Here to revive a ancient post again,
So I'm not going to advise here because every situation is different.
My runoff ppm is between 200-600.
That's about my feed. "Excess salts" are literally ions (and some) from either microbial excrement or what you just put in minus what bonded to the soil.
I've been running a 1 plant perpetual so collecting runoff is an everytime thing for me.
Check the ppm and ph.
The bad: excess in whatever wasn't used.
The good: battling nutrient companies BS.
PERSONALLY:
Instead of "Fresh" water in between feedings I collect runoff, dilute and ph it and use that. I PERSONALLY haven't had issues but I don't subscribe to large ppm feedings. I like to "spoon feed"
So, it can work but be wary if you aren't using a decently ratio and quality (Water soluble nutrients) fertilizer you may end up with excess of a certain nutrient. Which in high enough concentrations can cause a lock out.
EDIT: high volumes of phosphorus and calcium can create calcium phosphate, 95% water insoluble and useless to plants.
I'm sure you have heard to put water first then nutrients and to stir nutrients with each addition, and you may have wondered why some nutes have a part A and B blah blah blah. This is why, and it's why more and more fertilizer companies are using what's call "chelates" (Key-latest) because the chelators hold ion bonds together strong enough that they don't "fall apart" but loosely enough to be available to the plant. (Very laymans terms but accurate)
Will update after work.
Ps: I found this post looking for others using similar techniques.
EDIT:
But to summarize and continue with the negative possibilities;
1) nutrient lockout by inadvertent molecular fusion and diffusion
2) anything that is bad in your soil (Anerobic bacteria for example)
Is continuously introduced.
(Remedied by aeration of the stored runoff solution)
3) incalculable ratios only hypothesized by the nutrients that bond to soil (IE phosphorus) being left behind and all water soluble nutrients (usually nitrogen) that are leached when you get runoff.
Pros?
Yeah, it doesn't sound too good so far but if you think about it, the nutrients the plant did not absorb aren't down the drain.
The way I see it is, if you have 5 gallons of 600+ppm runoff and you throw it away
[unless you decide to feed your outdoor garden
like I did until they looked the happiest and went to feeding my cannabis with it]
That is an entire feed just wasted. In my eyes yourmixed up a homebrew of available nutrients, most have already been digested by microbes into an even more available form, and decided it wasn't the right mix and tossed it.
Does outdoor in-ground plants even get runoff?
Right...
