Dirt block turns to Grass block when rained on

I am proposing that when a dirt bock is rained on for about 6 minutes, it transforms into a grass block. It is kind of annoying that the only way you can get grass to grow on something like a man-made island is that you have to move a grass block using Silk Touch and then wait for it to spread, especially if you are too early in the game to have enchantments. Any suggestions would be appreciated! (The time doesn't really matter, 6 minutes just seems like a nice medium)