And indeed, fertilizing plants used to be as simple as applying fish and manure. First, read the label. Of the three primary nutrients, nitrogen is generally in shortest supply in Western soils and thus needs to be replenished most often.
Soluble nitrogen becomes available to plants quickly.
Insoluble nitrogen must be broken down slowly by microorganisms in the soil for plants to use it. Most fertilizers contain both forms of nitrogen, although labels don't always specify the percentages. You can buy fertilizers in either natural or chemical form. Plants can't distinguish nitrogen that came out of a chicken from nitrogen that came out of a chemical factory. People, however, seem to have their preferences.