Water and Plants

In last month's article we learned a couple of important facts about how water behaves in the soil. This month we will begin learning how to effectively water our plants - where to apply the water, how much to apply, and how to judge when we need to apply more water. Before we address those issues, however, we need to consider what happens when water is applied to the soil.

Water falling on the soil may infiltrate into the soil, run off to other locations, or evaporate back into the air. Only the water that infiltrates into the soil becomes potentially available to our plants. Water that runs off or evaporates is not available to the plant we are watering. We can prevent the runoff loss by applying water slowly and containing it around the plant with berms or depressions, and we can reduce the loss to evaporation by applying a thick layer of mulch around our plants.

Water that infiltrates into the soil initially moves downward under the force of gravity until it is grabbed and held to soil particles by electrostatic attraction, is taken up by the roots of plants, or reaches the ground water table. Where the water table is very deep - the usual case for us who live in the High Desert - none of this infiltrating water will reach the water table. Instead it will be captured by soil particles where it will remain until it is liberated by a plant or evaporated back into the atmosphere. Most of the water that is taken up by the roots of plants moves through the plants to the leaves where it is transpired back into the air as water vapor. Very little of the water that a plant takes in is actually held within the plant. It is estimated, for example, that a field of corn may transpire as much as 15 inches of water during a single growing season. We can envision a plant as a straw sucking water out of the soil and returning most of it to the atmosphere. As the water is depleted from soil particles around the roots, those particles steal water from particles farther away, replenishing the water available for the plant until the available water is used up.

Since the roots are the organs primarily responsible for providing water to plants, we should apply water where the roots are, more specifically where the water absorbing parts of the roots are. As obvious as this seems, applying water to the wrong location is a frequent mistake, especially with shrubs and trees. The roots of most plants have a branching structure that looks much like the branches that we see above ground. The roots are large in diameter near the plant and become smaller and smaller in diameter outward from the plant. The large root structures themselves are merely conduits. Water and nutrients are actually absorbed by tiny structures attached to the roots called root hairs. You might think of these as analogous to leaves on the branches of trees. Just as leaves are found near the ends of branches, so root hairs are found near the ends of roots. In the case of shrubs and trees, most of the root hairs are concentrated in a ring extending about half way to the drip line outward up to three times the radius of the drip line. The drip line marks the outer limit of where water dripping off the leaves of the tree would fall on the ground. This line may be of considerable distance from the trunk. For example, I have a mesquite tree in my yard with a drip line that is nearly 30 feet from the trunk. That means that most water absorption for that tree probably occurs starting at a distance of about 15 feet from the trunk and extending to as much as 150 feet from the trunk, an area of nearly 70,000 square feet! Compare these numbers with the sizes of the wells or the locations of drip emitters many people place around their trees and you can see that it's no wonder that their trees and shrubs often suffer stress from lack of water. In addition to spreading outward from the plant, most plant roots are found at relatively shallow depths. Many people seem to believe that trees, shrubs, and other plants have long tap roots that penetrate to the water table. In fact, most of the water absorbing roots of trees and shrubs are found within one to three feet of the surface.

For more information on watering your plants, the Michigan State University Web site at: http://msue.msu.edu/insue/imp/m od03/01800544.html.