Shade trees can be more attractive, more vigorous and more resistant to diseases and insects, particularly borers, if they are fertilized every year. The element usually needed most is nitrogen.
A tree with a good supply of nitrogen usually has dark green foliage during the growing season. One with a limited supply of nitrogen usually has smaller, light green leaves. The color turns to yellowish green as the nitrogen deficiency becomes more acute.
The method mostly employed by professionals to fertilize trees has been to drill or punch holes in the ground, 18 inches deep, spaced 12 to 18 inches apart, under the spread of the branches and fill the holes with a complete fertilizer such a 10-10-10.
Another way, much easier for do-it-yourselfers, is to apply nitrogen fertilizer to the soil surface, under the spread of the branches, will a spreader. Experiments at the University of Illinois by Dr. Dan Neely and Dr. E. B. Himelick, plant pathologists, have shown that nitrogen fertilizer applied to the soil surface is as effective, or even more effective, than that applied by any other method.
With rainfall, or watering, inorganic fertilizer moves readily down into the soil to the root zone.
One of the best times to apply the fertilizer is late fall. Early spring, before the buds start to open, is equally good.
The recommendation of Neely and Himelick is to apply nitrogen at the rate of six pounds per 1000 square feet. That would be the equivalent of 13 pounds of Urea (45-0-0), 18 pounds of Ammonium nitrate (33.5-0-0) or 29 pounds of Ammonium sulfate (21-0-0).
The fertilizer should be applied when the grass blades are dry. After the fertilizer has been distributed, it should be washed off the grass blades immediately using a lawn sprinkler or a spray nozzle on a hose. Fertilizer remaining on grass blades that become wet following a light rain or dew formation occasionally causes burning.
Research has shown that if the grass is dry, dormant and is watered thoroughly immediately after application, there should be no damage to the grass.
The first step is to measure accurately the area to be fertilized and determine its size in square feet. The area, of course, is where roots of the tree are most likely to occur: starting about two feet from the trunk and extending about six feet beyond the spread of the branches. Four stakes should be placed in the ground to designate the corners of the area.
To prevent the soil from becoming deficient in phosphorus and potash, it may be desirable to apply these elements every three to five years, preferably by putting them in holes in the soil.The amount can be determined by a soil test which is available at most state universities free of charge.
Dr. Francis Gouin, University of Maryland horticulturist, says he can see no compelling need to dig holes to fertilize trees.
"Mother Nature has been feeding her forest trees by simply dropping leaves on the surface of the ground," he says. "These leaves rot and release their nutrients into the soil and back to the roots. She has never used deep root feeding and she has been growing trees much longer than we have."