Answer: I am not aware of a rule of thumb for this situation. I have seen the effect more in newly laid walks and foundations rather than older established ones. After a few years, it stops. The concrete leaches where rain washes it and where the concrete contacts damp soil, so to some extent I do not think it would be leaching tremendously underneath as long as there was a good layer of gravel beneath it so water was not pooling there. Also, by adding ample organic matter and loosening the soil deeply to counter the compaction caused by laying the concrete, I think you might ameliorate any alkalinity. However, it is easier to run the basic tests and see ahead of time than to discover after planting that the plants are really suffering due to a pH that is out of whack. Having said that, most annuals and perennials will tolerate a range of pH so it may not be all that critically important. The reason I say this is that soils in your area are probably naturally very acidic, so any residual effect might be raising the pH into the acceptable range. Again, testing really would be the way to go.
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