Growing a tree should turn nearby sand next to water into clay

When you grow a tree, any nearby sand or gravel block that is adjacent to a water source should be converted into a clay block.

This would make sense, as in the real world, "Clay minerals most commonly form by prolonged chemical weathering of silicate-bearing rocks... Chemical weathering takes place largely by acid hydrolysis due to low concentrations of carbonic acid, dissolved in rainwater or released by plant roots" (from Wikipedia).

There is an existing precedent for this type of mechanic - large spruce trees convert nearby dirt into podzol.

This mechanic would also allow clay to be renewably obtained in larger quantities than with villager gifts.