NPK stands for the most important three minerals that must be present for crop growth. These are nitrogen, phosphorous, and potassium. Different crops remove different amounts of minerals. Bananas which are known to be high in potassium, remove high amounts of it. This potassium must be replaced.
Corn depletes high levels of maganese.
One problem environmentalist have with fertilizers is that agricultural run-off can be disasterous to water ecosystems. If an overbundence of nutrients is located within a water source, eutrophication can set in. The water's oxygen content is used up by microbial organisisms and fish die-off. Since fertilizers are highly necessary, improvements in its application are necessary to reduce collateral ecological damage. Precision agriculture is developing which will hopefully reduce ecological damage from agricultural practices.