Mulch is anything that protects the soil.  Mulch is created in nature by trees and plants.  As trees and plants shed their leaves or die the dead organic material that is left behind slowly rots away and enriches the soil.  Some trees shed their leaves every year.  This creates a weed barrier and helps the soil to conserve moisture.  The weed barrier cuts down on competition from other plants and keeps seeds and other plants from moving in and competing with the tree for nutrients.  Leaves make fantastic mulch but can create an almost impenetrable layer of on the soil. 

Gardeners realized the problem and came up with a solution.  Shredding the leaves still retains moisture but is much easier to work with.  Using shredded leaves instead of whole leaves gives the gardener more control and a faster decay.  Having mulch decay too quickly can be a problem in itself.  Store bought mulch can be expensive and having to replace it too often can cause you to spend your gardening budget on mulch instead of plants.

The response to this was to simply use the bark and other waste from tree harvesting and processing as mulch.  This mulch can last for several years instead of just one.  The funny thing about mulch is that when it decays it becomes compost.  No gardener can have enough compost in the garden and this is a nice by-product.  Some people simply use compost as mulch.  This isn’t the most common practice because compost in a precious commodity in the gardening world.  Trying to come up with enough compost to use as mulch can be difficult.  Compost is more likely to be used as a soil amendment or as a top dressing for the garden or lawn.

The next evolution in mulch was the green movement.  Environmentalists were concerned about folks chopping up trees willy-nilly to use as mulch.  The response to this was to use Cocoa bean hulls.  The problem with cocoa bean hull mulch is that it is expensive and decays in a season.  That’s funny, so did the leaves only they were free.

I’ve only mentioned a few of the different types of mulch at this point.  Wood and wood products are only a portion of the available mulches.  There are many inorganic mulching materials available for people who want their mulch to last a real long time.