Growth stops when the soil dries up completely, also freezing in the current growth stage, and then the plants begin to yellow and eventually die. Watering it then slowly turns it back to green and it begins growing again when it's fully green.
It's a mechanic that you really only see if you have a full field just planted on partially dry soil and then go afk for a bit. But watering your soil gives you something to do while you wait for plants to grow so you never really see it. I didn't want to punish players for not keeping the soil at max hydration since it's supposed to be more if a cathartic game where you watch YouTube videos or Netflix while lazily clicking.