A plant's roots come into contact with nutrient ions in 3 different ways.... one is osmotic pressure, another is transpiration, where photosynthesis literally pulls the water up through the plant and the final way is when the roots grow and just bump into the ions. Several things need to take place before nutrients are accepted into the plant... for example a plant sends carriers down to pick up the ions, if a plant has enough of one nutrient then the plant will not send the carriers responsible for picking it up. this is of course once the ions have entered the roots. Befroe the ions even enter the roots they will need to also get past the plants suberin. suberin is created specifically to block any unwanted ions from getting in. a certain consistency of suberin will block the passage of N and K whilst allowing the passage of P. also plants can separate water from nutrient ions.
Plants take nutrients like they would from a buffet. This is why ratios do not matter... all that matters is that there are enough available ions. certain things can get into the plant, like heavy metals or in some cases certain poisons, but in respect of nutrients our only concern with heavy feeding is the salinity content of the medium, as it is this that slows the passage of K. when we talk about over fertilisation we are talking about the medium and not the plant. In a high saline medium this slows the passage of K and we see necrotic edges on the leaves. you may call it 'burning'... in fact over fertilisation causes a deficiency int he plant, not a toxicity.
