5.4 Soil Chemistry, pH, CEC, Salinity, and Testing

Chemistry Controls Availability

A tree can show nutrient stress while the element sits in the soil. The key word is availability, and the master variable is soil pH — the negative log of hydrogen-ion activity on a scale of 0–14, where 7 is neutral. Most landscape trees do best in slightly acidic soil, pH 6.0–7.0, where the widest range of nutrients stays soluble. Acid-loving species (oak, rhododendron, pin oak, blueberry) prefer 4.5–6.0.

The single most exam-tested chemistry fact: as pH climbs above about 7.5, the availability of iron, manganese, and zinc plummets, producing interveinal chlorosis — yellow leaf tissue with green veins, worst on the newest leaves. This is high-pH (lime-induced) chlorosis, classic on pin oak and river birch in calcareous or over-limed urban soils. Below pH 5.5, availability of phosphorus drops and aluminum/manganese can become toxic.

The pH scale is logarithmic, a fact worth carrying into the exam: each whole unit is a tenfold change in hydrogen-ion activity, so pH 8.0 is ten times more alkaline than pH 7.0 and a hundred times more than pH 6.0. That is why nudging the pH of a large, well-buffered urban soil is so slow and why over-application of lime so easily pushes a soil into the chlorosis-prone zone.

A second pattern worth memorizing is the mobility of deficiency symptoms: deficiencies of mobile elements (nitrogen, potassium, magnesium) show first on older, lower leaves because the tree shuttles the scarce element to new growth, while deficiencies of immobile elements (iron, manganese, calcium, boron) show first on the youngest leaves. So chlorosis on new leaves points toward iron/manganese (often pH-driven), whereas uniform yellowing or scorch on old leaves points toward nitrogen or potassium. Reading which leaves are affected narrows the diagnosis before any lab work.

Cation exchange capacity (CEC) is the soil's capacity to hold positively charged nutrient ions — calcium (Ca²⁺), magnesium (Mg²⁺), potassium (K⁺), ammonium (NH₄⁺) — on negatively charged clay and organic-matter surfaces. It is reported in milliequivalents per 100 g (meq/100 g) or cmol(+)/kg. Sands carry roughly 1–5, loams 5–15, and clay or organic-rich soils 20–40+. Low-CEC sands leak nutrients quickly and need lighter, more frequent feeding; high-CEC soils buffer change and resist pH correction.

Chemistry Clues and Threshold Responses

Salinity is measured as electrical conductivity (EC) of a soil-water extract, in deciSiemens per meter (dS/m). Sensitive woody plants begin to stress around 2–4 dS/m, with marginal leaf scorch, twig dieback, and root injury. Urban salt comes from deicing (sodium chloride, NaCl), poor irrigation water, over-fertilization, coastal spray, and pet urine. Sodium is doubly harmful: chloride burns foliage while excess sodium disperses clay, collapsing structure and worsening drainage.

Salt stress also has an osmotic dimension that mimics drought: dissolved salts lower the soil-water potential, so even moist saline soil is physiologically dry to the root — it cannot pull water against the salt gradient. That is why salt-injured trees look drought-stressed in spring and why the management priority is to flush salts below the root zone with clean water where drainage permits, divert deicing runoff, and switch to salt-tolerant species (honeylocust, ginkgo, and many oaks tolerate more than maples or pines).

Where soil is sodic (high exchangeable sodium that has dispersed the clay), the remedy is not just leaching but adding a calcium source such as gypsum so calcium displaces sodium off the exchange sites, after which leaching can carry the freed sodium away. Note that gypsum does not change pH — a common distractor — it swaps cations and improves sodic-soil structure.

Testing is central. A representative soil sample combines several cores from the root zone (typically the top 15–30 cm), free of mulch and surface debris, mixed in a clean bucket. Sample by zone when conditions differ — a street side exposed to deicing salt versus a protected lawn side should be separate samples and separate bags. A standard report returns pH, buffer pH, CEC, percent base saturation, organic matter, and extractable nutrients; salt-suspect sites add an EC and soluble-salt panel.

Foliar (tissue) analysis complements soil data when a deficiency is suspected but soil levels look adequate — useful for confirming iron or manganese chlorosis driven by pH rather than supply.

Changing pH around an established tree is slow and often unrealistic in large or calcareous soil volumes; elemental sulfur lowers pH gradually and lime raises it, but buffering resists both. The honest, exam-favored answer is frequently species selection and management rather than a promised permanent correction. For lime-induced chlorosis, options include soil-applied chelated iron (the EDDHA chelate works best above pH 7), trunk injection of iron, or replacement with a tolerant species — not a blanket nitrogen application.

The strongest report states what was tested, what each number means for this tree and site, and one clear action: no fertilizer, a targeted correction, water management, salt-source reduction, mulch, or species change.