Would maybe be better to add wet soil to the soil group
I don't know what exactly is in soil group but it seem that wet soil is already in it.
(
EDIT : soil group contain blocks a sapling can grow on (=every kind of dirts and soil))
(
RE-EDIT : It seem that there are levels in the soil group that could be used to make a configurable value in the difficulty setting table definitions, I'll look into it)
I made this change because when using easy option food would grow on dirt or dirt with grass, even dirt with dry grass, as long as it was soil at the time it was panted...
Those blocks (dirt, dirt with grass, etc) turns to soil (not wet) if hoe is used on it.
They turn to wet soil if they are near water, but if not they turn back to what they were before
Requiring
only wet soil as a neighbor to allow growth means that it
only grows near water (like wheat or cotton from minetest_game farming)