Answer: The following information on fertilizing and watering deciduous fruit trees is from the University of Arizona Cooperative Extension:
Fruit trees should be fertilized annually in February or March. Nitrogen (N) is the most important nutrient to apply. If N is deficient then older leaves will turn yellow.
Amount of N per inch of Trunk Diameter
Apricot, Cherry, Peach/Nectarine, Plum 0.05
Example: An apple tree has a 2-inch trunk diameter, so it needs 0.2 lbs. of actual nitrogen. Ammonium phosphate (16-20-0) contains 16% actual nitrogen; i.e., there are 16 lbs. of actual nitrogen in 100 lbs. of fertilizer. 0.2 lbs.?16% = 1.25 lbs. You need to apply 1.25 lbs. of ammonium phosphate to supply 0.2 lbs of actual nitrogen to the tree.
Minor elements most often deficient are iron and zinc. Both nutrients, when applied to deficient soils, are quickly tied up chemically in the soil so that they are not available to the tree. One way to remedy micronutrient deficiencies is to apply them to the leaves in chelated form. Foliar sprays are most readily absorbed by young expanding foliage.
Iron deficiency causes a yellowing of new leaves while the veins remain green. Irrigating too frequently in the spring can induce iron deficiency. Symptoms of zinc deficiency include shortening of the space between leaves or nodes so many leaves are bunched at the twig end (rosetting), small leaves, and in severe cases brown areas on leaves.
For maximum tree and fruit growth, water needs must be satisfied from bud swelling through harvest. Watering frequency during the growing season can vary from 7 to 21 days, depending on the age of tree, climate, and soil type. Young plants have small root systems and require close attention. Be sure to use a soil probe ? a metal rod, or an auger to determine dryness of soil.
Mature fruit trees respond to deep watering; saturate the soil to a depth of 2-3 feet. Build basins to extend past the tree?s drip line (an imaginary line extending to the outer edge of the plant canopy). Mulches will help conserve moisture. Use an organic mulch, such as straw or bark chips, 6 inches thick; keep the mulch away from the tree trunk to lessen crown rot.
Normal fruit development depends on a continuous supply of water. For example, the red or yellow color of apple fruit will not develop properly if trees are stressed for water. Other problems aggravated by improper timing include split pits of peaches and cracks on prunes.
Not only does proper watering allow the fruit to develop normally on the tree, but summer irrigation helps the crop for the following year. Flower buds are initiated in the summer and develop in the fall. Dry soil this summer will cause apple and pear trees to have a heavier bloom and a reduced fruit set next spring. Peach, plum, and apricot trees react differently to summer water stress; they will have little or no bloom the following spring.
Once fruit has been harvested, continue periodic irrigation until leaves fall and then let the tree harden-off when going into winter. Remember that as long as the leaves remain green, trees will use water. Irrigation also may be needed in winter, especially with high temperatures or inadequate winter rains. Remember dry roots die!
Fruit trees are very sensitive to excess salt - either in the soil or in the irrigation water. Electrical conductivity, (EC) is a measure of salinity; the more salt ions in the water, the better it conducts electricity. If soil EC levels are greater than 2.5, salts may damage the tree. The first symptom of salt burn is brown or yellow margins on leaves. Frequent, shallow irrigations can cause salt to accumulate in the root zone. You can remove much of this salt by leaching the soil. To leach soil, apply four or five times the amount of water for a normal irrigation. Late fall is a good time to do this leaching because trees are dormant and less susceptible to waterlogging.
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