Definitely my favourite game I've played from this jam so far. It's simple, but effective. The gameplay is fun as is, but it could definitely be even better. I think that having rocks taking away less from your water, while having time take away way more from your water, would create a risk reward situation. You'd be trying to use the faster sand to get through so as not to waste time, but that makes avoiding rocks harder, whereas the slower dirt makes it easy to avoid rocks but would make time a much bigger threat. This would force the player to be more strategic between caring about the two sources of damage, rather than the game in its current state, where its almost always best to use the slowest type of dirt, because rocks pose a much bigger threat than the passing of time.
Also, nutrients increasing speed makes them more of a downgrade then an upgrade, since time is so much smaller a threat compared to the rocks. Those going for high scores would avoid the nutrients, but this would make the game more boring, creating a situation where the most effective way to play the game isn't the most fun way to play. Instead, I would remove nutrients and make the speed increase as the game goes on, naturally increasing challenge.
I'm pretty sure the way this challenge escalation is handled is through the water becoming more sparse, but there's such a heavy element of randomness to their spawning that it just makes the game feel less fair as it goes on, rather than more challenging. For this reason, I think that water should spawn in a predictable pattern, for example, every 5 seconds, though its horizontal position could still be random. Again, speed could be used instead as a way to increase challenge.
This all sounds quite negative, but I took the time to write this because if you fine tuned the gameplay a bit more, with better art and sound, I could legitimately see this being a successful mobile game. I also like how it's a unique take on the theme. Cool game!