Fragipan is one thing, but what about soils that are highly compacted from the top down? This is a problem common in urban landscaping, and not uncommon in parts of suburbia as well. Compaction makes it hard to start and grow plants, including trees, which serve many important functions, especially in cities. The solution is excavation and replacement of the compacted soil with a suitable growing medium.
Photo of urban landscaping by thanh.ha.dang (CC BY-NC-SA 2.0)

In Stockholm, Sweden, one enlightened leader grasped the choice of replacement growing medium as an opportunity to introduce biochar into the mix whenever trees are being planted in civic landscaping projects. Biochar brings many advantages over commonly used peat moss, including its resistance to compaction. The biochar techniques used by Bjorn Embrén, head of landscaping for the city, were developed over decades and have shown stupendous results in the growth rate and survival of trees. The mixes they use include relatively little soil, but massive amounts of various grades of gravel and biochar. Tree roots find the open structure very accommodating.

Embrén's advice to cities who are interested in these techniques is "Dare to try it and you will be convinced!"  What we, in Maryland, must do first is make a few changes to the specifications for growing media in the Stormwater Design Manual, which dictates a loam/sand mix for infiltration devices. Adopting these techniques would make it possible to select from a wider range of vegetation in bioretention and filter devices, as biochar-based filters drain more quickly than those that rely primarily on soil.

At any rate, we shouldn't have to take the full development and test approach to begin adopting these designs - that part has already been done. All we need is a few delegations visiting Stockholm to see how to make them successfully. They can start by reading this manual. It is a bit old and doesn't use the term "biochar." The term they use instead, "structural soil," could have been key to adoption of the changes since it sounds so common sense and familiar.

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