Last August I had chance to visit the city of Stockholm and hear about their biochar efforts. It is really impressive what they have succeeded within past five years or so, much thanks to the enthusiasm of Mr. Björn Embrén from City of Stockholm’s Landscaping Department.
It all started more than 15 years ago, when Björn noticed that trees perform poorly in compacted urban soils. It was costly to plant trees, fertilize and take care of them- and if they were not doing well, they had no landscaping value and they had to be replaced. On the other hand, there were trees growing on sides of rock walls or on top of old chimneys, where rarely any soil existed- so it was not soil that the roots needed but effective aeration (CO2 away and oxygen to the roots), moisture and nutrients.
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.