Fertilizer's Affect on Trees By Paul Martin, Growing Earth Tree Care Editing by Bonnie Appleton, VA Tech Believe it or not, fertilizers are do not directly provide food for trees. Trees produce their own food (sugars, carbohydrates and starches) in their leaves, through the process of photosynthesis. Fertilizers provide the source of the nutrients required for photosynthesis. Not all trees require fertilizer, but all trees require the nutrients available in fertilizers. In the forest, many of these nutrients are recycled as the leaves fall and decay back into the soil. Sunlight is converted into chemical energy by the choloroplasts in the leaves. The elements in fertilizers are necessary to maintain chloroplasts. They are necessary in the production of many other compounds, including enzymes, nucleic acids, proteins and alkaloids. Cell walls and membranes also require these elements. The three main elements of complete fertilizers -nitrogen, phosphorus and potassium -are essential for plant growth. These substances are listed on the packaging by their percentage of weight, such as 30-10-10. The first number is nitrogen, the second is phosphorus and the third is potassium. It's easy to remember the order because they're listed alphabetically. Many of the fertilizers available commercially contain small amounts of trace minerals including iron, manganese and zinc, but the small quantities are usually insufficient to correct a deficiency. The three main nutrients are usually present in the soil, in limited quantities. Lack of a proper amount in the soil can stunt a tree's growth. Nitrates, the major source of nitrogen, are soluble in water and can be easily leached by drainage water. Organic fertilizer is continually producing nitrogen as micro organisms break down the organic matter. Discolored leaves can signal a deficiency of certain nutrients, such as nitrogen. Too much nitrogen, though, can attract nitrogen feeding insects or produce sucker growth. Before you apply any fertilizer, have the soil analyzed, then apply the correct formula and rate.