Fertilizing Landscape Trees & Shrubs Part 2: Deficiency Diagnosis, Soil Testing & Fertilizer Selection

Effective fertilization begins with accurate diagnosis—visual symptoms, soil analysis, and tissue testing guide precise nutrient management. This installment covers deficiency identification, soil test interpretation, nitrogen dynamics, and fertilizer selection criteria.

Visual Deficiency Symptoms

Soil Testing Protocol

• Essential nutrients (18): C, O, H (air/water); N, P, K, Ca, Mg, S (macros); Fe, B, Cu, Mn, Mo, Zn, Co, Ni, Cl (micros)
• Sampling depth: 6-8″ for trees/shrubs (where feeder roots concentrate)
• Sampling pattern: 10-15 cores from dripline zone, mixed composite
• Lab provides: pH, buffer pH, P, K, Ca, Mg, CEC, base saturation, lime requirement
• Limitation: Soil N tests unreliable—N forms change rapidly. Use visual growth indicators instead.

Nitrogen: The Growth Driver

• Forms: Nitrate (NO₃⁻) mobile, leaches; Ammonium (NH₄⁺) held by CEC; Urea converts to ammonium
• Response: Greatest growth increase per unit applied—color, size, extension
• Risk: Over-application → excessive succulent growth, pest attraction, groundwater nitrate
• Strategy: Slow-release organic/inorganic sources; split applications; never exceed 3 lb N/1000 sq ft/yr

Fertilizer Selection Matrix

Label Interpretation (N-P-K)

Three numbers = % by weight: Nitrogen (N), Phosphate (P₂O₅), Potash (K₂O). Example: 10-10-10 = 10% N, 10% P₂O₅, 10% K₂O. Remainder = filler. Calculate actual nutrient: 50 lb bag 10-10-10 = 5 lb each N, P₂O₅, K₂O.

Phosphorus & Potassium Stability

Unlike nitrogen, P and K are relatively immobile in soil. Deficiencies develop slowly; applications persist years. Test before applying—excess P causes waterway eutrophication.