Nutrient consumption rates

Have a sciencey question...

Is there a method for estimating the relative quantity of nutrients consumed in a hydroponics system? When my TDS drops by 100 ppm, how does that correspond to molar loss of nutrient constituents? I imagine some empirical formulas for vegetative vs. flowering growth.

I remember reading something about this in a hydro text book. It lets you top up a recycled nutrient solution with a slightly different formulation. You could get by with very infrequent reservoir change-outs and maintain a better nutrient balance. Plants probably have a pretty consistent relative consumption rate through each stage of growth. The reservoir would still be changed out, but maybe only once-a-month or less. I've read that the Lucas Formula recommends change-outs when the system has consumed a reservoir's worth of water, or when the pH becomes unstable. Makes for a simple, more infrequent approach to system maintenance.

One would have to run used nutrients through a LC/MS to compare results to those from the original mix.

I suspect a variance in uptake of different elements depending on strain, age, stage of growth and environmental conditions.

Larger reservoir equals longer between changeouts.

Still looking for a cheap mass spec on ebay...

I'm sure these numbers have been found for a lot of plants (trying to dig that up now). Even very approximate numbers would help. For example, in my top-up solution, if I can comfortably cut my micronutrients in half during veg and by a third during flowering in my top-up solution, could this cut my flushes in half?

While reducing water change-outs is nice from a maintenance perspective, there's stuff like this too:

http://www.pressdemocrat.com/csp/mediapool/sites/PressDemocrat/News/story.csp?cid=1860712&sid=555&fid=181#page=0

Every once in a while, someone in the DWC section asks if they really need to change their reservoir.

What's interesting is is that the reason they usually ask is to report they've been doing it successfully for many complete grows. They already know the answer, and want to know what's wrong with doing it that way. Yes, it's possible without any high tech instruments to go very long periods in DWC without completely changing out the reservoir. It's the type of thing that with skill becomes just like watering and feeding a potted plant.

It's something I've been able to do myself for entire flowering periods, but it's usually due to laziness. For the time you're spending, it makes more sense in my opinion to change the nutrients every time the tank becomes low, because then you're not wasting much anyway, and the process is easy when it's low already. This is the best way to guarantee a consistent product without constantly having to think about what plant needs what specific additions.

My objective is to create a really stable system that requires infrequent or very simple maintenance. I'm introducing automation where necessary, though I'm also focusing on seeking stable equilibriums and self-check / balance mechanisms.

I presently have an RO water tank that auto-feeds my reservoir. As my system uses water, it's automatically topped up using a float valve. I've found that this does not impact pH much, only concentration. This slow-and-steady dilution gives me a slight buffer against overfeeding, and helps reduce salt build up.

I use an Excel spreadsheet where I keep track of my numbers. I developed some empirical formulas that allow me to enter in my current ppm and my target ppm. The spreadsheet tells me how many mL of each of my nutrients and supplements to add, based on the relative ratios of nutrients.

Right now my nutrient ratios are matched to what the supplier recommends, and I dial in the ppm I want. Because plants consume more macronutrients than micronutrients, I probably could reduce the fraction of micronutrients in my top-up formulation. If I had an idea concerning approximate consumption rates, this would go a long way towards maintaining a more balanced solution throughout the grow.

I still plan on flushing my system. I have a very good idea about the amount of water my system is consuming. I'm trying to determine a nutrient top-up and reservoir flush schedule as a function of system water consumption, and optimize this schedule as much as I can.

My grow is as much a research project as a means to grow some bud. So far things are looking pretty good, here's my bushy little control plant at just under 2 weeks veg, having kept my ppms below 150 until today.