Plants can't exactly pick & choose what nutrients they take up much better than you can separate clean water from pollutants..
Plants have specific needs at specific stages, the closer you can match your nutrient availability to the plant's needs, the healthier it will be.. Since they can be partially selective though, unused salts/nutrients will build up in the soil increasingly with each application..
Too much N late in the game can delay flowering, and stretch out the flowering period with no real benefit to yield.. It also wreaks havoc on the finished product, especially if not flushed thoroughly.. Buds grown in too much N tend to have a greyish cast to the trichromes, lower potency, and burn to a black ash rather than white, while literally smelling of ammonia.. Its a pipe smoker's nightmare because it burns your lips, fills your pipe with that gross liquid ammonia resin, and leaves a wet/oily black ash that solidifies in the bowl.. It kills any possible enjoyment of any hits after the initial one.. Its kind of like smoking shake, I'm sure everyone has seen it.. Especially if weed in their local scene is grown in corn fields..
You can use cheap ferts with good results if you know what you're doing, but you should definately get the proper formulation for each stage.. Even with that, and a really good eye for spotting problems FAST, its vital to flush often throughout its life to clean out toxic salt buildups, and to ultra flush before harvest..
