The ??? is some kind of lava or something. But the plant seems resistant to it.
Correct, its more of an all-purpose eraser I think, I don’t know if the plant immunity is intentional. If you look closely, a tiny bit of plant is destroyed each time you unleash it. However, you can use the plant to contain it as well.
If you pour salt on the ball it explodes. Then if you pour water on it, it rolls up the sides of the screen.
You can get a pretty long lasting cycle going by setting up rows of spouts, then seeding plants, then when the plants get thick enough, setting them on fire. The fire burns through the plants but slowly enough that they grow back in behind it.
Not quite that simple. If you put water on it it grows, and when it grows it starts bouncing around, then “jumping” up and down and moving around. (It is particularly fond of rolling up the sides of the screen.)
If you pour salt on it, it shrinks, and if it shrinks enough it will explode.
I make the little men work the salt mines or the oil rig. No beach for you!
You are a sick, sick individual. 
I love to turn off salt, sand, and oil, water to maximum, and get rid of zombies and keep the slug.
I assume that your “problem” was that the fire got into an unconnected pocket of plant, consumed it, and then there was no fire any more (but still plants).
If so, you can proof against this by turning the oil on at its maximum rate, and setting fire to the point where it comes out. This will burn perpetually, and act as a fuse for the plants in the event that the fire burns out (eventually the plants or the accumulated oil will build up to the “fuse” and the cycle will restart. The oil itself doesn’t matter much, fairly little gets through the burn zone, and it just flash-burns when the plants around it go.
I’ve been able to get one to run for hours this way (otherwise using a variant of your spouts-everywhere + plants + fire recipe).
The other, much rarer failure condition (of perpetual burn/growth) is the overly-successful uncontrolled burn, which manages to take out all of the plants, leaving no “seeds” for future growth. I haven’t figured out a trick to prevent this.