Plants require certain essential elements called “nutrients” to function and grow. In a natural forested setting leaves and other plant parts decompose and assimilate back into the surrounding soil and supply the contributing plants with sufficient quantities of essential elements. Landscape or urban plants are not afforded “nutrient cycling” because leaf and plant parts are customarily removed from the growing environment. Soil structure and texture is often negatively altered during construction processes creating poor growing conditions for plants including imbalances in soil pH further limiting nutrient availability. Trees and shrubs offer productive growth and vitality when soil pH is balanced and essential nutrients are available for uptake. It is important to recognize when a tree or shrub needs fertilization, which elements are needed, and when to apply them.
Plants take up nutrients dissolved in water through their roots. Certain elements, known as macronutrients, are required in relatively large quantities and include Nitrogen, Phosphorus, and Potassium. Most tree and shrub fertilizers contain varying ratios of these three macronutrients. Secondary and micronutrients are required in much lesser quantities and most notably include Iron, Manganese, and Zinc. Modern approaches to tree fertilization emphasize correlating observable deficiencies with soil and foliar analysis to provide an accurate prescription.
Goals of Fertilizing Tree and Shrubs
- Correct a visible nutrient deficiency
- Correct a deficiency detected through soil or foliar analysis
- Increase growth and vigor especially on young plants
- Increase plant vitality and recuperative capacity
Methods of Tree and Shrub Fertilization
- Liquid injection – fertilizer dissolved in water is injected in a well distributed pattern to the root zone.
- Surface application – simple technique for applying liquid or dry fertilizer
- Drill hole method – granular fertilizer is poured into 2” – 4” holes drilled in a grid pattern around a plant.
- Foliar spraying – used to correct minor element deficiencies like iron chlorosis.
- Trunk injection and implants – injects liquid fertilizer directly into the trees vascular system allowing for correction of minor element deficiencies. Should only be used when other methods are unproductive.
Qualifications for Fertilizing Trees and Shrubs
Fertilizing trees and shrubs should optimally be performed by qualified arborist with proper training and education. Many components go into an accurate application of tree and shrub fertilization including: fertilizer selection, timing, method and rate of application. An ISA Certified Arborist is extensively trained in the technical elements of tree and shrub fertilization and held to a continuing education requirement. All fertilization work performed by an arborist shall be completed in compliance with the ANSI A300 technical standard and the Z133.1 safety standard.