To date, soil tests have not been useful in determining tree needs. Additionally, leaf samples may be useful. In general, leaf nitrogen levels are higher in samples from trees carrying heavy crops. Biennial bearing trees in their off year or trees with a light crop typically have lower nitrogen levels. Table 2 shows leaf nitrogen standards for apple.
Fruit trees respond to any form of nitrogen fertilizer.
Certain considerations, however, should be addressed. For instance, ammonium and ammonium-forming fertilizers like ammonium sulfate, urea, and ammonium nitrate will decrease soil pH over time and are not recommended for soils with pH below 6.5. Nitrogen when applied as urea may be lost from the soil via ammonia volatilization when this material is applied to the surface of warm, moist, alkaline soils. Leaching of nitrates is another manner by which nitrogen is lost from the system. This is especially true with coarse-textured soils that are irrigated frequently.