ISA Certified Arborist Exam

Soil pH measures acidity or alkalinity on a scale of 0–14. Most trees prefer slightly acidic to neutral soils (pH 6.0–7.0). Soil pH affects nutrient availability; certain nutrients become unavailable at extreme pH levels.

At low pH (acidic conditions), aluminum and manganese may reach toxic levels while phosphorus, calcium, and magnesium become less available. At high pH (alkaline conditions), iron, manganese, zinc, copper, and boron become less available, potentially causing deficiency symptoms.

Iron chlorosis commonly occurs in alkaline soils when iron becomes unavailable despite adequate soil iron content. Affected trees show interveinal chlorosis (yellowing between veins while veins remain green) on young leaves.

Elemental sulfur lowers soil pH as soil bacteria oxidize it to sulfuric acid. Aluminum sulfate and iron sulfate also acidify soil but may cause aluminum toxicity if overused. The amount of amendment required depends on soil texture and buffering capacity; clay soils require more amendment than sandy soils.

Limestone (calcium carbonate) raises soil pH. Dolomitic limestone also supplies magnesium. Wood ash raises pH but should be used cautiously to avoid over-application.

Cation exchange capacity (CEC) measures the soil's ability to hold and exchange positively charged nutrients (cations) such as calcium, magnesium, potassium, and ammonium. Higher CEC indicates greater nutrient-holding capacity.

Clay particles and organic matter have high CEC due to their negative surface charges. Sandy soils have low CEC and require more frequent fertilization.

Base saturation refers to the percentage of CEC occupied by base cations (calcium, magnesium, potassium, sodium) versus acidic cations (hydrogen, aluminum). Higher base saturation generally correlates with higher pH.

Fertilization supplements soil nutrients to support tree growth and health. Proper fertilization requires understanding nutrient requirements, soil conditions, and application methods.

Macronutrients are required in large quantities:

Micronutrients are required in small quantities:

Nitrogen deficiency causes uniform chlorosis of older leaves first (mobile nutrient), reduced growth, and small leaves. Phosphorus deficiency causes purplish coloration, dark green color, and reduced growth. Potassium deficiency causes marginal leaf scorch on older leaves.

Iron deficiency causes interveinal chlorosis on young leaves (immobile nutrient). Manganese deficiency shows similar symptoms but with tan or gray spots in chlorotic areas. Zinc deficiency causes small, narrow leaves and shortened internodes.

Fertilizer labels display three numbers representing the guaranteed analysis: percentage of nitrogen (N), phosphate (P₂O₅), and potash (K₂O) by weight. A 10-10-10 fertilizer contains 10% nitrogen, 10% phosphate, and 10% potash.

To calculate actual nutrient content, multiply the bag weight by the percentage. A 50-pound bag of 10-10-10 contains 5 pounds each of N, P₂O₅, and K₂O.

Quick-release fertilizers provide nutrients immediately but may leach rapidly and risk root burn if over-applied. Examples include ammonium nitrate, urea, and potassium chloride.

Slow-release fertilizers release nutrients gradually over weeks or months, reducing leaching and burn risk. Types include sulfur-coated urea, polymer-coated fertilizers, and organic fertilizers. Slow-release products are generally preferred for trees.

Organic fertilizers derive from plant or animal sources and typically release nutrients slowly as soil microbes decompose them. Examples include composted manure, blood meal, bone meal, and cottonseed meal. Organic fertilizers also improve soil structure.

Complete fertilizers contain all three macronutrients (N-P-K). Incomplete fertilizers contain one or two nutrients and are used when soil tests indicate specific deficiencies.