ISA Soil Science and Management 2 — Questions and Answers

Question 1: What soil pH range is generally considered optimal for most landscape trees?

4.0-5.0
5.5-7.0 (Correct answer)
7.5-8.5
8.0-9.0

Correct answer: 5.5-7.0

Most landscape trees grow best in slightly acidic to neutral soil, with a pH range of approximately 5.5-7.0. This range allows optimal availability of essential nutrients.

Soil pH directly affects nutrient availability. In the 5.5-7.0 range, macronutrients (nitrogen, phosphorus, potassium) and most micronutrients are readily available for root uptake. Below pH 5.5, aluminum and manganese can reach toxic levels, and calcium, magnesium, and phosphorus become less available. Above pH 7.5, iron, manganese, zinc, and copper become increasingly unavailable, leading to deficiency symptoms like interveinal chlorosis. While some species are adapted to acidic soils (azaleas, blueberries) or alkaline soils (hackberry, bur oak), the 5.5-7.0 range serves the broadest range of landscape trees.

Question 2: What is the primary purpose of mycorrhizal fungi in relation to tree root systems?

They decompose dead root tissue to prevent disease
They form symbiotic associations with roots, dramatically extending the nutrient and water absorption network (Correct answer)
They produce antibiotics that kill root pathogens
They fix atmospheric nitrogen directly into the root tissue

Correct answer: They form symbiotic associations with roots, dramatically extending the nutrient and water absorption network

Mycorrhizal fungi form symbiotic relationships with tree roots, extending their hyphal networks far beyond the root zone to access water and nutrients — particularly phosphorus — in exchange for carbohydrates from the tree.

Mycorrhizal fungi are among the most important organisms in forest and landscape ecosystems. The two main types — ectomycorrhizae (ECM, common on oaks, pines, birches) and arbuscular mycorrhizae (AM, common on maples, ashes, most hardwoods) — colonize root surfaces or penetrate root cells and extend vast hyphal networks into the soil. These hyphae are much finer than root hairs and can access water and nutrients in soil pores too small for roots to penetrate. The fungi are particularly effective at mobilizing phosphorus, which is often bound to soil particles. In return, the tree provides the fungi with carbohydrates. This symbiosis is estimated to supply 80-90% of phosphorus and significant nitrogen to most trees.

Question 3: What does a soil bulk density measurement indicate about growing conditions for tree roots?

The concentration of organic matter in the soil
The degree of soil compaction, which affects root penetration, aeration, and water infiltration (Correct answer)
The amount of clay versus sand in the soil profile
The depth of the water table

Correct answer: The degree of soil compaction, which affects root penetration, aeration, and water infiltration

Bulk density measures the weight of dry soil per unit volume. Higher values indicate greater compaction, which restricts root growth, reduces pore space for air and water, and limits biological activity.

Soil bulk density is calculated as the dry weight of soil divided by its total volume (solids plus pore space). For tree growth, bulk density is a critical indicator of compaction. Sandy soils typically have bulk densities of 1.4-1.8 g/cm3, while clay soils range from 1.0-1.6 g/cm3. Root-limiting bulk densities vary by texture: approximately 1.75 g/cm3 for sandy soils and 1.4-1.5 g/cm3 for clay soils. Compacted soils have reduced macropore space, limiting gas exchange (oxygen in, carbon dioxide out), water infiltration, and physical root penetration. Urban soils are frequently compacted by construction equipment, foot traffic, and vehicle loading.

Question 4: Why is vertical mulching sometimes recommended for trees in compacted urban soils?

To improve the appearance of the landscape
To create channels in compacted soil that improve aeration, water infiltration, and root growth opportunities (Correct answer)
To provide a deep reservoir of fertilizer near the root zone
To prevent surface roots from developing

Correct answer: To create channels in compacted soil that improve aeration, water infiltration, and root growth opportunities

Vertical mulching involves drilling holes into compacted soil and filling them with organic matter or porous material, creating channels that improve oxygen availability, water movement, and root exploration into otherwise impenetrable soil.

Vertical mulching is a soil remediation technique for compacted urban soils where surface decompaction is impractical. Holes are drilled (typically 2-4 inches in diameter, 8-12 inches deep, on 2-3 foot centers) throughout the root zone and filled with a porous amendment such as coarse comite, expanded shale, perlite, or composted organic matter. These filled channels serve multiple functions: they break through compacted layers, create pathways for water infiltration and gas exchange, provide zones of low-resistance soil for root exploration, and introduce organic matter that supports beneficial soil organisms. The technique is often combined with radial trenching or air spading for more comprehensive soil remediation.

Question 5: What is cation exchange capacity (CEC) and why is it important for tree nutrition?

The rate at which soil can absorb rainwater
The soil's ability to hold and exchange positively charged nutrient ions on particle surfaces (Correct answer)
The percentage of soil particles smaller than 0.002mm
The amount of dissolved oxygen available in soil water

Correct answer: The soil's ability to hold and exchange positively charged nutrient ions on particle surfaces

CEC measures the soil's capacity to hold positively charged ions (cations) such as calcium, magnesium, potassium, and ammonium on clay and organic matter surfaces, making them available for root uptake.

Cation exchange capacity is a fundamental soil property that determines the soil's ability to retain and supply essential nutrients. Clay particles and organic matter have negatively charged surfaces that attract and hold positively charged nutrient ions (cations) like calcium (Ca2+), magnesium (Mg2+), potassium (K+), and ammonium (NH4+). Soils with higher CEC — typically clay soils and those rich in organic matter — hold more nutrients and resist leaching. Sandy soils with low CEC lose nutrients quickly through leaching. CEC also influences pH buffering capacity. Understanding a soil's CEC helps arborists design appropriate fertilization programs: low-CEC soils benefit from more frequent, lighter applications to minimize nutrient loss.

Question 6: What is the proper depth and extent for applying organic mulch around a landscape tree?

6-8 inches deep, piled against the trunk
2-4 inches deep, extending to the drip line or beyond, kept away from the trunk flare (Correct answer)
1 inch deep, only around the base of the trunk
8-12 inches deep in a raised bed around the tree

Correct answer: 2-4 inches deep, extending to the drip line or beyond, kept away from the trunk flare

Organic mulch should be applied 2-4 inches deep, extending as far toward the drip line as practical, and pulled back from the trunk flare to prevent moisture buildup against the bark.

Proper mulching provides numerous benefits: moisture retention, temperature moderation, weed suppression, and organic matter addition as it decomposes. The 2-4 inch depth is optimal — less provides insufficient coverage, while more can create anaerobic conditions, harbor rodents, and prevent gas exchange. Mulch should extend as far into the root zone as practical, ideally to the drip line, since this is where feeder roots are most active. Critically, mulch must be kept several inches away from the trunk flare. Mulch piled against the trunk (volcano mulching) traps moisture against the bark, promoting decay, canker diseases, and adventitious root growth into the mulch that can eventually girdle the trunk.

ISA Soil Science and Management 2

What soil pH range is generally considered optimal for most landscape trees?