to flourish under the onset of reducing conditions are those capable of using nitrate (NO₃^-) instead of oxygen (O₂) for respiration, and the process of soil denitrification begins with nitrate (NO₃^-) being reduced to nitrite (NO₂^-).

Usable organic matter powers the reduction process, so the most severe consequences of reduced oxygen content occurs where soil organic matter is most abundant and readily available.

The table shows important changes in soil chemistry and effects on soil nutrient status that occur after waterlogging. Nitrate availability is diminished because of reduction to nitrites by the denitrification process. The levels of soluble forms of iron (Fe²⁺) and manganese (Mn²⁺) increase.

Orange-brown staining deep in the soil profile indicates soil that undergoes regular waterlogging. The staining is the

(SO₄^2-) becomes reduced to hydrogen sulfide gas (H₂S).

Hydrogen sulfide gas has a distinctive rotten-egg smell and is highly toxic to grass roots. When the gas is produced initially, it will react with soluble iron (Fe²⁺) to form the insoluble iron-sulfide precipitate that causes black layer. The proportion of total iron as Fe²⁺ in anaerobic soils is greatly increased, and has been recorded as high as 50 percent. Ferrous iron is therefore readily available to react with hydrogen sulfide gas as the sulfur-reducing bacteria produce it. The chemical reaction of this process is expressed as:

When the majority of the reduced iron has reacted with hydrogen sulfide to form insoluble iron sulfide (FeS), gas concentrations within the soil profile will increase, resulting in increased damage to turfgrass roots; 1,000 ppm of H₂S in air is known to kill bentgrass. The toxicity of FeS precipitate to turfgrass is not fully understood. However, in greenhouse tests, creeping bentgrass did not perform well in FeS-amended root zones.

Black layer can often be confused with buried organic matter such as peat. If root zones are suspected of being anaerobic with severe reducing conditions, a simple test can identify true black layer. Iron sulfide is soluble in dilute hydrochloric acid and releases the hydrogen sulfide gas when treated, producing the distinctive rotten egg smell.

A simple field test can be undertaken by placing approximately 3.4 ounces (10 milliliters) of suspect soil into a glass or plastic container and adding 3.4 ounces of dilute hydrochloric acid (1 molar HCl). Close the lid, shake and then smell. (Handle the acid with care, and dispose of it in a responsible manner. Excessive inhalation of the gas may be dangerous.)

The test can be made more quantitative by placing a piece of lead acetate-

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result of reduced iron moving in solution through the soil profile and oxidizing to form iron-oxide precipitate. This happens where soil conditions are aerobic and redox potentials are generally greater than +200 mV.

When soils with a usable source of organic matter have been waterlogged for a prolonged period, reducing conditions become so severe that sulfate