Root Depths

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.

http://www.soilandhealth.org/01aglibrary/010137veg.roots/010137toc.html
http://www.soilandhealth.org/01aglibrary/010139fieldcroproots/010139toc.html

Some of them may surprise you: a common beet grows a 2½ foot tap root.

Hope this helps.

There is also quite a bit in Kourik's book Designing and Maintaining Your Edible Landscape Naturally

To follow-up on Katy's response, Kourik also has a book called Roots Demystified, which is an entire book devoted to exactly what you're after. A very valuable resource!

I found this information on the net some time ago. I was amazed at how deep some of the root actually go. http://www.soilandhealth.org/01aglibrary/010137veg.roots/010137toc.html The book was published in 1927 I believe, but I think plant roots are still about the same.