Irrigating tree with pipe in the ground

I have two 2"x12" PVC pipes buried at the dripline of each tree I have planted in my yard, with holes drilled in the side of the pipes and small rocks in the pipes. I have drippers in each pipe. This is how I water my trees. May I administer properly diluted chelated iron directly into the PVC pipes rather than pouring it on the ground around the tree? Is it possible to use these PVC pipes as a way to administer fertilizer, etc.—pouring it down the PVC pipe rather than just on the ground? I'm wondering if this would get help directly to the roots rather than on the grass around the tree?

I have heard of this technique since I was a child. There is some logic to the technique—the water is placed below the surface and less subject to evaporation. However, there are problems with this technique as well. You have largely avoided the first problem by not using a longer pipe. However, even a pipe that is one-foot long has you watering below many of the roots. If the pipe was perforated and the drip emitters provided water fast enough to fill the pipe, it would help by allowing some water to escape at different depths.

A second problem is the number of pipes. You have used two, but many more would be better. The larger the tree, the greater the root system. Most of the roots that actively absorb water and nutrients are further from the tree than the dripline. With only two pipes at the dripline, you are providing water to a very small percentage of the root system. If the pipes have been in place since the tree was first planted, the tree may have developed its root system only where the pipes provide water. This is especially true if there is never moisture in the soil at any other location. Nevertheless, the root system is quite limited in this circumstance. At least four to six pipes equally spaced around the tree (at and beyond the dripline) would encourage development of a more extensive root system.

If the soil is sandy, the water will quickly permeate the soil downward, spreading very little laterally. Once the water has permeated the soil to a depth greater than three feet, much of the water is unavailable to the tree and is wasted. A loam or clay soil will allow more lateral movement of the water and less rapid loss of the water below the tree's root zone. If you use this system of irrigation, the soil type will determine the number of pipes needed.

The lack of distribution will create a problem with the application of nutrients about which you asked. The nutrients will be localized, and there is a possibility of damage to the limited root system when fertilizer is applied to the roots through the pipes. (The chelated iron should not cause damage.) Properly diluted fertilizer will not damage the roots, but remember that the instructions for dilution are not designed for application in this manner. Those directions may allow for too highly concentrated nutrients. If lawn irrigation or rainfall has allowed development of roots outside the zone supplied by the pipes, then you will be providing nutrients to only a small portion of the root system and the effects will be less than desired. Be careful fertilizing your tree in this manner.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

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