Sometimes that's what I'd like to do with Gatorade. Drink the water and spit the salt out, but I haven't figured out how. The roots aren't that selectively permeable, so the plant gets much of the nutrients. Whether the plant can pee them out unmodified is another question. The EC meter won't distinguish between ions, so it may register a moderate conductivity even when the composition is no longer as nutritious as necessary sicne roots maintain electrical equilibrium mostly, meaning they are experts in cap and trade politics.

Rules of thumb generally say, just measure the water you add and when it is about half or two-thirds replaced in demand situations, just change then.

Unless you add demineralized water the water minerals start increasing in proportion and accumulate driving up the EC also. The loss of water only is mostly from non plant evaporation, due to environmental factors like temp, illumination and your design's exposing water. One way to separate that would be to set up with no plants but simulate covering, and then watch how the levels change. It won't be completely accurate since the transpiration of the plants cools the general area and increases humidity. Plant transpited water has retained all nutrients in the plant since only pure water evaporates...if mediated by the plant, the nutrients get taken up, but if not, it is that sort of water loss you are worrying about.

This post was edited by PupillaCharites on Sat, Nov 16, 13 at 17:32

Using solution is a healthy thing. The extent to which they take up mostly water is influenced most greatly by temperature.

Temp is most important to tomatoes in the setting of fruit. They will live at a very wide range of temps, but it is fruit set that is affected.

Most plants modulate the uptake of nutrients by the availability of nitrates and ammonium ions. The root is like a hydraulic pump. You prime it with a little magic powder (in the form of dissolved nitrogen-bearing ions) and the water goes in, nutrients and all minerals, it is unstoppable. The plant cannot "leave" nutrients (in our case ionic salts) and only drink pure water. The plant can cut it's overall uptake of the juice or increase it.

Temperature is a secondary effect, like all the rest of the environmental conditions, not a principal driving effect. When the plant starts transpiring more, it goes through more solution, but it is not leaving the salt behind. That is why in hot conditions we need to cut back on the EC, since the sweating plant's going through more water. In the winter the nutrient solution is stronger ... in neither case is the plant principally and selectively filtering proportionally 'more water' from the salts, and that is why counting the replaced liquid is a good practicaly way to decide when to change the water.

Don't worry about the plant leaving the nutrients, it just can't happen, the plant is stuck with drinking it all. Set your fertilizer strength and the plant will adjust how much it drinks by its need for nitrogen and ability to pump it, along with all the salty bathwater through the roots and into the plant.

You can add some fertilizer to worry about EC, but definitely not every time, some people alternate very successfully, or add 1/3 or 1/2 strength depending on the evaporating rate in general, but since you are also building up a kiddie end of the swimming pool, you may waste good fertilizer towards the end when the plant will be much happier if you just change the diapers and put the addition toward a quicker change, though different plants behave differently depending on what sort of microbial life begins to teem in there...

Many strategies exist for people that insist on recycling nutrients to 'keep it going', which I admit not being one. One of them is removing old nutrients and using them somewhere else, but perhaps blending in 25% of them back into the new juice. The practical decision you are faced with ifyou have a properly sized reservoir it usually isn't such a big deal and small additions of nutrient are commonly done if there is a well known strength of solution that is part of your recipe/strategy.