Black layer, a major problem for golf courses around the world, is associated with poor turfgrass performance, and its identification and prevention are of considerable importance.

Black layer is a continuous layer or pockets of metal sulfides accumulated as a result of anaerobic respiration by sulfur-reducing bacteria, Desulfovibrio species (heterotrophic gram-negative bacteria), under severe reducing conditions brought about by prolonged anaerobic (without oxygen) soil conditions. Sulfur-reducing bacteria metabolize soil organic matter and use sulfate or elemental sulfur instead of oxygen in the respiration process to produce hydrogen sulfide (H₂S) gas. The chemical reaction between hydrogen sulfide and ferrous iron (Fe²⁺) results in the formation of the black precipitate FeS, which is often associated with organic matter and fine pore systems.

Anaerobic soil conditions normally occur when the soil becomes waterlogged and the movement of oxygen into the soil profile is severely restricted. The diffusion rate of oxygen through water is approximately 10,000 times more limited than through air. Therefore, when soils become waterlogged, diffusion of oxygen into soil pores is severely reduced and aerobic soil organisms and grass roots will

rapidly use the available oxygen needed for normal aerobic respiration.

As oxygen becomes depleted from the soil, conditions change from oxidizing to reducing. The rate at which oxygen is consumed by soil organisms depends on a number of factors, including soil microbial biomass, soil air-filled porosity, temperature, moisture, pH, and type and availability of organic matter. If oxygen is depleted from the soil for a prolonged period, the soil chemistry will change.

The reducing or oxidizing status of a soil is measured with a device known as a redox probe. Soil oxidizing conditions are measured at approximately +800 mV, and reducing conditions start to occur at approximately +400 mV, with severe reducing conditions at ­200 mV.

Before waterlogging, a soil would typically have a redox potential of +800 mV, and within 24 hours, redox potential could be down to +400 mV, indicating that soil oxygen content is greatly depleted and reducing conditions have begun to be established. At this point, normal aerobic soil respiration cannot be sustained.

Once this stage has been reached, the onset of low redox potentials is induced by microbial respiration, and microbes that can use alternatives to oxygen for respiration start to prosper. The first group of bacteria that are able

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