All healthy fruit trees are heavy feeders and remove large quantities of mineral nutrients from the soil. Even though all plants require 16 elements that are referred to as essential, the macro nutrients are the elements most heavily removed from the soil. It is important to replace these macro nutrients on a regular basis to keep trees vigorous and healthy. The macro nutrients we should apply regularly are nitrogen (N), phosphorus (P), potassium (K), calcium (CA), magnesium (MG) and sulfur (S). Carbon, hydrogen, and oxygen are also macro nutrients that are 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 would include manures, composts, blood meal, bone meal, fish meal, alfalfa pellets, etc.
- 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 your plant needs. Fertilizer application timing is very important to protect our water tables and waterways from unintentional contamination due to run-off and leaching. You should only apply fertilizer at the time of year when the fruit tree will utilize the fertilizer that you've made available in the soil.
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 know accurately what nutrients are present and what nutrients are lacking at your orchard site you should 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 macro nutrients you may decide to just apply compost or aged manure to keep the soil healthy and porous.
If your soil lacks phosphorus, calcium, or potassium it is recommended to work these nutrients into the bottom of the planting hole with the soil to be available for the root system of the new tree. These elements do not move rapidly in the soil so incorporating them at planting makes them more quickly available to the tree.
Do not use nitrogen fertilizer in the planting hole, this can burn new roots. Instead, 1 pound of triple super phosphate per tree is sufficient. Or, 3 pounds of bone meal per planting hole will work for organic growers. If your soil's pH is below 6 or your soil sample indicates a deficiency of calcium, 1-2 pounds of lime per planting hole will 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. Using 5-10 pounds of aged manure around the drip line of the tree will be a good organic alternative.
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 in to 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 when deciding how much nitrogen to apply each year. You should use about 1/8 th of a pound of actual nitrogen per year of tree age. In the second year that means use a quarter pound of actual nitrogen or 1.25 lbs of ammonium sulfate 21-0-0, 1.75 lbs of 16-16-16, 9 lbs of rabbit manure, and 17 lbs of steer manure. For a tree that reaches eight years old you should add one pound of actual nitrogen which would come from 5 lbs of 21-0-0, 7 lbs of 16-16-16, 35 lbs of rabbit manure and 70 lbs of steer manure. Once your tree has reached eight years old you do not need to increase the amount of nitrogen fertilizer any longer. Stay with one pound 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 eight 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 12-18 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 over stimulated will be an insect magnet because of the succulent new growth. The fruit on apple trees with excessive nitrogen fertilization will also have a tendency to have bitter pit, an apple spotting disorder.
With genus prunus trees, (cherry, plum, peach, apricot, nectarine) your growth targets for new wood will be a little different than apples and pears. A young prunus tree should be making about two feet of new shoot growth per year. If the growth is less than 18 inches you should fertilize more and if the growth is more than 30 inches you can 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. Doing a soil test is very important to get you started in the right direction toward a good fertilization program.