Mulching your garden is one of the best things you can do for your plants and your soil. There are many forms of mulch from organic types such as wood chips to man-made plastics. But keep in mind that not all mulch products are created equal.
For trees and shrubs the type of mulch is less important than it is for vegetable gardens. Using the wrong mulch around vegetable plants can cause the plants to grow poorly and be more susceptible to diseases. Pick a mulch that has a low Carbon-to-Nitrogen (C:N) ratio. The main reason to use a mulch with a low C:N ratio is to ensure all your available nitrogen does not get tied up trying to decompose the high C materials.
In nature the first place a high C:N molecule looks for N is within the product itself. In the example of a wood chip there is not enough N available for proper microbial growth so the microbes search for sufficient N in the surrounding soil. Imagine the effect of available Nitrogen for your vegetable plants if the entire soil surface is covered with several inches of wood mulch?
Even worse, imagine if your entire soil structure was very high in carbon products such as wood chips? This would virtually tie up all the nitrogen in your soil trying to digest all the wood chips leaving very little for your plants.
Never use wood chips, bark, pine needles or sawdust. The best types of compost are those with a low C:N ratio such as straw, dried grass clippings that have not been treated or fertilized with chemicals, or home-made compost.
If your choice of mulch is going to be rotted manure compost make sure you only use manures from animals where you know what they are eating. If an animal has a diet of hay that happens to be high in salts, a common occurrence, then your compost will also be high in salts. A high salt medium will prevent or slow plant growth.
The best manure to compost is from cattle, pigs, sheep, rabbit, llama, alpaca, and goats. The worst manure to use is from horses as it is very high in weed seeds. Chicken and duck manure is good to use, but unless you have a professional soil testing kit to test the Nitrogen content it is best to stay away from any fowl manures.
Straw mulch is a great choice because it is very low in carbon and adds fantastic structure to the soil as it decomposes. In addition straw is inexpensive and a bale of straw will last most home gardeners an entire season.