Why suburbs will never have tall trees | 10:42 am | 7 June 2006
Now consider what’s happened at the subdivision. Once the topsoil is removed, you’re left with the rock and clay underneath, which hasn’t seen the light of day for thousands of years. Landscapers call it “hardpan,” and from an engineering point of view it’s an ideal material to mould into the site’s drainage plan.
Run heavy equipment over material like that, and it quickly gets compacted into something with much the same consistency as concrete.
Once the houses are in place, the topsoil gets put back, but usually to a depth of only 20 cm., which is the typical municipal standard and enough to support healthy turf.
The rest of the stockpiled topsoil is usually sold off and eventually ends up in nurseries, but only after it’s been rehabilitated by adding manure or peat moss or sand. That’s because the soil became anaerobic after sitting in a pile for so long. “There’s no oxygen within that pile anymore, and eventually all the living microbes and organisms in that soil die,” says Ubbens.
So you end up with less-than-ideal topsoil spread thinly over a layer of clay hardpan that often includes pieces of brick and other debris. “In our business, we call it `builder’s loam,’” says Ubbens. “It’s unfortunate that it’s so bad that it’s even got a name.”
Planting trees in that is like sticking them in a clay pot. “We bore a hole in that heavily compacted clay, put the tree in with a certain amount of soil, but the tree will eventually start to decline,” says Andy Kenney, senior lecturer in urban and community forestry at the University of Toronto.