Soils in the Mahanadi Delta are less fertile than those in the Godavari Delta because of

Mahanadi delta soils are less fertile than Godavari's chiefly because annual floods scour away the fertile top soil.

Inundation by sea water is not the main cause here, as the real issue is loss of nutrient-rich topsoil.

The soil is not simply derived from a red-soil hinterland in a way that explains the gap.

Because flood erosion is the true reason, none-of-these does not apply.

Repeated flooding removes humus faster than it can rebuild.

The Godavari delta by contrast receives steadier, richer alluvial deposits.