Grassland soil and forest soil

Apparently, if an area has a lot of rainfall, it can have forest soil, but if it doesn't get as much rain that makes it a grassland soil. This is because if an area has a lot of rain, it makes very good soil and trees can grow there. It's wet enough in these areas so trees can survive. In grassland areas, there might be enough rain to produce good soil, but only enough so that grass can grow. Grass doesn't need as much water as trees, so if there is less rain trees can't grow, but grass will, making it a grassland. More water in an area means that the rocks weathers more quickly and forms more topsoil faster, which is better for growing things.