I am working on a new food forest in Oregon and I have been thinking a lot about root depths. I am focusing on creating guilds with varying root depth, but cannot find a lot of root information. Is there an easy rule or observation that can be made or applied in order to guesstimate the root depth of a plant? Are the roots generally as big as the plant? Perhaps a good book that has some insight?
There is no simple rule to answer your question. For the most part, annual plants will have a smaller root structure than perennials mainly because they are only there for a season. Desert plants often have huge root systems because they need to go so far to find water. Mountain plants often have shallow roots because the soils are shallow.
Here are two books which will give you a lot of info regarding vegetable roots, and crop plant roots. John Weaver was a well respected plant researcher in the 1920's. Lots of good graphical diagrams.