Iron-Cemented Layers Form at Soil Eh and pH Boundaries.

Iron-cemented layers have been documented in the putting green soils of over 30 golf courses across the USA, but their formation is still poorly understood. The layers can form at textural boundaries in less than five years and lead to decreased water infiltration rates, saturated soils, and a decline in turfgrass density. The objective of this study was to elucidate how boundaries of soil pH and redox potential (Eh) influence iron redox chemistry and solubility. Putting green soils were constructed inside columns to meet the recommendations of the United States Golf Association. The study was a 2x2x3 factorial design with two sand sources (low or high pH), two gravel sources (low or high pH), and three rates of ferrous sulfate (0, 3, or 50 kg ha^-1) applied weekly. In columns with low pH sand and high pH gravel, iron-oxide accumulation was visible after just two iron applications at the high rate. Layers formed more slowly in soils with low pH sand and low pH gravel, where an Eh boundary existed at the interface. In soils with high pH sand, iron accumulated homogenously in the top 10 cm of the profile, and layers never formed at the interface of sand and gravel. The results of this study suggest that high pH gravel layers increase the likelihood of layer formation due to decreased iron solubility in high pH environments. When low pH sand overlies high pH gravel, iron remains soluble until it reaches the gravel layer, where it precipitates and forms a thin pan layer. Layers can still form in soils without pH boundaries, but the rate of formation is likely slower. This research will help inform putting green construction recommendations that consider soil chemical properties in addition to physical properties.