Soil Extractable Organic Sulfur and Sulfate Increases Under Dairy Cow Feces and Urine Patches: The Consequences for Soil Testing

The usual method to determine the sulfur (S) status of soil in a grazed pasture system is to measure extractable soil sulfate. Plants annually uptake more S than is present as soil sulfate. Additional S comes from rainfall, fertilizer, excreta, or mineralization of soil organic S. The amount of soil organic S available for mineralization is measured by the amount of extractable soil organic S in 0.02 M KH2PO4 (the ‘organic S test’). Organic S comprised 67% of total S in fresh dairy cow urine and 84% of total S in feces. Feces and urine patches from grazing dairy cows are a potential source of error in the soil testing for sulfate and organic S. Recovery of organic S from soil beneath feces and urine patches was much lower than sulfate. Our results show that the organic S soil test was little influenced by soil sampling under dung and urine patches.