in each container (appriximately 1/6th and 1/12th cup, respectively) was replaced several times over. This is a very encouraging result; despite the extensive replacement and near-exhaustion of the sugar, both cups remained, essentially, a concentration of their respective "nutrient", boding well for the proposed hydrponic nutrient balancing system.
One untested issue that remains is the effect of time on the solutions in the concentrate containers. Some nutrient solutions come in A-B mixtures to prevent the slow side reactions that could render certain nutrients either unavailable or of limited availability to plants. How significant of an impact this will have on an implementation of this system is unknown. An additional issue in regards to time is to whether some nutrients will prefer to precipitate out and be replaced in solution by their more-soluble concentrate over prolonged periods not experienced in this test. An actual implementation of such an irrigation system will need to be built to determine whether this is a problem.
Thoughts?
freemangreens
grizzman
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