I prefer how Fertile Fields handles resource costs of terraforming, but while using e.g. Alpha Animals, the sand squids can be used to convert sand into fertile sand, which can be designated as growing zone, yet it is still inferior to actual dirt. Only problem, when converting the sand, you wont be able to further improve the soil with Fertile Fields later on.

When trained in obedience, the squids can also improve the fertile sand to a slightly better version, though still inferior to actual dirt.

I am unaware if there are more creatures in the mod that can change the terrain, but for now, I would like to request these two be added, maybe include them in the "change sand to other terrain"?, cheers, and keep up the good work :).