The Chambers Co. Agricultural Extension Agent might be able to help. Might be a good way to get some use out of your tax dollars. A lot of them are very helpful and very knowledgeable. Typically they have a degree in either animal or crop science, so they should be knowledgeable about either ruminants or crops, or both. Your local agent might even be a hunter. While your question probably isn't the type of question they get everyday, they get some unusual questions from people, and the extension agent might be able to help you. In fact, he might enjoy the challenge of finding an answer for you.
At one time the extension agents in Arkansas would send off soil samples for free soil testing to determine what kind of treatment a person's garden plot needed (i.e., whether it needed lime, what kind of fertilizer would be best, etc.). Don't know if they still do, but if you're concerned about the pH or nutritional needs of the soil under the trees, that might be a service that the extension agent could provide even if he couldn't answer your question.