Fertilizing Fruit Trees
When you are planning a fertilization program for fruit trees, your goal should be high yields of high-quality fruit for the life of your trees. It is possible to obtain above-normal yields for one year, but succeeding crops could suffer as a result. While it is very important that nutrients be present in large enough quantities for the best tree growth and fruit production, the ratios of these nutrients is even more important. For example, fruit from trees with access to too much nitrogen and too little potash would be poorly coloured and have poor storage qualities.
Essential Elements:
There are 13 essential nutrient elements that come from the soil, and three elements that come from the air and water. A deficiency of one of those essential nutrients will result in poor growth and perhaps even precipitate the death of the tree.
Nitrogen
Most of the tree growth takes place in a six-to-eight-week period in the spring and early summer. Fruit buds for the following year are already formed before mid-July. For this reason, nitrogen requirements are high in the early part of the season since nitrogen stimulates growth and especially fruit set. Too much nitrogen late in the season may result in large poorly-coloured fruit that does not store well. It may also cause increased winter injury of trees.
Phosphorus
Phosphorus deficiency is rare in fruit trees, since trees are better able to utilize soil phosphorus than other plants. Phosphorus is, however, essential for tree growth and fruit production.
Potassium
Fruit trees are heavy potassium feeders, and potassium deficiency is very common in fruit trees. Signs of deficiency are small leaves which tend to curl. As the deficiency becomes more extreme, the edges of the leaves look scorched. Potassium increases hardiness and diseases resistance. Because it helps carry sugars within the tree, adequate potassium levels mean sweeter fruit.
Calcium
Trees may show calcium deficiency even though the soil in which they grow is high in calcium. Calcium available to the tree through its root system sometimes moves too slowly through the tree. Calcium sprays are often recommended as a corrective measure. A deficiency of calcium will show up as bitter pit in apples. They will not store well.
Magnesium
After nitrogen and potassium, magnesium is the most common deficiency found in orchards. Apples deficient in magnesium will be small and immature in appearance. On some varieties, the fruit will drop prematurely.
Micronutrients
Other essential nutrients, micronutrients such as iron, boron, zinc, copper, manganese, molybdenum and chlorine, are required in very minute amounts and are usually obtained from the soil.
Fertilizing
Testing the soil in which your fruit trees grow is the most reliable way to understand the nutrients available and the fertilizer needs. If it is not possible to do a soil test, use a fertilizer with a 1-1-2 ratio. A common tomato fertilizer, such as 7-11-17, provides a good ratio for fruit trees. Make the first of two applications of this fertilizer in April. Punch holes under the drip line (the line around the tree where rain would drip from the branches/foliage) every 12 inches. The holes should be about nine inches deep. Pour fertilizer evenly into each hole according to this rule of thumb: one pound of fertilizer for every inch of diameter of trunk. Thus, a tree with a five-inch diameter trunk would require a total of five pounds of 7-11-17 divided evenly among the punched holes. Top up the holes with sand or compost. Repeat again before June 1, at half the rate (i.e. half a pound of 7-11-17 for each inch of diameter in the trunk.
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