Let's take a look at the effects of organic mulches in soil:

Organic mulches are derived from plant material. This mulch type will decompose in time which has two results:
one it means it must be replaced over time, and two it means that it will enrich and improve the soil. The fact that it will enrich the soil results in increased aeration of silt or clay loam soils and added water-holding capacity of sandy loam soils, what a plus!

Mulching improves and stabilizes soil structure (arrangement of soil particles) by reducing the impact of rain, weight (people walking on the soil) and cultivation of soils, especially when wet. In short, compaction of the soil is reduced, and erosion is reduced as well. Mulching with organic mulches does a better job of improving and stabilizing soil then any other mulch.

Another soil factor that is effected by organic mulches is the pH. The pH (acidity or alkalinity) of soil can be changed depending on the mulch selected. Thus, you can improve the pH of your soil, make it more or less acidic or alkaline, depending on what you need, by choosing the appropriate organic mulch. For example, most composts will be slightly alkaline and thus are excellent for use in acidic soil regions, while continuous use of oak leaves, pine needles, pine bark and sphagnum peat moss will increase acidity.

While organic mulches contain both major and minor mineral elements essential for plant growth, they should not be considered substitutes for fertilizer. They do decompose and add essential and helpful nutrients to the soil, but those should be considered "extra" not the only nutrients the soil receives.

The truth is that with certain organic mulches more than just the mulch is going to be required to obtain the optimum soil conditions. Often you should use mulch along with other products to get the best results you can for your soil. For example, materials such as straw, wood chips and sawdust (which are considered organic mulches) have high carbon to nitrogen ratios; therefore, nitrogen must be added to the soil at the time of mulching to keep it balanced. The high carbon to nitrogen ratio materials are easily decomposed by microorganisms. The microorganisms require nitrogen to multiply and survive. As a guide, 1 to 2 pounds of actual nitrogen per 1,000 square feet should be applied. If additional nitrogen is not supplied, the nitrogen in the soil will be used by the microorganisms in the root zone of desired landscape plants, and nitrogen deficiency will likely occur. Thus, as you can see, it is important to know what else, if anything, you should do with your soil, what fertilizers to use, etc. when using organic mulches, as some give lots of extra benefits, and others can detract from the soil if not used properly and with the proper other products.

Mulches of the organic variety require more upkeep, but they make your plants healthier, and your yard beautiful, so look into your options, and consider your soil conditions to determine which organic mulch is best for you.