Q: I know the numbers on the fertilizer bag represent the amount of nitrogen, phosphate, and potassium contained in the fertilizer, but how do these nutrients help my plants?
A: The numbers on the fertilizer bag do represent the elements you listed nitrogen (N), phosphorus (P), and potassium (K) which are all important for normal plant and animal processes such as growth, reproduction and protection. In the most general of terms nitrogen is necessary for growth and production of new leaves. Phosphorus is vital for production of flowers and fruit. Potassium assists the plant build root structures and enables it to withstand harsh stresses such as heat, cold and drought.
It is good to remember fertilizers are not food but rather it assists the plant with the normal system functions of reproduction, growth and survival. Leaves are the food factories of the plant. Fertilizers may not need to be applied every spring and fall or even every year if the plants are healthy and established, which is especially true of mature trees. Do not apply fertilizer when the tree is fruiting or flowering as this may cause the flowers and fruit to drop.
If plants are consistently showing signs of nutrient deficiencies it may be best to move them to a more appropriate site. Fertilizer should not be applied during times of drought unless supplemental irrigation can be supplied. Nor should fertilizer be applied when storms are imminent as the fertilizer will end up in storm water drains rather than its intended target. I always found the process of adding nitrogen to shrubs a curious activity. The added nitrogen increases the plant’s leaf and stem production which causes someone to prune the new growth off a few weeks later. It seems counter-productive – yes, a curious activity indeed.