Over the past 25 years the traditional macronutrient trio, nitrogen (N), phosphorus (P) and potassium (K) has slowly begun to share its NPK analysis shorthand with what has become a fourth “major” crop nutrient — sulfur (S)

While still relegated to many university “micronutrient” lists, sulfur has taken an important part of fertilizer prescriptions across the nation for many agricultural crops since 2000 as atmospheric sulfur dioxide gas (SO2) has been significantly reduced by regulations eliminating the compound from the exhaust stream of heavy industry and transportation. (See Figure 1)

Today, industrial scrubbers on coal and natural gas-fired power plants collect elemental sulfur and divert it for industrial use, as well as similar systems in the refining industry which remove sulfur from motor fuels.

The result is tons of sulfur that once returned to earth’s soils in precipitation are now sequestered, leaving farm fields increasingly shy of the nutrient. (The changes brought about by clean air standards set by the EPA also substantially reduced the acid-rain phenomenon which was effectively lowering the pH of many lakes and streams.) 

The shift in atmospheric sulfur supply is largely regulatory, but The Sulphur Institute says field deficiencies also result from more sulfur being removed from topsoil through high-yielding, fertilizer-centric farming operations, overall intensified crop production and high-yielding crop varieties, along with improved irrigation management. 

Adding to the “S drain” is the use of increasing proportions of high-analysis, sulfur-free fertilizers such as urea, diammonium phosphate (DAP) and potassium chloride and the decreasing use of…