Loamy soil seems to work slightly different in generation IV than it did in Generation III, based on what I've seen in the game, The soil changes color to indicate how moist it is, in three phases. A dark brown for spots where berries have just been planted or freshly watered soil, An orange color for soil that has dried a little, and pale, sand-like soil when it dries out completely. The rate the soil dries at, as far as I can tell, is twice the rate the plant grows a step, meaning berries need to be watered twice as much for the maximum yield, which also seems to have increased.
Also the drying seems based on when the plant was last watered, rather than being in synch with growth.
I'll try and test some of this tomorrow. Kendai