Verdure: Sulfur-reducing bacteria blues

Black layer is a physical accumulation of metal sulfides within the pore spaces of soil particles. How do they form?


Black layer sample sitting on top of healthy turfgrass

“Black layer” is the term used to describe the visual appearance of a dark-colored layer or banding in the soil profile of putting greens. Black layer was also called “the black plague,” “the black death” and “anaerobiosis” (meaning life in the absence of oxygen). When black layer forms within the critical upper root zone of putting greens, turfgrass root growth and function is negatively affected, and water infiltration is compromised. The result is damaged turf and poor surface playing conditions, which will cause golf course superintendents to sing the blues.

If black layer formation is associated with water-logged root zones and anaerobic (no oxygen) root zones, how does this happen? William “Lee” Berndt, Ph.D., (1957-2024) was one of the first turfgrass scientists to extensively investigate the nature, cause and management of black layer in putting green root zones. Let’s take a look at black layer and Berndt’s research into this topic.

Black layer is typically defined as a physical accumulation of metal sulfides (for example, iron sulfide) within the pore spaces of soil particles. But how do metal sulfides form? Let’s start with the environment. A prolonged wet and saturated root zone results in soil that contains little or no oxygen. Thus, sulfur-reducing bacteria (SRB) grow and thrive in that wet and anaerobic environment. Under those conditions and circumstances, organic matter and sulfur present in the soil are broken down by SRB to produce hydrogen sulfide gas as a byproduct of their activity.

Those SRB essentially “breathe-in” sulfur-like soil microorganisms like humans utilize oxygen, but SRB “expel” hydrogen sulfide. The hydrogen sulfide is responsible for that rotten egg smell often associated with black layer. Next, the hydrogen sulfide (H2S) reacts with iron (Fe2+) present in the soil to produce iron sulfide (FeS), and that visual dark band or distinct darkened layer is formed and now visible. In agricultural soils, seasonal high-water tables contribute to anaerobic conditions and SRB making their own version of black layer called soil mottling.

Berndt demonstrated that hydrogen sulfide is in fact of a biological origin. Creeping bentgrass (Agrostis stolonifera) soil cores with naturally occurring black layer were utilized in a series of controlled experiments. Radiolabeled sulfate (35SO42-) was injected into those cores so that it could be detected and followed through the soil, and most of that sulfate was converted to sulfide. But what was converting that sulfate to sulfide? More radiolabeled sulfate was injected into those soil cores containing black layer, and this time sodium azide was also injected. Sodium azide is a soil sterilant, and therefore no sulfide was produced because the SRB were prevented from making hydrogen sulfide.

In another experiment, radioactive sulfide was again injected into black layer cores, along with sodium molybdate, which specifically inhibits SRB. Almost no sulfide was formed because the sodium molybdate prevented the SRB from producing any hydrogen sulfide.  Again, this demonstrated that within turfgrass soils, hydrogen sulfide is produced by a biological process, and therefore hydrogen sulfide is important for the formation of black layer.  

But how toxic is hydrogen sulfide to turfgrass? In a controlled study, healthy creeping bentgrass soil cores were exposed to hydrogen sulfide at 1,000 parts per million for up to seven days. At 12 hours of exposure, the turfgrass was visually stunted. Seven days later, the turfgrass was necrotic, which progressed to a dead, straw-brown color. The roots were stunted and necrotic, and the roots and thatch began to develop a blackened appearance. Metal sulfide was detected within the black layer, thus further supporting evidence that hydrogen sulfide was in fact produced there. With black layer, hydrogen sulfide also contributes to turfgrass injury and necrosis.  

So, black layer develops in turfgrass root zones when oxygen levels decline because of saturated or wet conditions, and SRB get to work making hydrogen sulfide, which then reacts to form that distinct layer. Within soil pore spaces, it is more difficult for oxygen to diffuse through water than it is through air. As the roots decline because of the harsh environment within that black layer, turfgrass health and function are seriously diminished.

Thank you, Lee Berndt, for your contributions to turfgrass science.

Source: Berndt, W.L., and J.M. Vargas Jr. 2006. Dissimilatory reduction in sulfate in black layer. HortScience 41:815-817 (

Mike Fidanza, Ph.D., is a professor of plant and soil science in the Division of Science, Berks Campus, at Pennsylvania State University in Reading, Pa. He is a 23-year member of GCSAA.