Ask Mr. Smarty Plants

We agree, the look of shredded hardwood mulch is very attractive. It is of greatest benefit in that it protects the roots of your plants from heat and cold. When you put it into your garden, it will begin to decompose "in place" and become an enriching amendment to the soil. This is why you usually need to replenish mulch about once a year or so.

We can remember some discussion a few years ago about mulches pulling nutrients from the soil, but I think that was referring to chipped wood from tree trimmers. It is slow to decompose and does need some help from the material in the soil. We have personally nurtured a compost pile of mostly fallen oak leaves, of which we had a lot, adding water and cottonseed meal for nitrogen and stirring the pile for oxidation. It took at least a year to achieve the level of composting we liked, but we certainly used it as mulch. If you don't have the room or the inclination for a compost pile, you would be better to use the composted mulch you can purchase. However, we have also put 2 to 3 inches of commercial shredded hardwood bark on gardens with good soil and not felt any need to supplement the nutrients. 

This is one of those things that is as much personal taste in gardening as anything else. Read these two articles and form your own opinion. 

Agresource Using Compost as Mulch to Increase Soil Nutrient Level, Microbacterial Activity and Plant Growth. 

Bear Path Farm Compost as Mulch