Why are waterlogged soils often considered infertile?

Waterlogged soils are often considered infertile primarily because the saturation of soil with water leads to a decrease in oxygen availability. In healthy soil, oxygen is crucial for various biological processes, especially for nitrifying bacteria that convert ammonium into nitrite and then into nitrate, which are essential forms of nitrogen that plants can absorb and use for growth. When soils are waterlogged, the pores that would normally be filled with air become filled with water, creating an anaerobic environment. Without sufficient oxygen, these nitrifying bacteria cannot function effectively, resulting in reduced nitrogen availability for plants. This limitation ultimately affects plant health and crop yields, leading to the classification of waterlogged soils as infertile.

The other options highlight conditions that can indeed impact soil productivity but do not directly relate to the primary mechanism of nitrogen availability being hindered in waterlogged conditions.