They then started experimenting with structural soils, a planting holes with a layer of big, 3-7 cm diameter rocks in the bottom and soil washed between the rocks.
This was a success, and as the next step, in 2009, they switched to using biochar instead of soil for all of their urban tree planting projects. They have found that the main advantage of biochar for urban tree projects over other soil amendments is that it resists compression and compaction. The crushed stones and biochar mix results in an improved porosity in the soil (roughly 40%). Increased porosity facilitates better gas exchange, permeability leading to improved root penetration.