Mulch is the most cost effective and efficient long-term fertilizer program. It is the preferred method to deliver nutrients and improve soil. If naturally occurring leaf- litter is routinely removed through normal maintenance procedures, organic mulch should be applied on top of the soil within the drip-line of new and existing trees to improve the health of the soil and tree over time. Mulch reduces water loss from evaporation, minimizes weed competition, improves soil structure, provides nutrients for plant roots, and protects tree trunks from damage associated with mechanical devices like lawn mowers or weed-eaters. Mulch also insulates the soil from extreme temperature fluctuations endemic to southern California.
If properly applied and maintained over time, a layer of organic mulch is more beneficial than routine fertilizer or soil amendment programs. In frequently used areas, mulch protects the soil from compaction, particularly if it contains larger size pieces of organic material such as tree bark. Soil compaction from foot or vehicle traffic not only ruins soil structure and future root development, but it can physically damage or kill existing tree roots.
Debris that falls from trees sometimes requires clean-up based on safety, health, or aesthetic concerns; organic material otherwise improves the soil and should be maintained under a tree, preferably covering the entire critical/root zone (the area immediately under the canopy).
Vegetation should not be planted or allowed to grow under a tree, if possible; this includes turf grass and color installations (flowers). An approximate two-inch layer of organic material (naturally occurring leaf-litter or mulch) should cover the soil and be maintained as needed.
To learn more about specific Mulching techniques, you may want to look over this additional resource provided by the International Society of Arboriculture.