I'm a relative new player but I played minecraft and TerrafirmaCraft enough to get some experience this type of progression. I think small patches of Chalk/Limestone in all biomes with an uncommon or rare occurrence could solve this issue (like how peat/clay spawns, but much less common and could be covered by soil). Also like how ores spawn, there could be a hint by a chalk/lime rock on it.
It could solve the "luck" issue with this.