How often use fertilizer

sounds like you are in a soilless medium based on the fertilizers you are using. always get 10% runoff and you reset the equilibrium each time -- if religious about this, it's impossible to have a buildup. any toxicity seen is caused by the formula and not a buildup. You may still need to adjust the formula over time, but eventually 99% of plants should be happy with a properly balanced diet. if you are in soil, it's more of a trial and error thing as far as how often you fertilize and how you have to supplement the soil more over time as the original amendments are used up by the plant. in this situation you often see people fertilize every other irrigation or even every 3rd. if in soilless. alwys fertigate. always get 10% runoff. each time you do it resets to roughly the same equilibrium of nutes as before. this is why soilless/hydro grows plants so efficiently. i'd suggest tracking ppm of each nutrient molecule you feed. Free apps on your phone or touchpad can do the math and tabulation for you simply by typing in dose and %s off the guaranateed analysis labels. when you learn a good ration, it travels well regardless of brand of fertilizer you buy. as long as you can get the ppms at or near what you had before, you'll have the same results. the ingredients are commodities. there is no difference in cal nitrate from cal nitrate. mkp is always mkp. the molecule won't change, thereofre there's no variance in quality and no need to overpay for it. anyway, like i said... if you have good fertigation habits, the cause of any problem seen in a soilless/hydro context is 100% about the formula needing a tweak. buildup in substrate over time is impossible, but that doesn't mean you can't give too much in your feed.