In a healthy woodlot, the nutrient cycle is balanced and stable for the most part. Through the decomposition of leaves, organic material is naturally returned to the soil- for the benefit of the plant- on an ongoing basis. Conversely, in an urban environment, this natural cycle of nutrient availability is dramatically disrupted due to the collection and disposal of leaves. As a result, in the fall, there is a continuous loss of nutrients from the soil. Fertilizing turf grass in this environment limitedly helps to replenish this loss but replenishment of nutrients for plant growth is generally not satisfactory. Lawns tend to prevent good soil aeration naturally found in wooded surroundings. Asphalt, building foundations and other impervious surfaces prevent roots from getting nutrients due to a high percentage of water run-off. The effects of undernourishment are evident in reduced vigor of the tree as well as reduced ability to withstand insect and disease attack.

In the re-created landscape, soil and environmental conditions are dramatically changed which necessitates the addition of natural or synthetic fertilization in order to maintain the general health of trees.

Considering the dry summers that we have experienced in Niagara in 1998 and 1999 as well as a projected hot summer for this coming summer, it is very important that municipalities, park agencies, estates and individual home owners consider tree fertilization as a practical means of maintaining vigorous tree growth. As a general rule, trees that are in a healthy state of growth are less susceptible to decline and more able to withstand environmental stresses. Root systems of established, solitary shade trees growing in a natural state have roots that extend far beyond the leaf canopy. In addition to this, up to 90 percent of the fibrous- or feeder- roots are located in the upper 12 inches of the soil profile.

Plant Feeding

There are three major mineral elements and at least nine trace or minor elements that are required for plant growth. Even to this day, a "complete fertilizer" is often considered to be one that contains Nitrogen, Phosphorous and Potassium. It is important at this point to indicate that these three elements do not make a complete fertilizer. The other nine elements, while important in their own right, will not be mentioned in this article. Only N-P-K will be covered.


Nitrogen in most soils is the one element most likely to become depleted (due to excessive rainfall or over irrigation). An under-supply of nitrogen will result in plants having stunted growth and yellow foliage. Yellowed foliage may also be caused by lack of water, disease, or too much water. Yellowing of foliage from these causes is oftentimes followed by leaf drop.

Lack of nitrogen will bring on the same yellowing, but the leaves will remain clinging to the plant. An excess of nitrogen will result in very rapid soft growth, which gives a weak plant structure. An oversupply of nitrogen is therefore as damaging as an undersupply. Observe your plants. If the growth is slow and the leaves turn from a dark green to a yellowish tinge about the time of maturity, you may be fairly sure the nitrogen supply is limited.


Lack of phosphorus is evident when normal growth is obtained, but foliage is yellowish in colour instead of the healthy dark green of normal growth. Phosphorus stimulates flower and seed production and hastens maturity, which is generally desirable. Phosphorus encourages deep rooting and, therefore, helps develop a plant that can and will provide and support heavy healthy foliage.

Potassium (Potash)

Lack of potassium makes a plant very susceptible to disease. Plants grown in the absence of the proper quantities of this element have foliage that is dull and yellowish in colour, and the plant itself is not robust. You will find the stems brittle, the roots small, and the colour in general far from healthy looking. Sufficient potassium will mean strong roots, a healthy and sturdy plant with blooms of intense colour- providing there are ample quantities of the other elements also available.

Tree Fertilization: Easy to Calculate

To determine the actual nitrogen in each bag of fertilizer, multiply the first percentage number on the bag label by the weight of the bag. Take the example of 20-5-15. Say this bag is 55.5lbs (25kg).

Therefore: 55.5 lbs x 20% N =11.1 lbs of actual nitrogen in each bag.

The Universal Tree Fertilization Calculation helps clear the confusion caused by the multitude of shade tree fertilization recommendations.

Universal Fertilization Calculation Table.
Root System Radius (feet) Weight of 18-6-12 Weight of 33-0-0
2' 5 oz. 3 oz.
3' 11 oz. 6.5 oz.
4' 1 lb. 4 oz. 11 oz.
5' 2 lb. 1 lb. 1 oz.
6' 2 lb. 13 oz. 1 lb. 8 oz.
7' 3 lb. 14 oz. 2 lb. 13 oz.
8' 5 lb. 1 oz. 2 lb. 13 oz.
9' 6 lb. 8 oz. 3 lb. 8 oz.
10' 8 lb. 4 lb. 5 oz.
11' 9 lb. 8 oz. 5 lb. 3 oz.
12' 11 lb. 8 oz. 6 lb. 4 oz.
13' 13 lb. 8 oz. 7 lb. 5 oz.
14' 15 lb. 8 oz. 8 lb. 8 oz.
15' 18 lb. 10 lb.
16' 20 lb. 11 lb.
17' 23 lb. 12 lb. 8 oz.
18' 25 lb. 8 oz. 14 lb.
19' 28 lb. 8 oz. 15 lb. 8 oz.
20' 32 lb. 17 lb. 8 oz.

The Cultivation Method

This method of fertilizing deciduous trees consists primarily of broadcasting fertilizer over the effective feeding area of the roots and working it into the soil as deeply as the roots will permit. As a rough guideline, approximately 3 to 5 pounds of actual nitrogen fertilizer should be broadcast to a 10' circle, 75 to 100 pounds to a 50' circle, and 300 to 400 pounds to a 100' circle. Application time can occur immediately after the frost leaves the ground and well before the commencement of tree growth in the spring. This results in maximum plant response.

For evergreen trees the rate is 2 to 3 pounds of actual nitrogen per 1000 square feet. This method of fertilization is practical for any grounds maintenance program, is cost effective in terms of reduced labour, and results in a tree that exhibits good vigor.

In future articles of Hort-Pro, the liquid injection technique will be described in addition to the "punch bar" method of applying granular fertilizers.