All healthy fruit trees are heavy feeders that remove large quantities of mineral nutrients from the soil. Even though all plants require 16 elements that are referred to as essential, fruit trees draw most heavily on macronutrients.
It is important to replace these macronutrients on a regular basis to keep trees vigorous and healthy. The macronutrients we should apply regularly are nitrogen, phosphorus, potassium, calcium, magnesium and sulfur. Carbon, hydrogen and oxygen are also needed in large quantities, but these are supplied by air, water and organic matter.
There are two categories of fertilizer that differentiate the source of the plant nutrients:
- One category is referred to as organic fertilizer. These fertilizers originate from plant or animal sources and are naturally occurring. These include manures, composts, blood meal, bone meal, fish meal, alfalfa pellets and others.
- The other category of fertilizer is chemical or conventional. These fertilizers are either processed minerals or chemical salts.
Whether you apply organic or conventional fertilizers to the soil, the important thing to remember is to apply only what your soil lacks and what your plant needs.
The timing of application is important to protect our waterways from unintentional contamination due to runoff and leaching. Apply fertilizer only at the time of year when the fruit tree will use it.
Fruit trees in western Oregon are able to grow on a wide variety of soils, but in most locations fertilization will improve plant vigor and health. To accurately determine what nutrients are present and what nutrients are lacking, take a soil sample. After you have your soil test results, you can supply just what nutrients your soil lacks. If your soil has sufficient levels of macronutrients, you may decide to apply just compost or aged manure to keep the soil healthy and porous.
If your soil lacks phosphorus, calcium or potassium, work these nutrients into the bottom of the planting hole with the soil, so that they will be available for the root system of the new tree. These elements do not move rapidly in the soil, so incorporating them at planting time makes them more quickly available to the tree.
Do not use nitrogen fertilizer in the planting hole; this can burn new roots. Instead, apply 1 pound of triple super phosphate per tree. Organic growers can use 3 pounds of bone meal per planting hole. If your soil's pH is below 6 or your soil sample indicates a deficiency of calcium, add 1-2 pounds of lime per planting hole to sweeten the soil.
It is a good practice to hold off using any nitrogen fertilizer for several months in the first growing season. Once the root system becomes established, you can put a light application (one cup of sulfate of ammonium) around the drip line of the young tree. For a good organic alternative, apply 5-10 pounds of aged manure around the drip line of the tree.
Nitrogen or other macronutrients should be applied in the second year. Research indicates that applying fertilizers in August or early September maximizes nutrient uptake and is more efficient than late winter fertilization. Early season fertilization is often leached. Late summer to fall fertilization at the time of your last irrigation is taken more directly into the tree, helping to make healthy buds, spurs and shoots for the coming year without stimulating late growth.
So, how much nitrogen should you use for the first few years with your fruit trees? There is an easy rule to follow. Use about one-eighth of a pound of actual nitrogen per year of tree age. In the second year, that means to use a quarter-pound of actual nitrogen, 1.25 lbs of ammonium sulfate 21-0-0, or 1.75 lbs of 16-16-16, or 9 lbs of rabbit manure, or 17 lbs of steer manure.
For a tree that reaches 8 years old, add 1 lb of actual nitrogen, which would come from 5 lbs of 21-0-0, or 7 lbs of 16-16-16, or 35 lbs of rabbit manure or 70 lbs of steer manure. Once your tree has reached 8 years old, you no longer need to increase the amount of nitrogen fertilizer. Stay with 1 lb per year going forward.
Another way to gauge if your fertilization is adequate is to measure the amount of new wood your tree is making each year. If your apple or pear tree is under 8 years old and is making less than 12 inches of new shoot growth each year, you should apply more nitrogen. If your tree is making between 12 and 18 inches of new growth per year, you are fertilizing correctly.
If your tree is making over 18 inches of new growth per year, you can reduce your nitrogen inputs. A fruit tree that is overstimulated will be an insect magnet because of the succulent new growth. The fruit on apple trees with excessive nitrogen fertilization may also have a tendency to have bitter pit, an apple spotting disorder that is also linked to a calcium deficiency.
With trees in the genus prunus (cherry, plum, peach, apricot and nectarine), growth targets for new wood will be a little different from that of apples and pears. A young prunus tree should make about 2 feet of new shoot growth per year. If the growth is less than 18 inches, fertilize more. If the growth is more than 30 inches, reduce your nitrogen fertilization.
Remember to research your own areas for local fertility knowledge. Many soils in western Oregon are deficient in potassium, phosphorus and boron. Our local soils can also have excess amounts of magnesium.
A soil test is an important start in a good fertilization program.