My last article in the summer issue of “Branching Out” was on tree fertilization, a topic that is near and dear to my heart. On the surface this topic appears to be fairly straightforward. Homeowners supply trees with resources in the hopes that we will achieve tree growth. In fact, one could argue that improved tree growth is the only reason to fertilize trees. In the last issue, I highlighted tree growth as an outcome of tree fertilization so that it would not be confused with improved tree health. Too often, we inappropriately use these terms interchangeably.
All trees require the same 16 essential mineral elements to live and grow; yet over 95 percent of a tree’s mass consists of only carbon, hydrogen and oxygen. Through the process of photosynthesis, these three elements are combined to produce a raw material called glucose. Glucose is used to build the complex structure of the tree and supply living cells with the energy to function. Surprisingly, the amount of water required by a tree vastly exceeds its requirements for essential elements. Trees require 500 grams or more of water to produce one gram of dry weight tissue. This means that less than five percent of the tree’s mass consists of the essential elements. Some elements, like nitrogen, phosphorus, potassium and calcium, accumulate in greater amounts and are called macro-elements. Those that accumulate in smaller amounts are termed micro-elements. Regardless of their designation, all of these elements are required; if one is missing, the tree will not be able to complete its life cycle. With the exception of calcium, the majority of elements are concentrated in the trees actively growing parts and therefore, affect growth potential.
Tree fertilization is the result of the tree site’s ability to supply resources, such as water and the essential elements, and the tree’s demand for these resources. The soil’s native fertility and current land management practices are used to evaluate the supply side of the equation. For example, the native fertility of a sandy soil is much lower than the native fertility of a soil possessing smaller, finer particles like loams and silt loams. Additionally, many cultural landscape management practices, such as the annual removal of tree leaves or the collection of turf grass clippings have the potential to reduce the supplying power of the site. The demand for resources is a function of tree age, health and its inherent growth strategy. For example, healthy, young trees add tremendous amounts of new tissue on an annual basis. These new tissues demand new resources from the soil when the tree is growing. Remarkably, the new resources acquired to support this rapid rate of growth build up and are recycled within the tree to help meet future demands. This is particularly true for elements like nitrogen, phosphorus and potassium. As a result, the demand for new resources from the soil does not increase exponentially as the tree gets larger since they are stored within the tree.
Hopefully, it has become apparent that fertility management should not be a one-size fits all program. Our understanding of how and why trees grow and the changes that occur over time has improved tremendously over the years. Modern arboricultural companies recognize this and tailor fertility management based on evaluations of site quality, land management practices and the trees in question.