Alot of my soil is this way and so when I build with cob I often need to buy in clay. Be careful that the local fireclay you buy at the building store is just clay and does not have any cement in it. I have the best luck buying clay from a
pottery supply house and yes it is still bags of dry powdered clay. I have the best luck when mixing the dry clay to add the clay to the water, not the other way around. SO, you put the water in a
bucket or old bathtub or what have you and add the pwdered clay on top and mix and then when that is mixed, add more, etc... You can even use a mixing rod on your drill to help with this, you can buy these mixing tools at the hardware store. Donot breath in the powder. Look for instruction for making clay slip for light
straw clay construction for tips on making a clay slip.
Then, you use this wet clay slip with your
native soil to make your cob mix instead of some or all of the water. you will need to experiment a bit for your ratio