I hate to say it bro but that is a myth. I have all clay, when you add sand to clay and till it you get a bunch of little pebble like clay blobs coated in sand.
Yes this myth dose holds some water but it is not impossible to add sand to make your soil amendment better.
It has some truth because of the different sized particles of sand and clay. Clay is silt like and has very small particles while sand is course and their particles are much larger. I am about to find a diagram to help me explain but if you know how metallic molecules work and know lattice structure works you know what i mean. For those who don't what happens basically is that the smaller clay particles fill the holes in the empty space of the large sand particles so it become more dense and less porous.
If you have 100% clay soil just make sure you have at least
50% sand
50% clay
that combo will work just fine at this ratio you will keep the fertile rich clay with the good drainage of sand.
When I till new clay for a new garden I add river pebbles , sand, mulch & compost. Never lets me down, you will always have to take drainage into consideration because under what you till is still all clay and will act like a cement pool. Just learn how much water the soil will pool and adjust as your plants grow.
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