I'm able to get sawdust from a lumber shop and ashes and charcoal from a restaurant. We have been piling on the sawdust and I'm careful not to overdo the ashes. There is a surprising amount of charcoal which I ground up for terra preta.
Corn seems to grow well, so we will grow several back to back crops of sweet corn to help starve out the root knot and build up more OM.
MY wife grew up on a mountain farm with those neat looking rice terraces. She said they never had disease problems. The soils there are a red clay, slick as grease when wet. The subsoil is a white silt (I thought it was silt), but actually it is a rather low fertile volcanic ash packed hard into a subsoil, what looks like a silt pan in the USA. Where it is exposed not much will grow.