Step 1: Picture what waterlogged soil looks like.
When the water table rises too close to the surface, the pore spaces in the root zone that would normally hold air get filled with water instead.
Step 2: Think about what roots need to function.
Plant roots need oxygen for respiration, and with the pores flooded there is simply no room left for air to reach them.
Step 3: Connect this to yield.
Starved of oxygen, roots cannot respire properly, which weakens plant growth and lowers crop yield, more directly than salinity buildup, erosion, or rainfall alone would in this scenario.
\[ \boxed{\text{Oxygen deficiency in roots}} \]