Nitrogen degradation is primarily caused by:

The correct answer indicates that nitrogen degradation is primarily caused by thatch. Thatch is a layer of organic matter, mainly composed of dead and decaying grass clippings, roots, and other plant material, that accumulates on the surface of the soil. When thatch builds up, it can create an impenetrable barrier that limits moisture and nutrient absorption for plants. As this organic material breaks down, it can consume significant amounts of nitrogen from the soil, which can lead to nitrogen depletion.

While soil pH, diseases, and weather conditions can influence plant health and nutrient availability, their effects are more indirect compared to the specific impact that thatch has on nitrogen degradation. For example, if the thatch layer is too thick, it can create an anaerobic environment that inhibits microbial activity necessary for nitrogen mineralization. In contrast, issues related to pH or diseases might limit overall plant health but do not directly lead to nitrogen degradation in the same way that an excess of thatch would. Weather conditions may affect soil moisture and other factors but do not primarily drive the degradation of nitrogen independently.