"Green manure," or planting a cover crop, is an excellent way to add organic matter to your soil. Any time you have soil "like cement," it is a good idea to add organic matter to break up the soil to help it drain and help your plants or lawn grow. Depending on how poor your soil is, you may also want to add compost or black dirt in addition to planting a green manure crop.
You'll want to pick a cover crop that will add organic matter to your soil. Your choices include legumes, such as vetch, clover and peas, grasses such as annual ryegrass, winter wheat, and winter rye, and buckwheat.
Basically, you'll spread the seed this spring and then till the plant under either later in the summer or fall. If you grow buckwheat, for example, you'll want to till the plants under when they start to flower but before they go to seed. You don't want the plants to go to seed, or they will self-seed and become weeds coming up where you don't want them.
For more information on cover crops, here is a good link: Green Manure Cover Crops for Minnesota.
