To test your soil, squeeze a handful of it into a ball. If you can break the ball easily by poking it with your index finger, your soil is dry enough to be worked. When you've thoroughly worked the soil, it should be loose, friable and free of clumps. If you're breaking up lawn or areas that have been in sod, make sure that the clumps of grass are turned over so that the roots are facing the sun. This will help prevent the unwanted grass from making a reappearance in the garden.
Organic matter serves as a glue, holding sandy soil particles together.
In clay soils, it wedges in between soil particles, loosening or lightening the soil, allowing water and air to reach plant roots.