Loamy soil is actually the best for your plants, such as Arbor Day Nursery's Garden Mix. Heavy clay is not good and it needs to be broken up to allow the roots of your plants to move and grow. Here are the best ways to combat the heaviness of clay:

1. Mix in and replace some of the soil with good compost or mixed soils
2. Add gypsum, minerals and organic materials to soil when planting and fertilizing 2-3 times every year. Gypsum aids in breaking down the clay so the roots can grow, while the fertilizer gives nutrients to the soil for the plants to grow. A product Arbor Day carries called Sledge Hammer will also help break down the heavy clay.
3. Use mulch and organic materials to cover the top layer of soil, keeping moisture in. In time, the organic materials will absorb into the soil and help mix in with the clay.