New potting soil doesn't stay moist in the bag. It's subject to dry air and warm temperatures, which evaporates the moisture from the soil. Old potting soil sitting around in unused planters, pots and containers is subject to the same conditions. Dried potting soil clumps together and resists becoming moist again. Even though dry, it still has various uses.
Worn out potting soil used over and over again doesn't have many nutrients left for growing plants. Moisten both used and new dried out potting soil by adding water. The soil may be resistant and the water may run off. If that's the case, put the soil into a watertight container such as bucket or tub. Add water and let the soil sit until the water has been absorbed. Squish the soil through your fingers, working the water into the fibers of the soil.
If there is too much soil to put in a container, such as a large planter, make a trough in the middle of the planter. Fill the trough with water. It will take awhile to become absorbed. When it has, loosen the soil on the sides of the planter and refill with water. A final option is to spread ice cubes over the surface of the soil. As the ice slowly melts, it gives the soil time to absorb the water.
Use the dried out soil as filler in the bottom of large container you plan on planting. Mix with slow-release fertilizer. Potting soil is not inexpensive. A large planter can take more than a bag or so to fill. Most annuals only have roots that penetrate to a depth of 12 to 24 inches. After planting and watering, the dried out soil will become moist. By the time the plant's roots reach it, it will be able to provide nutrients from the slow-release fertilizer as well as moisture. If the planter is very deep and the dried out soil is in large clods or still in the shape of the planter it was previously used in, break it apart, put in the bottom of the container and fill in the gaps with new potting soil.
Cover a new compost pile with the dried out potting soil. Break apart any clods. As you add more compost materials, the soil will integrate with the compost. Alternate layers of potting soil with green material such as lawn clippings, plant trimmings, kitchen peelings, coffee grounds and brown material such as dried leaves.
Potting soil will loosen up clay soil so it allows water to drain and the garden soil doesn't stay soggy. On the opposite side of the coin, adding dried out potting soil to sandy soil adds materials that help sandy soil retain water.