To make choosing a fertilizer easier, let’s look first at the label. This label must give you the percentage of three major nutrients, nitrogen, phosphorus, and potassium. You will hear these referred to as NPK, in reference to their chemical element symbols. You may also see notations of numbers such as 15-5-10. These numbers indicate that the fertilizer is 15% nitrogen, 5% phosphorus and 10% potassium. Fertilizers that include all three major nutrients are referred to as complete fertilizers. Percentages of other minor nutrients may also be indicated on the label.
Plant nutrients are commonly divided into 3 groups:
- Trace nutrients
The major nutrients are nitrogen, phosphorus and potassium. All three are crucial for plant growth.
Nitrogen, as a major nutrient, is primarily responsible for shoot growth and green color in the leaves. The fruit tree uses nitrogen that is in the chemical form of nitrate nitrogen. Calcium nitrate (15-0-0), sodium nitrate, and potassium nitrate (13-0-44) are common fertilizers that supply this immediately available form of nitrogen.
A less readily available form of nitrogen is ammonium nitrogen. It can be found in ammonium sulfate (21-0-0, monoammonium phosphate (11-52-0), or diammonium phosphate (18-46-0). Over a period of several months, ammonium nitrogen is converted by chemical processes in the soil, to nitrate nitrogen. Another fertilizer, ammonium nitrate (33-0-0), provides both nitrate and ammonium nitrogen for combined quick and slow release of nitrogen.
A third form of nitrogen is organic nitrogen. In this case, the term organic is meant as its chemical primitive form – that carbon is included in its makeup – not the current popular connotation of “organic”. Organic nitrogen must be converted first to ammonium nitrogen and then to nitrate nitrogen. This process takes time, sometimes upto several years to complete. Urea fertilizer (45-0-0) is a common form of organic nitrogen.
Nitrogen is normally in low supply and heavily used by the fruit tree. In most regions of the country, it will be necessary to supplement the soil with a nitrogen source on a regular basis, usually every year. At the same time, it is important to apply nitrogen judiciously. It can easily burn the tree roots or cause excess shoot growth at the expense of producing fruit buds.
Nitrogen is a constituent of many cellular molecules, in particular proteins and nucleic acids. There are many lower molecular weight nitrogenous organic compounds vital to cell metabolism – vitamins, cofactors, hormones, the chlorophyll pigments and phytochrome photoreceptors.
Nitrogen deficiency appears as lightening of the normal green color. The first symptoms will appear on the older leaves. As the deficiency intensifies, the leaves will turn yellow and die.
The fruit tree relies on phosphorus to help in the growth of roots, seeds, and early leaves. Phosphorus is not used directly, but rather converted to nucleic acids fro use in the growth process. Slow to work in the soil, since it is not readily dissolved, phosphorus should be applied to the soil ahead of planting when needed. It can be found in rock phosphate and complete fertilizers.
Potassium, the third major nutrient, is used by a tree to produce fruit, growing roots and resisting disease. It works by helping to transport starch and sugar through the tree. It is found in potassium nitrate, potassium sulfate, and Muriate of potash.
Calcium, magnesium, and sulfur are often called secondary nutrients. They are as important to tree growth as the major nutrients but needed in much smaller quantities. Calcium nitrate fertilizer is a common source of calcium, and Epsom salt is often used as a source for magnesium.
Both can be applied as a foliar spray, or dry calcium nitrate is often broadcast on the ground as well. Secondary nutrients may need to be supplied to your tree on an occasional basis. Soil or tissue tests can be used to determine when and how much is needed.
Trace nutrients are all necessary for plant growth functions but in very small, or “trace,” amounts. An excess of trace nutrients can even cause poisoning or toxicity in your fruit tree. The trace minerals are boron, copper, chlorine, iron, manganese, molybdenum, and zinc.
Seaweed emulsions are sometimes used as a source of trace minerals. Deficiencies and excesses of these minerals vary considerably in different parts of the country. Tissue testing and consulting with your local extension agent will be the safest way to determine if and when additional trace nutrients are needed in you fruit tree planting.
If the leaves of your shrubs begin to yellow, your plants may be suffering from an iron deficiency. The leaf margins and the tissue between the veins turn light in color; while the veins themselves may remain dark green. This yellowing can progress to the point where the leaves become very pale and turn brown at the tips.
Iron deficiencies can occur under of any number of circumstances: roots damaged by tilling or by dry soil; sandy soil and insufficient organic matter; an overabundance of hard coal ash in the soil; poor soil drainage; or alkaline soil caused by too much lime or by setting plants near cement walls. To quickly remedy an iron deficiency, apply an iron chelate, adding it to the soil or spraying it on the foliage.