newSplotchDecoration doesn't work

[somebush]

I want to create dirt patches on the surface of the Mangrove biome (Biomes O' Plenty). This will diversify the default monochromatic surface of the biome.
I wrote this script, and also included the script file in BiomeTweaker.cfg, but I did not see the result. Maybe newSplotchDecoration generates spots on the bottom of shallow bodies of water rather than on the surface of the ground?

mangroveBiome = forBiomes("biomesoplenty:mangrove")
mangroveBiome.addDecoration(splotchDec)

splotchDec = newSplotchDecoration()
splotchDec.addBaseBlock("biomesoplenty:grass")