Here's an easy test to see whether you need to improve your soil. Take a round-headed shovel, place its tip on the surface of your planting bed, jump high into the air, and stomp with your feet on the blade's flat end. If you briefly lose consciousness, as well as some fillings, your soil could use some improvement.
Rock-hard soil could mean a couple of things. You could be digging in rock. Most likely, though, you're dealing with dirt that has lots of clay in it.
Clays of Many Colors
Folks in the Southeast often curse red clay, but clay comes in lots of colors--black, brown, white, and even blue. No matter the color, it is tough on plant roots. When clay is dry, it's as hard as concrete. When wet, it's as sticky as taffy. Clay drains poorly, which leads to rot, and contains little air, which suffocates roots. If you want plants to grow, you'll have to amend the soil.
What's the best way? Some people claim you can loosen clay by adding gypsum (calcium sulfate) to it. The theory is that gypsum binds clay particles together to make bigger particles, providing more space for air, water, and roots. While this works to some extent, adding gypsum alone isn't enough for most gardens. The best way to loosen and improve clay soil is by adding lots of organic matter.