Black layer is a symptom associated with anaerobic conditions in the soil, characterized by the formation of black layers or pockets within the soil profile. These layers are composed of metal sulfides, primarily ferrous sulfide, indicating the presence of sulfur-reducing bacteria such as Desulfovibrio spp. When oxygen is lacking, these bacteria convert elemental sulfur into hydrogen sulfide gas, which is toxic to turfgrass roots and contributes to turf decline.
Causal agents
Black layer is associated with anaerobic conditions in the soil. Sulfur-reducing bacteria are involved in the conversion of elemental sulfur to hydrogen sulfide gas. The primary causal agent is Desulfovibrio spp.
Primary Hosts
Black layer often occurs in artificially constructed sites with high sand content, such as golf course putting greens. Particularly susceptible areas are those where incompatible soils are layered and water movement through the profile is restricted. However, black layer can be found anywhere anaerobic conditions are present.
Disease Cycle
Black layer forms when oxygen levels in the soil are low, creating anaerobic conditions. Sulfur-reducing bacteria convert elemental sulfur to toxic hydrogen sulfide gas.
Epidemiology
Black layer can occur in both native soils and sites with high sand content, such as golf course putting greens. It is more commonly found in the upper 10 cm of the soil profile.
Symptoms
Turf affected by black layer exhibits symptoms of yellowing, bronzing, or wilting. Thinning and eventual death of turfgrass may occur. Symptoms are more severe in areas subjected to additional stresses like shade, poor air movement, or mechanical wear.
Credit: Peter Dernoeden/University of Maryland
Figure 15.4.1: (a) Symptoms of black layer generally include thinning turf in irregular patterns. (b&c) However, identification of the problem can be found beneath the surface where a physical black layer will appear in the soil or (d) around the plant itself. A good diagnostic feature is to smell the soil. If it smells like sulfur or rotten eggs then it’s a good indication of anaerobic conditions.
Signs
Black layers or pockets, composed of metal sulfides, can be observed within the soil profile. Dark deposits of metal sulfides may be visible on belowground tissues.
Management
Black layer is usually an issue caused by the physical properties of the soil. Fertilizer sources can also contribute by exacerbating the problem. Correction requires improving drainage, reducing organic matter in the soil, and evaluating the fertilizer being used.
Cultural
Maintain proper soil aeration through design, construction, and maintenance practices. Provide surface and subsurface drainage to minimize waterlogging. Ensure uniform irrigation coverage. Implement routine aerification and sand topdressing to reduce organic matter accumulation and increase soil porosity.
Chemical
Avoid excessive application of elemental sulfur, sulfate fertilizers, or organic forms of nitrogen. Consider spoon-feeding with nitrate forms of nitrogen to temporarily increase soil redox potential.