Adobe soils are soils with high clay content that swell and shrink
dramatically and bare ground commonly has cracks in it in summer.
"An adobe soil may be defined as a heavy soil which readily cracks, and
certain adobe soils have not only high wilting and hygroscopic
coefficients, but also high shrinkage coefficients. For example, some
adobe soils shrink so much on drying that the air-dry volume is only
half the volume of the moist soil: hence on drying, cracking is
inevitable." Monthly bulletin of the Department of Agriculture, State
of California: Volume 10 - Page 42, 1921. It took me about an hour to
find an actual definition of adobe soils.
Back in the 1970's when we lived in San Luis Obispo our house would
rise and fall about an inch each year. I went to put a sliding
door on the bath tub to make a shower encloser and the wall was out of
square by almost an inch. Best fig tree I have ever seen was standing
in mud for a month each winter, bone dry in summer. I trucked about 15
yards of beach sand in to make a six inch deep vegetable garden as
everything would not grow in standing water during the winter.
The carrots had hooked bottoms from hitting the adobe and deflecting
back up into the sand.
We've since learned that mulch goes a long ways to fix the adobe soils
so they will absorb water and hold it. In vegetable gardens use
compost, in native gardens use shredded redwood, oak, pine or
cedar and the soil will open up.