[From Garden Design magazine, June/July 1996, p. 13:] Organic mulches (bark, wood chips, pine needles, leaves, and the like)... add humus to the soil as they decompose, but microbes that perform this transformation draw nitrogen from the soil in the process. This, in turn, causes a deficiency where the mulch meets the soil. Fortunately, deep-rooted trees and shrubs are rarely affected by surface nitrogen depletion. Organic mulch suits them because most have evolved with a natural layer of decaying forest litter or meadow debris... nitrogen deficiency can be a problem with shallow-rooted herbaceous perennials. And some organic mulches -- oak leaves, for example -- release acids or volatile oils that can burn foliage growing near the ground.