Which nitrogen form would result in the largest drop in soil pH?

Anhydrous ammonia results in the largest drop in soil pH primarily due to its conversion to ammonium in the soil. When anhydrous ammonia is applied, it reacts with water to form ammonium ions, which can then be converted by soil bacteria into nitrate through nitrification. This nitrification process releases hydrogen ions, which significantly contributes to a decrease in soil pH.

The extent of pH drop is particularly pronounced with anhydrous ammonia because it does not contain any calcium or alkaline materials that could buffer the acidity. In contrast, other forms of nitrogen, such as ammonium nitrate and ammonium sulfate, release ammonium as well, but they also contain components which can have less impact on soil acidity compared to the pure ammonium form released from anhydrous ammonia. Calcium nitrate contains calcium, which can help to mitigate changes in pH as it has a buffering effect.

Thus, the application of anhydrous ammonia leads to a greater accumulation of hydrogen ions through the nitrification process, causing the largest drop in soil pH among the nitrogen sources listed.