Many nutrients are needed for tree growth, but most are available in the soil in sufficient quantity to satisfy the needs of fruit trees. Where soils lack fertility, the addition of compost or fertilizers can prevent nutrient deficiency. However, lack of fertility is not a common cause for lack of fruit bearing or poor tree growth. Trees that are marginal in health, attacked by insects and disease or have not been recently pruned are not likely to benefit from fertilization. If you are unsure about fertilizer, a soil test will provide information on which nutrients are in short supply.
When applying chemical fertilizers, protect your hands with latex, nitrile or neoprene gloves. For safety, store fertilizers in their original containers and keep away from open flames or gasoline. Fertilizers should not be stored with food or drink.
Spring is the best time to fertilize fruit trees when the tree is most rapidly growing. There are many fertilizer products available for fruit trees, but fertilizer content will vary. When using commercial fertilizers, follow the instructions on the package label. Most products contain a mix of different elements, usually nitrogen, phosphorus and potassium with additional micronutrients.
Nitrogen-containing fertilizers should be used conservatively since nitrogen can promote lush growth and increase disease susceptibility of fruit trees. Nitrogen fertilizer should be applied before June so that the tree acquires winter hardiness in time for winter. Each fertilizer product contains a different amount of nitrogen, so the amount of fertilizer needed depends on its nitrogen content. The amount of fertilizer to add will also vary with tree age and size. The recommended rate in the first few years after planting is ½ ounce nitrogen per tree. Fertilizer that contains 10% nitrogen (10-10-10) should be applied at a rate of five ounces of fertilizer to get ½ oz. of actual nitrogen. For mature semidwarf trees, apply 3 oz. of nitrogen in a three-foot diameter circle around the base of the tree. Fertilizer that contains 10% nitrogen (10-10-10) can be applied at a rate of 30 oz. of fertilizer per tree to get 3 oz. of nitrogen. This same rate can be applied to peaches. Apply only 10 oz. of 10-10-10 to dwarf apple trees. You may find that trees grow just as well without fertilizer every year. Sweet cherries and pears are very vigorous and should be fertilized conservatively or not at all.
Potassium and magnesium are in short supply in New England orchards. This can be fixed by applying potassium magnesium sulfate (sul-po-mag) once a year at a rate of no more than 1 lb. per tree for young trees and dwarf trees. Up to 2 lbs. per tree can be applied to fully grown semi-dwarf trees. Full-sized standard trees can receive 2-3 lbs. per tree. To avoid an excess of potassium, 10-10-10 should not be applied with potassium-magnesium-sulfate. Compost and manure-based fertilizers are typically rich in potassium.
Boron is commonly deficient in New England soils. Agricultural borax is a good source of boron when applied to the soil once every three years, but must be used correctly to prevent the addition of too much boron which can damage trees. Always wear gloves when applying boron-containing fertilizers. Borax can be applied at a rate of 0.5 oz. per tree for young apple trees and dwarf trees. Use a rate of 1 oz. for semi-dwarf trees and a rate of 3.5 oz. for full sized trees. These rates are for borax that contains 11% boron. Rates should be cut by a third for borax containing 15% boron and by half for borax containing 20% boron. Stone fruit are sensitive to excess boron. Use half the rate suggested for apple. Never apply more than the recommended rate since excessive boron is harmful to the tree.
Composts and manures are variable in nutrient content, but can be a slow-release source of nitrogen and other nutrients. The rate to apply is not clear because of their variability, but can slow the growth of trees when applied in excess.