Fertilization of apple and peach-bearing trees, particularly those growing in lawns, is almost as important as spraying them during spring and summer to protect the fruit from insects and diseases. One of the best times to apply the fertilizer is late November or early December in the Washington area.

Non-bearing trees should be fertilized only if the soil is poor and the trees are not making good growth. Go easy on fertilizing pear trees, especially with nitrogen fertilizer because of the danger of the tree becoming infected with the fire blight disease to which succulent new pear growth is very susceptible.

Moderate vigor is best for good fruit production. Too much as well as too little new growth is detrimental. The amount of fertilizer to apply with depend on a number of varying factors. A good safe general rule for fruit trees in the home garden is four pounds of 10-6-4 fertilizer per 100 square feet, spread evenly on the soil surface from near the trunk to slightly beyond the ends of the branches.

Work the fertilizer into the mulch if there is one, water it in if there is sod, and if there is neither mulch nor sod, work it gently into the soil because deep cultivation may cause injury to the roots.

Applying the fertilizer too early in the fall can result in severe winter injury on some apple varieties. If applied after complete formancy, fall fertilization is completely safe and effective, according to specialists. Dormancy occurs in most fruit growing areas between the middle of November and early December.

A month or two of good weather usually occurs between dormancy and freeze-up. Fertilizer applied during this time will move into the root zone but not into the top of the tree to cause regrowth or soft bark. It will be taken up by the roots ready to push vigorous new growth as soon as spring arrives.

An abundance of nitrogen, just enough and not too much in the tree early in the growing season for setting of fruit and growth of a large leaf surface is necessary for maximum yield.

Early spring also is a satisfactory time to apply the fertilizer. However, with its spray schedules and numerous other tasks, it is not the most convenient time to make the application.

Used early and at reasonable rates, nitrogen will cause no harm to trees. As a matter of fact, Michigan State University research shows that nitrogen increases tolerance to severe winter weather.

Peach trees that were low in nitrogen were killed in the Michigan cold climate. This means that keeping trees on a low nitrogen diet predisposes them to winter injury.

Research at the University of Maine has shown that fruits from trees adequately fertilized with nitrogen were definitely better flavored than those from low nitrogen trees. Good color can be maintained with sensible rates of nitrogen properly applied and balanced with adequate potassium and other minerals.

Since nutrients do not transfer from one side of a tree to another, it is important to apply the fertilizer all around the tree. A given section or limb of a tree is nourished from a corresponding section or branch of the root system. The root system may not grown in the same manner and direction as the above-ground part which it nourishes. The roots may originate on the same side of the tree but later extension and branching may develop in any direction.