Why is agriculture in grasslands more productive than agriculture in rainforests?

Soils in grasslands have a deep A horizon from the accumulation of organic material that dies back every year. This is what makes these soils so good for agriculture. In tropical rain forests the A horizon is very shallow because of rapid decomposition of dead material and leaching due to heavy rainfall. This, and the fact that cleared forest land bakes to a brick-like consistency, is why tropical soils have limited agricultural potential.

Grassland soil (mollisols) is way more fertile than rainforests (oxisols). In the rainforest there is a lot less productivity.