Fertilizing Trees

Fertilizing your trees to promote faster growth may seem like the logical thing to do, however, this is not always the case. Ornamental trees may require fertilizing because many grow under poor conditions having to tolerate extensive pruning, large areas of pavement and concrete, and sod cover which can use up most of the available nutrients and moisture. On the other hand, shelterbelts usually grow under better soil conditions.

There is no general agreement on the best formulation of fertilizer for trees, however, it is agreed that the fertilizer should contain the three major elements, N, P and K. Nitrogen will move downward in the soil, however, phosphorous and potassium move very little in the soil.

Trees have a deep and extensive root system as compared to annual plants. The location of feeding roots varies with species and soil type but generally they are present in a band around the tree, extending from the dripline (the farthest spread of branches) inwards two-thirds of the way. Occasionally the roots will extend 1 to 3 times beyond the dripline. Fertilizer must be placed near the feeding roots so that it is available to the tree. The recommended method of doing this is discussed below.

The fertilizer is put into many holes placed around the tree and then the holes are filled in with peat moss, shredded manure, or topsoil. The holes should be 1 to 2 feet deep, about 2 feet apart and slanted toward the tree. For the homeowner, a crowbar is the most readily available tool for making these holes. The best time of the year to do this is in the early spring. Summer applications are not recommended because soft, succulent tissue is produced which does not have enough time to harden off before fall.

This method of fertilizing trees may be useful for high value ornamental or shade trees, however, this is not a feasible method of application for shelterbelts. There are several other reasons why fertilization is not recommended for shelterbelts. First of all, shelterbelts grow in a more favourable environment than city trees.

Secondly, soil tests outlining nutrient deficiencies for crops do not necessarily apply to trees growing in the same area. This is because tree roots go much deeper, thus having more nutrients available to them. Also, trees do not necessarily require the same amount of nutrients as crops.

Thirdly, shallow fertilization for a few years as is done with crops, will probably do more harm than good over the long run. The fertilizer does not move much in the soil so root growth near the soil surface is promoted. This leads to a shelterbelt with a shallow root system which is more likely to suffer from drought during dry spells. Trees such as poplar and Siberian elm normally have a shallow root system and surface fertilizing can aggravate this.

In conclusion, fertilizing ornamental trees may be done but fertilizing shelterbelt is not a recommended practise.