I read that too, Savbo, but I don't believe it. Wherever the tree branches spread out to, then add another half measure of length for the actual root for that side. EG if a branch stands out 6 ft from the trunk, then the main root on that side is 9 ft long, which casts serious doubts to the Kew theory. I could see roots with no food at all and sitting in heavy clay going round and round, but because the loosened area is only that big.
And if there's soil slump, it's because the person planting neglected to firm in the soil as they filled several levels, which causes the future slump.
I believe a lot has to do with the technique used and knowledge of the person doing the planting.
Mycorrhizal bacteria, an innoculant, is sold over here under the trade name of Mykes, but good garden centres will know what the local brand is. It's a natural bacteria fungus, which looks like a brown powder, that you dust directly onto roots to encourage feeder roots. The basic idea is much like the nitrogen fixing nodules you get with legume plants, which also comes as innoculant in package form for areas never planted before with beans. More and more public gardens are regularly using mycorrhiza because it gives the plants a great natural boost without the stress that fertilizers give plants, and plants are more successful and resistant to pests and diseases. It doesn't guarantee resistance, but definitely helps.
Here's more info:
http://www.bunchgrapes.com/mycorrhiza.html
