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Determination Of Exchangeable Nitrogen In Soil


Inorganic nitrogen primarily exists in soil as the ammonium ion (NH4+) and the Nitrate ion (NO3-) and represents less than 2% of the total nitrogen in a typical soil. The bulk of the nitrogen in soils is “fixed’ to organic molecules and is not readily available to plants. To estimate the amount of nitrogen that is available for plant growth extraction procedures have been developed and the amount of nitrogen determined under the conditions of the extraction have been termed Exchangeable Nitrogen. The amount of Ammonia or Nitrate extracted is highly dependent upon the extraction solution and over the years a 2M Potassium Chloride solution has become the extraction solution of choice. Colorimetric procedures such as the indophenol blue method for ammonium ion and the cadmium reduction method for nitrate are virtually standard for testing these potassium chloride extracts because they are not subject to interferences from common soil constituents, are extremely sensitive, and are the common automated methods used for the determination of these ions in water and wastewater.

Extraction Procedure

Place 10 grams of soil sample in a 250 milliliter wide-mouth bottle and add 100 milliliters of 2M Potassium Chloride Solution (150 g KCL/L). Stopper the bottle and shake for one hour. Allow soil to settle, filter, and perform analysis on the liquid.

Method Summary and Results

Nitrate + Nitrite Nitrogen:

Nitrate Nitrogen is reduced to nitrite by a small tubular reaction coil made from cadmium metal. The nitrite formed, in addition to any nitrite originally present is determined as an azo dye at 540 nm following a reaction with sulfanilamide and N-(1-napthyl) ethylenediamine dihydrochloride. A range of 0.2 – 10 mg/kg Nitrate + Nitrite Nitrogen can be measured.

Ammonia Nitrogen:

Ammonia Nitrogen reacts with alkaline phenol and hypochlorite to form indophenol blue in an amount that is proportional to ammonia concentration. The color is intensified with sodium nitroferricyanide and the absorbance is measured at 660 nm. Precipitation by of calcium and magnesium hydroxides is prevented by the addition of EDTA. A range of 0.2 – 50 mg/kg Ammonia Nitrogen can be measured.


Precision was evaluated by repetitive analysis of standard solutions. Potential interferences will vary depending upon sample matrices that are specific to each individual sample and sampling locations. An alternative buffering system used for nitrate prevents potential carryover from the EPA ammonium chloride buffer into the ammonia test.

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