I'm no expert but the cracks in the surface of the soil in the pictures are I beleive caused by the clay content.

Our soils bake and crack like that (we have a fair amount of clay in our topsoils) and from what I have been told by our extension folks it is because the clay particles expand when they are wet and shrink when they get dry, causing the cracks. This is what causes the clay to be able to hold water. It swells and shuts up the pores when it gets wet.

John

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