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Other soil improvements
Other than temporarily raising soil pH, improving drainage, or loosening compacted soil, it is difficult to permanently improve most existing soils for trees. Most other so called "improvements" are temporary and have not been shown to impact species selection.
Colloidal phosphate, a byproduct of phosphate mining, can be used as an inorganic amendment for sandy soils. It is expensive and only locally available, but its improving affects (enhanced water holding and exchange capacity) can last 10 years or more.
Synthetic, organic, and other soil amendments are being developed and marketed, and may be useful in the future. Research shows little, if any, benefit from products currently on the market. If soil amendments are used they should be mixed into at least the top 12 inches of large areas of soil before planting.
Amending only backfill soil around an individual plant provides no benefit to trees unless roots will be confined to this small area by adverse conditions in surrounding soil. For example, it may pay to amend the soil in a sidewalk cutout if you believe the compacted soil beneath the sidewalk will prevent root growth there.
Expanded slate or sintered fly ash incorporated into a compactable soil can help prevent future compaction. At least 20 percent of the soil volume should be comprised of this soil amendment in order to be effective. Long-term testing in Washington, D.C. indicates that this strategy helps prevents compaction for at least 22 years, even in heavily used turf areas (Patterson, 1993).
Organic matter incorporated into sandy soil can temporarily improve water and nutrient holding capacities. Organic matter can help aerate clay soil by separating aggregates or clumps. If clumps remain separated, air and water should penetrate better.
By keeping people and heavy equipment off the soil after organic matter is mixed in, clay soil clumps may stay apart and some of the new airways may remain intact even after organic matter has decayed and disappeared. This might improve tree growth.
