ISA Certified Arborist Exam

Soil is the foundation of tree health. Understanding soil composition, structure, and chemistry is essential for proper tree care and management. Arborists must evaluate soil conditions to diagnose tree problems and recommend appropriate treatments.

Soil consists of four primary components: mineral particles (45%), organic matter (5%), water (25%), and air (25%). These percentages represent ideal conditions; actual ratios vary by location and soil type.

Mineral particles are classified by size into three categories:

Soil texture refers to the relative proportions of sand, silt, and clay. The soil textural triangle classifies soils into categories such as sandy loam, silty clay, or clay loam based on these proportions. Loam soils contain roughly equal proportions of sand, silt, and clay and are generally ideal for tree growth.

Organic matter consists of decomposed plant and animal materials. Despite comprising only a small percentage of soil volume, organic matter significantly influences soil fertility, water retention, and structure. Humus is the stable, fully decomposed fraction of organic matter that binds soil particles into aggregates and improves cation exchange capacity.

Soil structure refers to the arrangement of soil particles into aggregates or peds. Good soil structure creates pore spaces for air and water movement. Structure types include granular, blocky, platy, and columnar. Granular structure is most favorable for root growth.

Aggregation occurs when mineral particles bind together through organic matter, clay particles, fungal hyphae, and root exudates. Compaction destroys soil structure by collapsing pore spaces, reducing aeration and drainage.

Soil aeration refers to the exchange of gases between the soil atmosphere and the ambient atmosphere. Adequate oxygen supply to roots is critical for respiration, nutrient uptake, and overall tree health.

Roots require oxygen for aerobic respiration, which produces the energy needed for nutrient absorption, water uptake, and growth. In well-aerated soils, oxygen concentrations range from 15–20%, similar to atmospheric levels. When oxygen drops below 10%, root function becomes impaired. Concentrations below 3% cause root death.

Carbon dioxide produced by root respiration and microbial activity must diffuse out of the soil. Poorly aerated soils accumulate carbon dioxide to toxic levels.

Soil compaction is the primary cause of poor aeration in urban environments. Compaction results from foot traffic, vehicle traffic, construction activity, and heavy equipment operation. Compacted soils have reduced pore space, limiting gas exchange and water infiltration.

Waterlogging occurs when soils remain saturated for extended periods. Water displaces air from pore spaces, creating anaerobic conditions. Poorly drained soils, over-irrigation, and altered drainage patterns contribute to waterlogging.

Fine-textured soils (high clay content) have smaller pore spaces and are more susceptible to poor aeration than coarse-textured soils.

Trees in poorly aerated soils exhibit reduced growth, chlorosis (yellowing of leaves), dieback of branches, and increased susceptibility to pests and diseases. Root systems become shallow and may develop surface roots. Bark may become loose or discolored at the root flare.

Vertical mulching involves drilling holes 2–4 inches in diameter and 12–18 inches deep in a grid pattern throughout the root zone. Holes are filled with porous materials such as pea gravel, coarse sand, or composted organic matter.

Radial trenching creates narrow trenches radiating outward from the trunk through the root zone. Trenches are filled with amended soil or porous materials. This technique is more disruptive to roots than vertical mulching.

Air excavation (air spading) uses compressed air to remove soil without damaging roots. This technique allows inspection of root systems, removal of compacted soil, and replacement with amended backfill.

Core aeration removes small plugs of soil to create channels for air and water. This technique is less effective for trees than for turf but may provide some benefit in mildly compacted soils.

Organic mulch application improves soil structure over time as organic matter decomposes and incorporates into the soil. Mulch also protects soil from further compaction.

Water moves through soil by gravity (gravitational water), is held in pore spaces by capillary forces (capillary water), or is held tightly to soil particles (hygroscopic water). Only capillary water is available to plants.

Field capacity is the amount of water held in soil after gravitational water has drained. Permanent wilting point is the moisture level at which plants can no longer extract water. Available water is the difference between field capacity and permanent wilting point.

Sandy soils drain quickly and have low water-holding capacity. Clay soils drain slowly and hold water tightly, making less available to plants despite higher total water content.