Clay soil makes it difficult for seedlings to thrive. These young plants, just starting to thrive after being hardened off and after emerging from its seed state, don't have the strength to push their tiny, weak roots through clay soil's hard, compacted form. Amend the soil well to loosen the soil and give your seedlings a chance at survival.
Till the clay soil with a rake or rototiller when it's dry—it compacts beyond being workable when wet. Till to a depth of 8 inches, working in a steady motion until the soil completely breaks up.
Sort through the soil and remove all rocks and weeds. Place in a trash bag.
Shovel 1 inch each of compost, peat moss and manure onto the soil. Till the garden area again, to a depth of 8 inches, to mix in the amendments.
Shovel 1 inch of coarse builder's sand onto the top of the soil after adding amendments. Adding sand before amending can cause the soil to change to a cement-like consistency. Till in the sand, working to a depth of 8 inches again to mix all amendments consistently.