Why don't trees make the ground subside?
I am looking out of my window at two very large oak trees in my yard and I am wondering how they are able to grow so large without making the ground below them shrink. I understand that they use minerals from the Earth, but how can they use enough of these minerals to make a 20 meter tree without essentially eating up all of the dirt? The Earth around them in fact seems a little higher than the surrounding yard.
We put this question to plant scientist Howard Griffiths...
Howard - Well, it's probably more likely that if you've got a tree adjacent to your house that it might be your house that's subsiding somewhat because, if you've got a tree growing on a clay soil, they can abstract water from it and that can sometimes in a dry summer make the clay shrink and cause subsidence. So maybe it's the other way round. But to answer your question which is where to plants get their minerals from? Why don't plants by taking out the minerals out of the soil cause the soil to reduce? You have to remember that plants are still 90-95% water and the mineral content is only perhaps about 1,000th of the plant dry weight. When you burn a log you get ash out which is very light, isn't it and that's the mineral content that's left after you've burnt that log. So, in actual fact, because of the relative uptake of carbon from the atmosphere which makes the plants grow, they don't absorb very much from the soil. At the same time, they're putting roots below ground which are building up the volume and so the earthworms are busy replenishing the minerals from the ground rock below.