What is the primary reason for the decreased levels of sulfur on cropland in recent years?

The primary reason for the decreased levels of sulfur on cropland in recent years can be attributed to the declining sulfur dioxide emissions from coal-burning facilities. As regulations aimed at reducing air pollution have become stricter, industries have significantly cut back on emissions of sulfur dioxide, which historically deposited sulfur in the soil through atmospheric deposition. This natural source of sulfur for agricultural lands has diminished, leading to lower sulfur availability in crops and soils.

While the use of synthetic fertilizers has increased, many of these fertilizers are low in sulfur since they primarily supply nitrogen, phosphorus, and potassium. Soil erosion can contribute to nutrient loss, but it is not specifically linked to sulfur decline. The conversion of cropland to urban land is a separate issue that may impact overall agricultural productivity but does not specifically account for the decrease in sulfur levels in remaining agricultural areas. Hence, the reduction in atmospheric sulfur deposition due to stricter emissions controls is the key factor driving the decrease in sulfur levels in cropland.