If you have clay soil you need to loosen it to have any success at all with gardening. Bark mulch is a good way to do this. If you are working on a large area, it is recommended that you use a garden tiller as digging clay soil can be a back breaking job. You can rent one at your local home and garden store for a small fee.
Prepare Your Soil
Clay soil is very heavy and dense. Watering and digging most times will only result in it compacting again and killing the roots on any plants or grass you try to grow. Before adding anything to the soil it needs to be loosened. Using either a tiller or doing it by hand, you will need to dig to a depth of at least 8 inches and turn the soil over of the entire area you are working. Once it is dug up, use a hoe to break up any dirt clods until the area is relatively smooth. Clay soil will seem heavy and wet even if it hasn't been rained upon for sometime.
Once the soil is turned over, you will need to start adding your amendments. If you are adding only mulch you should add at least a depth of 6 inches of mulch and turn the soil over again to mix it well. You can then add any plants, shrubs or flowers you want to plant. It is also a good idea to add mulch to the top of the flower bed or planting area to a depth of at least 4 inches. Each year, work this top mulch into the soil to further amend it, and then put another 4 inch layer on top. It can take several years to get a truly good planting area with rich, dark soil; this won't be a one time job.
Adding Other Amendments
If you wish to assist the mulch in its job and not have to amend the soil as often, you can also add about 3 inches of peat moss, 2 inches of manure and an inch of sand with the mulch. Broadcast lime over the entire area in Spring and again in Fall to further loosen the soil.
You should also consider having a compost heap to create rich dark compost to add to your mulch to loosen the soil more. Compost is free since you create it yourself with grass clippings, vegetable waste and other things you would normally discard. It is also earth friendly and helps you avoid having to use chemical fertilizers.
Clay soil can take several seasons to loosen enough to create a good planting base. It will compact easily. Don't be discouraged, it will be well worth your time for the success you will have in growing plants and grass and also for the drainage advantages that loose soil in good condition will give you.