The correct answer is - humus.
The humus is the result of the decomposition of the biomass, the more biomass there, the bigger the humus layer will be. It has a distinct dark coloration, varying from dark brown to almost black, and this is the part of the soil that is by far the most fertile. So in this case, since the farmer noticed dark colored soil that is filled with organic material, it is a sign that the soil has a nice deep humus layer that will bring him high productivity.