Tree Nutrition and Fertilization
Elements and mobility
Know the macronutrients (N, P, K, Ca, Mg, S) and micronutrients (Fe, Mn, Zn, Cu, B, Mo, Cl, Ni). Mobility inside the plant drives symptom location: nitrogen, phosphorus, potassium, and magnesium are mobile, so deficiency appears first on older leaves; iron, calcium, boron, manganese, zinc, and copper are relatively immobile, so deficiency shows on new growth.
Availability
Soil pH controls availability more than total soil content does. Iron, manganese, and zinc become less available as pH rises; phosphorus and molybdenum become less available at low pH. Diagnose with soil test, tissue analysis, and site history together, not with a guess from leaf color alone.
Decisions
Fertilize only when a deficiency is confirmed or growth targets justify it. Do not fertilize a newly planted tree, a drought-stressed tree, or one compromised by root loss. Nitrogen drives shoot growth, not root growth, and excess nitrogen can increase succulent growth and pest susceptibility. Match the ratio to the deficiency; do not simply choose the highest analysis on the shelf.
Traps
"Deep root feeding" is a delivery method, not proof of superiority. Applying iron to a high-pH soil without acidifying or chelating may not correct chlorosis. Phosphorus applied unnecessarily risks runoff and water quality problems.
Worked example
A silver maple with interveinal chlorosis on the newest leaves while soil pH is 7.8 is showing an iron availability problem, not an iron shortage. The fix is acidification and/or chelated iron applied where roots can absorb it, then verifying with tissue testing the following season.