Why is the topsoil in tropical rainforests thin?

Answer 1

it is because, tropical rain forests face continuous rain fall, which leaches away the nutrients, along with the top soil. but still it has a dense vegetation, how? its simple it has a lot of trees and animals, so the remains of the waste materials, settles down and breaks down to release nutrients, so the cycle begins. the soil also has a lot of carbon in it, [as the whole place is filled with carbondioxide] it speeds up the reaction, so the dead remains quickly break down and fulfill the needs of the lush vegetation.