There's more to it than just large amounts of biomass to the creation of fertile soil because there's multiple factors that determine what will happen to that biomass and how it will affect the soil.
We can take the temperature, precipitation, and gradient of fall as examples.
The temperature is very important, if its high the decomposition of the biomass will be quick, and if it's very low the decomposition may take years to be completed and the soil to get the benefits of it. But, even if the temperature is high, if there isn't enough precipitation, than the decomposition will not have any real effect on the soil and the soil will be sandy.
The precipitation is very important because it helps in the decomposition and for getting the nutrients deeper into the soil, so the bigger the amount of it, the better. But if the temperature is relatively low, than it will be a pretty slow process, and the soil will not get the full benefit of it.
The gradient of the fall, as a physical feature is very important because if the gradient is too big, than the biomass will gradually just get downwards with any wind and rain, and the slopes will not see, pretty much, anything from it. On the other hand, if the gradient of the fall is very low, than the biomass will be more stable on the ground, and its movement will be restricted, so the soil will benefit more, depending of course on the other factors as well.
