The benefits of WOOD ASH in the garden. Unlike the decomposed remains of leaves, stems and other green plant parts, burned wood doesn’t contain nitrogen. But it does provide phosphorous, potassium, calcium, boron and other elements that growing plants need. It’s also very alkaline and useful for raising the pH in gardens. You’ll need about twice as much of it as lime, but it will supply nutrients at the same time, and if you’re a wood-burner it’s free.