Soil Management
Core concepts
Texture is fixed - sand 2.0–0.05 mm, silt 0.05–0.002 mm, clay <0.002 mm - while structure (aggregation) can be improved. Clay holds water and nutrients well (high cation exchange capacity) but has tiny pores and poor aeration; sand drains fast but retains little. Bulk density above roughly 1.4–1.6 g/cm³ begins restricting root growth, and above about 1.8 g/cm³ roots may not penetrate at all. Understand pH effects on nutrient availability (iron and manganese tie up at high pH), organic matter benefits, and why a soil test precedes any fertilization decision. Urban soils are commonly compacted, layered, alkaline, low in organic matter, and sometimes contaminated.
Traps
- Confusing texture with structure, or 'fixing' clay by adding sand - that makes brick.
- Assuming clay is infertile; it has high CEC but low aeration.
- Fertilizing without a soil test, or pushing nitrogen and inviting pests and weak growth.
- Mulch volcanoes; keep mulch 5–10 cm deep and pulled back from the trunk.
Worked example
A tree declining in a new parking-lot island shows shallow, flattened roots, standing water after rain, and a penetrometer reading above 2.0 g/cm³. Diagnose compaction and poor drainage rather than disease. Mitigation: relieve compaction with air excavation or radial trenching, add organic matter to the surface (not the backfill), mulch, and redirect water - do not simply fertilize, which does nothing for aeration.
Rule of thumb
Dig the planting hole 2–3 times the root ball width but no deeper than the root ball, and treat the soil around the tree - not just the hole - as the real root zone.