In Scotland, they call it the Goodman's Croft: the little corner of unplowed land that you leave in every field.
The Goodman, of course, is the Devil. Well, we know Who that is.
A croft is a farm, especially a small one. So the Devil's half-acre is land left wild, sacrosanct. The Wild is his field, as the deer are his cattle.
Plow if you must, but leave some for the wild. It's ancient tradition and soundest ecology, both.
The custom lives on here in the secular US Midwest. You'll notice that lots of fields have one lone tree standing in them, often with a cairn beneath. In any traditional society, you'd look at this and say: field shrine.