It's more that plants use different minerals at different rates and depending on environmental conditions. For instance, in grow rooms expected to rise above ideal temperatures, a simple trick is to reduce the amount of nitrogen by up to 20% (depending on severity of swing). Conversely, amounts can also be increased when temperatures swing low. I notice you didn't mention that nutrient absorption occurs in direct correlation to other factors such as CO2 levels, light wave length etc etc.

Also, it is important to distinguish between spider mites. I mean, are we talking red spider mites, two spotted mites (AKA Borg mites)? Insecticidal soaps and pesticides are both poor choices when dealing with serious infestations as successive populations will build immunity like you won't believe. By far the best way to deal with an infestation is by removing the infected plants altogether and doing a through clean of the grow space. Change filters, scrub fans. Then, the second anything green is re-introduced, release a colony of predatory mites.