An organic mulch, such as wood chips, bark chunks, pole peelings, straw or pine needles, applied over the soil in a thickness of 2-3 inches, and placed in a circle from the trunk to under the branch ends, is very beneficial to young trees. This will help keep the underlying soil moist, an important consideration during drought. The mulch will also inhibit weed growth and promote growth of beneficial soil fungi called mycorrhizae. Organic mulches allow water from irrigation or rain to pass through to the soil, while the lower layer of mulch, in contact with soil, breaks down slowly to improve soil texture. Organic mulches mimic forest conditions, where dropped leaves and twigs provide the same benefits. Additionally, research indicates that tree roots under mulches are denser and more extensive than tree roots not mulched or tree roots under a lawn.
Disadvantages of organic mulches are far outweighed by advantages, but may include slugs and occasional slime molds developing on mulch surfaces after rainy weather. One infrequent possibility is a nitrogen deficiency in the tree, induced by the mulch, as organic mulches tend to be high in carbon and low in nitrogen. Nitrogen deficiency is more likely when low-nitrogen materials like wood chips are tilled into the soil as a soil amendment. When the mulch is left on the soil surface, nitrogen deficiency is much less likely. Should it occur, a fertilizer like ammonium sulfate provides the needed nitrogen.
