By Keith Brown,

Certified Arborist

 

Nutrition

Trees have basic nutritional requirements that are necessary for survival. Trees have other basic nutritional requirements that are necessary for proper development. Nothing, including trees, can live with out carbon, hydrogen or oxygen; fortunately these three things can be obtained from the air. The other thirteen elements needed by trees for proper development must be obtained from the soil. Yes, thirteen. Nitrogen, phosphorus and potassium are only the three most growth limiting factors. There are actually eight "macro-nutrients" that trees require in relatively large quantities. The other five elements are only required in trace amounts. Now we have a basic understanding of what trees need: lets discuss the rest of the tree's table setting.

Before you eat dinner with your family in the evenings the meal must be prepared, right. Food is prepared, or cooked, so that it is eatable; the food is put on a plate or bowl, where it can be properly eaten from; drinks are placed into glasses, so they can be accessed easily; and there are utensils to eat with. How well would you be able to eat if raw food was scattered everywhere and your water was a sitting puddle? Trees require organization, too. Simply pouring fertilizer and spraying water everywhere does little or no good for a tree.

What kind of place setting does a tree need to feed from? A soil composition plate. There are three types of soil: sand, silt and clay. Each one has different water and nutrient holding capacity. Therefore, before you feed the tree, you must know what kind of plate it is eating from.. You don't eat soup off a plate do you? Check out our articles on spring fertilization and fall fertilization for more information about fertilizing recommendations for the Austin area.

Next, let's prepare our tall friend's food. The first step is to do a complete soil and foliar nutrient analysis. After we know what to feed the tree, it's time to provide utensils for the tree to eat with. Most slow release nitrogens and other nutritional molecules in the soil are tied up in stable molecules which cannot be dissolved in water (anything absorbed by the tree must be dissolved in water). Soil organisms solubilize these stable molecules so that the tree may use them. Often, however, because of construction damage these organisms and the water holding ability of
the soil are greatly reduced. New technology has provided a way to place the soil organism back in the soil and to increase the availability of water.

In bringing this to an end, let me remind you that by not doing these things, your trees are not necessarily going to die. Try comparing an uncared for tree to a person from a third world country. The tree will be unhealthy most of its life and will not live as long as other well looked after trees. What I have outlined is a best case scenario. Carbon, hydrogen and oxygen will always be available to the tree via air.

SPRING FERTILIZATION

The limestone we live on cause very high competition for nutrients plants need to ensure a happy healthy life. Many necessary elements (i.e. Fe, Mn, Zn, etc.) are not available at the high pH of limestone. During rains and other forms of erosion small amounts of these nutrients are released for trees and other plants to use; however, in many landscape situations there are too many plants in a small area to be supported by such limited resources. Irrigation systems help, but this quickly depletes resources. A well-timed fertilization in the spring is the best solution for this problem. A foliar spray of these limited nutrients in the spring shortly after bud break will ensure
that your trees will have the nutrients they need to develop bigger, greener leaves. Bigger, greener leaves means more sugar production and better water conducting abilities for the tree. This means better insect resistance, better disease resistance and better drought resistance during our hot summer months. Remember to take care of your grass, too. The less your grass depends on natural resources, the less it will be robbing from your trees.

FERTILIZING IN THE FALL

Did you know that trees do most of their growing in the fall, not in the spring? Most of the cell division that produces new growth takes place after the leaves drop or, in the case of live oaks, in the fall after the leaves have become non-functional. In the spring when you see the tree growing, it is actually "expanding." The cells divide in the fall, remain dormant during the winter and in the spring when water is available the cells finish development. Also, what you don't see is that most of the root growth takes place in the fall as well. Now that we know how important fall fertilizations are, your next question might be, "I fertilize my grass, doesn't that help my trees too?" The answer is yes and no. Phosphorous, one of the three most growth limiting nutrients, does not move down through the soil when you spread it on the surface for the grass. Nitrogen and potassium (the other two most growth limiting factors) do move down through the soil, but if you are using the fertilizer sparingly as you should, very little of this makes it down to the tree roots. Of the two annual fertilizations, the one in the spring is really more important. Fertilizing in the fall produces more growth the following year, which creates a higher demand for the micronutrients that are provided in the spring fertilization. Fertilizing in the fall and not in the spring can produce a tree that is taller, but less healthy. On the other hand fertilizing in the spring and not in the fall will produce a tree that is healthier, which can stimulate growth.