Without pulling out my crystal ball, I would say that if you dig down or take some core samples you will find organic matter. Most likely tree roots are in that area. Even if a tree is not within 50' of the home it could still send roots out. Also large shrubs can produce some huge roots. If this was a newer home I would have bet that the builder covered over tree stumps.

You might not see any trees or shrubs if they were cut down. But their roots are still in place and rotting or decomposing away. This is why you see the oyster or shelf mushrooms growing. Roots underground can cause major subsidence when they rot away.

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Scott Patterson
Spring Hill, TN
http://www.traceinspections.com