Alpha-alpha-Dipyridyl solution is used to confirm the presence of ferrous (Fe++) iron in soils. It is used to indicate reducing conditions and the possibility of aquic conditions. Spraying several drops on freshly exposed, uncontaminated soil material will develop a bright pink (or red) color within a few seconds if Fe++ is present at adequate concentration levels. This reaction indicates the soil is reduced and anaerobic at the time of application at least in the area where the reaction occurred. Soil material must be at least moist (usually saturated) for positive reactions to occur.
A negative reaction to alpha-alpha-Dipyridyl indicates the soil contains no apparent Fe++. This may be because: 1) the soil is aerobic and all Fe++ exists as Fe+++; 2) the soil is anaerobic but Fe+++ have not been reduced to Fe++; or 3) the soil is anaerobic and contains no reducible Fe.
Some soils (sands, coarse sands, organic soils, etc.) may be so low in iron that Fe++ is not present in adequate concentration levels to react with the dye.
Duration of reduction/anaerobiosis is not determined by one positive reaction but by a series of positive reactions made at weekly or more frequent intervals.
Due to traffic pans that restrict water movement through the soil, cropped and pastured areas may exhibit brief periods of reducing conditions near the surface following heavy precipitation.
Alpha-alpha-Dipyridyl solution has a short shelf life (if not properly maintained) and should be stored in amber or opaque bottles, preferably under refrigeration and away from direct light. The solution can be tested by adding a few drops to a few grains of ferrous ammonia sulfate; a bright pink (or red) color develops within a few seconds. The solution may be tested in field conditions by placing a few drops on the shiny portion of field equipment such as shovels and spades; a bright pink (or red) color develops within a few seconds.
False positive reactions can occur if the soil is contaminated with metal particles containing iron that came from shovels, knives, or hands or if the solution is pink (or red) not colorless in the bottle.
Positive reactions to the dye are expected in the soil where roots grow and may never occur in the soil where roots are not found. In soils where roots are concentrated along cracks when moist to dry and former cracks when saturated or nearly saturated (such as in some Vertisols), a positive reaction is considered to be the dominant condition if it occurs across 60% or more of the former crack surfaces.
For some soils high in organic matter (Histosols and soils with mollic or umbric epipedons), the recommended test procedure is to place a small amount of soil in a small vial that contains the indicator solution and a small amount of table salt; then shake. The presence of Fe++ is confirmed if a bright pink (or red) color develops within a few seconds. The sodium in the table salt moves the ferrous iron from the soil exchange complex into solution.
U.S. Department of Agriculture, Natural Resources Conservation Service. 1998. Field book for describing and sampling soils. Compiled by P.J. Schoeneberger, D.A. Wysocki, E.C. Benham, and W.D. Broderson. National Soil Survey Center, Lincoln, NE.
U.S. Department of Agriculture, Soil Survey Staff. 1999. Soil taxonomy: a basic system of soil classification for making and interpreting soil surveys. USDA Agricultural Handbook 436. US Govt. Printing Off. Washington, DC.
Vepraskas, M. J. 1994. Redoximorphic Features for Identifying Aquic Conditions. Tech. Bulletin 301. North Carolina Ag. Research Service, North Carolina State Univ., Raleigh, North Carolina.