Should we fertilize trees? They rarely need it.
Fertilizer is always a contentious issue, because a lot of businesses sell fertilizer in myriad forms. These sellers will all tell you that you need more and more, which is a load of… uh… fertilizer.
Fertilizer is basically a mixture of essential elements. Nitrogen (N), Phosphorus (P), and Potassium (K) are the big three, and any mixture of fertilizer that uses a number sequence like 20-20-20 is referring to the percentage of N P K within that container. There are about a dozen other required elements, called micro-nutrients, which are needed sometimes in very small amounts.
All soils contain some or most of these elements. All plants require most of these elements in greater or lesser degrees. When a plant is grown in soil that is deficient in an element that it needs, that plant or crop will not thrive. Careful soil analysis can be very important.
Some plants thrive and respond better to fertilizer than others. Annual flowers, lawns and vegetables respond well to regular fertilizing and the application of organic compost, which contains a lot of the elements a fertilizer would contain, with the added benefit of organic plant material for the soil.
Any tree growing in a healthy, good-looking lawn has already received as much fertilizer as it needs, or more. The problem with too much fertilizer starts here. All plants have very complicated bodies and, like ours, they have hormones, growth regulators, one of the most important being auxin. Auxin is responsible for shoot extension. Long-lived trees have learned to regulate their growth to match environmental conditions and to act differently as the tree ages. In a sense, the growth regulator is a locked system. But locks can be picked. This is exactly what too much nitrogen does. It forces the growth regulator to go beyond its natural safe limits.
Shoot extensions can be forced to grow abnormally long. Forced to grow beyond its natural limits, a shoot is stretched and the subsequent branch and trunk can be much weaker than its naturally slower growing brother. You may have great growth but the shoot is not stronger.
How many broken trees have failed in weak trunk sections that were forced to grow like turf? This is the classic “higher-faster-farther” that trips us up. It’s a holdover of 50s thinking—the same ideas that intended to improve the world with DDT.