According to SFGATE, here are the best fertilizers for apple trees:
Nitrogen - The recommended application rate is 0.1 pounds of nitrogen per inch of trunk diameter measured 1 foot above the ground and never more than 0.7 pounds. You can provide 0.1 pounds of nitrogen with approximately 0.5 pounds of ammonium sulfate, 0.3 pounds of ammonium nitrate, 0.8 pounds of blood meal or 1.5 pounds of cottonseed meal.
Potassium and Phosphorus - Apply approximately 0.4 pounds of phosphate or 0.2 pounds of potash per tree. You can provide the phosphate with 0.9 pounds of triple superphosphate or 3 pounds of bone meal, or the potash with 0.4 pounds of potassium sulfate or 4 pounds of wood ash.
Minor Nutrients - Apple trees require small quantities of many other nutrients, including boron, iron, zinc and manganese. Fertilize your trees with a broad-spectrum mineral amendment such as greensand or azomite. Mix these products into the soil at a rate of about 5 pounds per tree.
Soil pH Levels - For apples, the appropriate pH range is 6.0 to 6.5, or slightly acidic. The only reliable way to accurately adjust your pH is to obtain a soil test that reports pH and provides amendment recommendations that are suited to your soil and growing conditions.
Find out more about apple tree fertilization, when is the right time to fertilize, where to fertilize and what other kinds of fertilizer to use, all at SFGATE.
Tip! Learn the proper fertilization practices for apple orchards at Spectrum Analytic Inc.