Thursday 21 July 2016

Tree Fertilization Part One

Posted by at 4:01 PM

Summer appears to finally be here, and with it is the peak of the growing season in Wisconsin.

In my last message I indicated that this article would be devoted to tree fertilization. I must amend this; the next two articles will address the issue of tree fertilization.

I will say up front, that I believe there is a tendency to over fertilize our landscapes. In other words, we apply more fertilizer to a site than it is capable of using. The excess fertilizer, unfortunately, makes its way into our ground water and surface bodies of water, thus degrading these valuable resources.

I would also like to clarify a few misconceptions that I believe will aid in the development of an environmentally responsible nutrient management program.

  • All trees need 16 elements to survive; collectively they are called essential elements. 
  • Some elements, called macronutrients, are required in great amounts on an annual basis. These macronutrients are Nitrogen (N), Phosphorous (P), Potassium (K) and calcium. N, P, and K are the primary constituents of most common fertilizers.

Nitrogen and phosphorus are the elements that most often limit tree growth. I used the term growth limiting,which may be unfamiliar to some readers. From a fertilization perspective, applying nutrients that limit growth (i.e. nitrogen) will initiate an increase in annual growth. Too often we use the term nutrient deficiency to mean the same thing. This is incorrect, nutrient deficiencies cause dysfunction and if left untreated, will lead to death. 

Nutrients and Tree Growth

  • Practically speaking, if the leaves on the tree are of normal color, size, and distribution it is probably not deficient in any of the 16 essential elements.
  • The amount of annual tree growth is not positively related to tree health (i.e. more growth does not equal better health). Health reflects the tree's ability to persist within a landscape. Healthy trees chemically and physically defend themselves against the pressures of the environment in which they live. The amount of annual growth is a reflection of resource availability and growing climate. While rapid increase in size, or growth may be beneficial for visual impact and survival early in the life of a tree, it is less important as a tree ages and defends itself against insects, disease and decay issues. Remarkably, trees regulate the amount of annual growth in response to these factors.
  • Leaf area within the canopy becomes stable over time. In other words, at some point in time the total area of the leaves will not get larger even though the tree continues to increase in height and diameter. This is important because the leaves are the biggest consumer of N, P, and K.
  • The statement that all trees require the same essential elements does not mean that all trees require these elements in the same amount. Young trees that lose their leaves every year, deciduous  trees, have a higher demand for essential elements than old trees that retain their leaves for multiple years, coniferous trees. As trees grow larger they accumulate essential elements that are continually recycled internally to support annual growth. Functionally, large, old trees have a larger internal bank account of essential elements to support growth.

In my mind, the aim of tree fertilization programs should be to promote tree growth in an environmentally sustainable manner. The next article will address how to evaluate the tree's demand for water and nutrients relative to the ability of its surroundings to supply these resources. 

Dr. Les Werner, University of Wisconsin - Stevens Point