When clay soil is dry, it's almost as hard as concrete. Plant roots can't push through it. No air can get in from the surface. Most bacteria and other soil organisms that need oxygen can't breathe. But clay is important because it can change the soil chemistry.
Clays give off minerals and absorb acids.
The perfect soil for plants and soil organisms has about the same amount of sand and silt, plus a smaller amount of clay. This soil has enough large and small spaces for air and water to flow in. It also has enough clay to let it stick together and hold humus . These clumps make another size of space. Plant roots can easily grow through these spaces.