The importance of fertilizing young trees cannot be taken too lightly. It is during the early years that trees develop the root systems and the basic skeletal structure that will support them throughout their lives. Applying fertilizer during these early years can give them a boost that will ensure their health and long life. When you shop for fertilizer, be sure to select a brand that specifically designates it for trees, and if possible, for your particular type of tree. The requirements of a maple, pine and dogwood, for example, may differ.
Fertilizer is used to replenish soil nutrients so that the soil can sustain plant life. Plants require a combination of 17 nutrients in order to survive. Most fertilizers contain varying amounts of nitrogen, potassium and phosphorus, the three most commonly needed plant nutrients. In nature, fallen leaves and other plant matter replenish these nutrients as they decay. In home landscapes these materials are removed, creating soil deficiencies that fertilizer replenishes. Fertilizing young trees helps them build the sound root systems they will need to sustain them.
Some fertilizers come with a percentage of their nitrogen in a "quick release" formula. This means the nutrient is more readily available to the tree. This should not be used to mix in with back-fill soil when planting a new tree, as direct contact between the fertilizer and the roots can burn the roots. Quick release formulas should be broadcast spread and watered in to the topsoil after planting to provide quick access to nitrogen for the roots. This will help them flourish and take hold quickly.
Slow release nitrogen fertilizers can be mixed into the back-fill soil and used when planting new trees. This form of nitrogen is not readily available and takes longer to release. There is less chance of any excess leaching into ground water. It provides a steady supply of nitrogen throughout the growing season.
All trees, but especially young trees, do the bulk of their growth in the spring and then settle down to a slower rate of growth for the rest of the growing season. While some recommend fertilizing in the fall too, this is not always necessary or even a good idea. In areas where sudden frosts can occur early or where trees are sensitive to cold, fertilizing in the fall can lead to growth at a time when the tree should be preparing to become dormant. A frost at this time could cause significant damage.
It is always a good idea to check the soil's nutrient levels before applying fertilizer. A soil test, which can be handled your local extension office, can reveal a great many things. Not only will it show any nutrient deficiencies, it will show the pH of the soil and the percentage of organic material found in the soil. Your results will also be accompanied by recommendations for curing any deficiencies.