Next time you do a nutrient change, mark the water level inside of the reservoir (SWC system). Then all you need to do is add fresh water back to that line, and you'll know that the nutrient solution is not becoming concentrated or diluted. If you have not done a nutrient change yet, I'd recommend doing them every 1 to 4 weeks (mostly depending on plant size compared to reservoir size). In other words, a 5 gallon reservoir will become out of balance twice as fast as a 10 gallon reservoir for the same size plant/s.

I realize that general of a statement, along with everyone's personal opinions, that opens the door and leaves lots of room for debate. But the question was quite general in nature, so there wasn't much to go on. But unused nutrients will remain in the solution, and can/and will make the nutrient solution unbalanced over time (the longer you go without changing it) even if you top it off (to the mark) daily. How fast depends on many factors.

As for the question about using tap water, well that's a debate in itself. I don't know anything about the water quality of your tap water, that can change from house to house even in the same city (not to mention across states and country's). But I would recommend using the best water quality you can, and use the same water when topping off your reservoirs.

Homehydro,

Thanks for the info. The tomatoes aren't planted in a hydroponic type medium, rather potting soil mixed with 20% perlite, which is why I'm more worried about nutrient buildup in the soil rather than the resevoir. Also, the containers are fairly small (growing a smallish determinate cherry tomato type Market Red), 10l (2.5 gallons) of medium with about 3l (0.7 gallons) water resevoir. The plant seems to use about 2l per day when the sun is out, so the nutrient solution is pretty much completely replaced every 2 or 3 days.

I've started some new seedlings in a soilless medium (perlite and vermiculite), but they are still quite young.

OK, I'm not sure if you are concerned about "nutrient buildup" or the nutrients the roots get being out of "balance." Buildup happens with hydroponic growing media also. Sounds like your talking about salt buildup in the growing media (soil). I don't recommend using soil instead of a growing medium for hydroponics, but if that's the case I don't believe that they will build up any faster in soil then than they would in a hydroponic growing medium. The keyword being "buildup."

The difference is soil already has nutrients in it, hydroponic growing medium does not. If you water your plants (in soil) with miracle-grow, you are still adding nutrients to it. The difference there is miracle-grow does not have the small amounts of trace elements that's in soil. I'm probably wrong but 2.5 gallons of growing media (soil) and .7 gallons of water sounds like a bad ratio.

P.S.I pour all my old nutrient solution on the plants and trees around the house (4 trees and about 20 bushes), I try to do it evenly, although the two trees in the back yard get most of it. But I can't say I have seen any ill effects, I can't say they are doing better either, I wasn't expecting to do a comparison on that. Also our soil sucks and is hardy fertile here.

Hi brickza,

I am using a similar technique (actually plant bags of different sizes and pots) to grow determinate tomato plants and other various crops. I also use perlite (or rice hulls) mixed with potting soil for the purpose. Except that I use this method as a final destination for my depleted nutrients - actually alternate depleted nutrient solution with "fresh-one" plus occasional watering. I used to have an extended drip system (to little waste) installed for the purpose, but have switched to manual since the production of this part has decreased. Most depleted nutrients are now used for bush and tree seedling production, which thrive amazingly well in it (as they fix nitrogen from the atmosphere and are anyway very forgiving in matters of pH, nutrient quality and composition).

Anyway, the hybrid system with soil/perlite mix has proven to be not only a simple, economical and effective method, - but also very forgiving and hassle free. The soil part is promoting natural ion-exchange and self regulating pH in the root zone. Besides that, nutrient temperatures are apparently not so much of an issue.

I would recommend to simply water your plants once a week, (or in your case between nutrient changes) that should be sufficient. Build up of "unused" nutrients is anyway overrated from my understanding and experience. With a potting soil/perlite mix it seems to be even less of an issue. I'd keep nutrient concentrations reasonable, though. In fact I guess that nutrient/salt build up is mostly the result of people pushing and overdoing it with nutrient concentration. I've never knowingly experienced it in any kind of system nor with any medium.

I remember having read an older study about tomatoes grown in both, amorphous substrate (was it perlite?) and potting substrate/soil, a while back. Both fed with a standard drip system. There was no notable difference in growth and crop size/quality and no issue with the plants grown in "potting soil". No indication for salt build up in the potting soil either.

Also your strategy with having only a small amount of nutrients seems fine to me, as you don't need to worry much about permanent changes in concentration. A frequent and (almost) complete change of a small amount is sometimes the better alternative. As I don't know the reason why you actually recycle or have your system "automated" (have you?), but I guess you could even simplify things and try/consider drip to (little) waste as well. Why? Well because if you dose the spent nutrient amount well, there is little loss and you can extend your "garden" easily and without any technical change or modification as there is no need for a run back then. Just a suggestion that seems useful to me in this particular case.

Cheers and good luck with the final results,
Lucas

PS: the "drip to little waste" what I proposed is obviously only possible outdoors. It goes actually without saying - but I wanted to be sure to be understood properly by everyone. Also - with getting oftenly little or incomplete information, some guesswork and selected (although coming with some risk) anticipation from my side is necessary. Thanks for the understanding, Lucas.