Whether the original source was grass clippings, sawdust, animal manure, or vegetable scraps from your kitchen, all organic matter eventually becomes compost. Whether you make your own compost or buy it ready-made, you can add finished compost any time to the garden or around plants.

Why not just add raw organic matter to your garden instead of composting it? Be composting the materials first, the final product is uniform in color, nutrients and texture; is odor free; and contains far fewer viable weed seeds and potential disease organisms (depending on how it was composted). Your plants will be happy with you for treating them so well.

As gardeners have become more aware of the value of compost, more sources of it have become available. You can buy compost from a number of sources either in bulk (back the truck right up) or bagged, depending on where you live.

Bagged compost is obviously the easiest way to buy it, especially if you have a small yard or container garden. The downside, however, is that you don't really know the quality of the compost until you get it home.

For larger quantities of compost, buy it in bulk. The price is less per quantity and you can check the quality of the compost as well. Many private companies, municipalities and community groups make and see compost. Often they will even deliver the compost to your yard for a fee.

Here are a few tips to evaluate the quality of bulk compost.

Consider the source. Before buying the compost, ask about the primary organic matter sources that were used to make it. Compost made from yard waste (leaves and grass clippings) is usually considered the safest and best. Other compost may contain ingredients that had contaminants, such as herbicides from agricultural crop residues and heavy metals from municipal wastes, which may affect the growth of your plants or accumulate toxins in your soil.

Look at the color and texture. Finished compost should look dark and have a crumbly texture without any large pieces of undecomposed organic matter, such as branched or pieces of wood.

Squeeze it. If water oozes out when you squeeze a handful of the material, it's too wet.; if it blows away easily, it's too dry.

Give it a whiff. The smell should be earthy without a strong ammonia or sour smell.

Private and university laboratories can test compost for organic matter content, maturity and impurities. An organic matter content test indicates whether you're buying compost or soil mixed with compost. Good quality compost should be made of at least 40 percent organic matter. A compost maturity test also tells the rate of decomposition of the compost. If the rate is high, the material may not be finished decomposing. The ideal compost is slightly warm and "approaching maturity". The laboratories can also test your garden soil for organic matter and make recommendations about how much more you need to add.