How do trees get the nutrients needed to grow and stay healthy? As you all know, trees don’t “eat” – at least not in the typical sense, but they do consume and like all living things they need proper nutrition. Trees consume some basic raw materials and elements needed for growth and development from their roots, stems and leaves. The main source of water and chemical nutrients is from roots and the main source of carbon dioxide and other gasses is from the leaves. Trees, as with all green plants, are able to take sunlight and carbon dioxide, add some water and chlorophyll and make their own food. If trees can make their own food, why do we fertilize them? And, what is in typical tree fertilizer?
There is more to tree nutrition than just throwing some fertilizer on the ground. Among other things, trees need a healthy root system in order to absorb any fertilizer we throw at it and the soil pH needs to be correct. For a wide range of pH values, element availability for trees is adequate, but there are some specific pH requirements for certain nutrients to be absorbed and used by trees. Most homeowners try and have their trees fertilized each year; usually with an all-purpose fertilizer – But trees often need nutrients that are not in a standard store bought fertilizer preparation. If you really want to know what types of elements your trees need and if the pH is correct you should have a soil test done. Like a blood test, this will provide a reading of the areas that your tree may deficient in. The following elements are required in small amounts for the growth and development of healthy trees. These are referred to as micronutrients: iron (Fe), manganese (Mn) copper (Cu), zinc (Zn), molybdenum (Mo), and boron (B). Iron and manganese are required for chlorophyll production and photosynthesis. A deficiency results in yellowing between the veins, also known as interveinal chlorosis (see photo above – pin oak with a micronutrient deficiency). The other elements are need for the various physiological functions within a tree. Of course, all trees need the “macro” elements – N, P and K, but these are in most fertilizer preparations. (Note: recent research has shown the phosphorus is abundant in our soils of the Northeast and should not be included in fertilizer preparations).
Many elements are essential to tree life and each element has a unique job to perform in a tree. All of these essential elements are needed in different proportions. Too much or too little of any single element disrupts tree life. To determine the proper concentration of an element your tree needs and to determine the pH, soil samples should be collected and analyzed.
As you can see, there is more to fertilization than just putting some spikes into the ground. I highly recommend having a soil analysis done if any of your trees are showing decline symptoms. If the site is wrong for the plant – as in a high pH, all the iron in the world will not improve the condition of your tree. In that instance, you need to replace with a more suitable species or take steps to lower the pH.