When to Fertilize a Tree:
Ideally, growing trees should be fertilized throughout the year. The greatest amounts of nitrogen (N) based fertilizer should be applied during the early spring and summer months. Several light applications a year are preferred as the tree gets older. A soil test may be needed to determine the amounts of phosphorus (P), potassium (K). Read the label for proper ratios and application rates of N, P and K for trees.
Again, for young trees, the time to put out fertilizer is late March through early June. When a tree reaches the desired height you may want to decrease the fertilizer application to only once a year.
How to Fertilize a Tree:
An application of between .10 and .20 pounds of nitrogen per 100 sq. ft. will be adequate.
Again, read the lable. Keep solid or concentrated fertilizer off stems and leaves and adequately water the fertilizer into the soil as that prevents fertilizer burn injury to roots. Stick with the higher ratio nitrogen fertilizers unless your tree is determined to be deficient in potassium or phosphorus (soil test). N-P-K rates of 18-5-9, 27-3-3, or 16-4-8 are good bets. Conifers rarely need high rates of fertilizer so you might want to skip applications or stop feeding after a year.
Organic fertilizers come from plant and animal sources. These fertilizers have a slower release of nutrients as they need to be decomposed by soil microorganisms. They are easy on plant roots but take longer to become effective.
Organic fertilizers are harder to find than inorganic fertilizers and often more expensive but they are the least harmful and less exacting when applying. The best organic fertilizers are cottonseed meal, bone meal, manure and chicken litter. Read the label (if packaged) for application methods and amounts to use.
Inorganic fertilizers are inexpensive and are the most frequently used fertilizers for trees. Inorganic nitrogen based tree food sources are sodium nitrate, ammonium nitrate and ammonium sulfate.
General purpose fertilizers are complete with N-P-K which is usually defined as the ratio of nitrogen, phosphorus and potassium in the mixture. You can use these excellent fertilizers but don't overdo. Use high-ratio nitrogen products unless a soil test suggests a lack of other nutrients. Inorganic fertilizers can come in slow-release, liquid or water-soluble for foliar application.
Read the label for application rates.
Remember Organic Soil Amendments:
The greatest value of most organic materials is in the change they bring to soil structure. Remember that chemical fertilizers have no positive physical effect on soil structure.
Peat moss, leaf mold, pine bark, sawdust, and stable manure can improve the soil while adding nutrients. These amendments increase the fertilizer and water-holding capacity of many soils. Mulching with these amendments aid in root development.