Orchard Nutrition: Macronutrients
Fruit trees need to maintain an appropriate balance between vegetative growth and fruit growth. Too much vegetative growth may reduce fruit set and yield the following year. This balance is partially influenced by the availability of nitrogen (N), phosphorus (P), and potassium (K). Nitrogen, P, and K are used by plants for structure, nutrient transport, and movement of water, which are among many other important functions. A lack of macro-nutrients or a nutrient imbalance may result in decreases in both vegetative growth as well as fruit yield. Also, fruit ripening and quality can be negatively affected when nutrient deficiencies are present. Generally, leaf tissue nutrient content is a good basis for determining plant needs.
Nitrogen deficiency can be detected visually. Trees will have little to no new shoot growth. Deficient leaves are pale green to yellow. Symptoms first appear in older leaves because N moves from older tissue into actively growing younger leaves. Leaves from deficient trees tend to drop earlier in the fall. Fruit set might be light, and mature fruits can be smaller and mature somewhat earlier than usual. Excess nitrogen can also cause problems. Fruit will color poorly and lose firmness in storage. Leaves will remain dark green into fall, and leaf drop will be delayed. As a result, the tree’s process of entering winter dormancy will also be delayed, increasing susceptibility to possible winter injury.
Nitrogen applications are ideally applied in spring. Summer applications should be made at least 6 weeks prior to fruit ripening to ensure optimum fruit quality at harvest. Typical nitrogen needs are between 0.01 to 0.04 lbs N per tree, per year of age with an annual limit of 0.3 lbs N per tree. The amount that needs to be applied to reach this range will depend on soil texture, soil organic matter content, and leaf tissue content at the start of the growing season, among other indicators. Vegetative growth is the primary indicator for nitrogen requirements. Depending on the crop, new growth in younger trees should be between 10 and 30 inches per year, and in older trees it should be between 4 to 18 inches.
Phosphorus (P) and Potassium (K)
The level of phosphorus and potassium in the soil does not change as rapidly as that of nitrogen, so their management is monitored more effectively by soil testing (at a depth of 1 foot and 2 feet, within the tree row) and periodic tissue sampling (at each important stage of growth) for sufficiency.
Phosphorus is critical to root growth and function and the proper cycling of energy in the plant. Phosphorus deficiency affects older leaves first, turning them small and bluish green on the margins. Other symptoms might include reduced flowering, a decrease in fruit quality, and delayed fruit maturity. Excess phosphorus can cause imbalances in the uptake of zinc and iron.
Phosphorus is not very mobile in the soil, so it should be applied within the root zone before planting a new orchard, or when renovation of orchard sections. Mid-season adjustment of phosphorus levels in soils is generally not practical, so providing adequate levels at the beginning of the season is the best strategy for management. Annual adjustment
of phosphorus nutrition is recommended with an application of mono-ammonium phosphate (11-52- 0).
Potassium is critical in the water relations of plants and in the assimilation and cell-to-cell transfer of other nutrients, particularly calcium, which is important for fruit quality, particularly in pome fruits. Levels of potassium in Utah soils are regulated by the weathering of clay minerals and are generally sufficient without fertilizer application. However, on
sandy or gravelly soils low in clay content, potassium deficiencies may be expressed by calcium or other micronutrient imbalances in the plant. The primary deficiency symptom is yellowing and bronzing of the margins of older leaves.
In-season adjustment of potassium nutrition is possible with foliar sprays of potassium chloride or potassium sulfate solutions, or injection of these materials into the irrigation water.