Determination of exchangeable nitrogen in the soil

Introduction

Inorganic nitrogen exists primarily in the soil as the ammonium ion (NH4+) and nitrate ion (NO3-) and represents less than 2% of the total nitrogen in a typical soil. Most of the nitrogen in soils is “fixed” in 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 extraction conditions has been termed exchangeable nitrogen. The amount of ammonia or nitrate extracted is highly dependent on the extraction solution, and over the years a 2M potassium chloride solution has become the preferred extraction solution. Colorimetric procedures, such as the indophenol blue method for ammonium ions and the cadmium reduction method for nitrate are practically standard for testing these potassium chloride extracts because they are not subject to interference 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). Stop the bottle and shake for one hour. Allow the soil to settle, filter, and perform an analysis of the liquid.

Summary of the method and results

Nitrate + Nitrite Nitrogen:

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

Ammonia nitrogen:

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

Conclusion

Precision was assessed by repetitive analysis of standard solutions. Potential interferences will vary depending on which sample matrices are specific to each individual sample and sampling locations. An alternate buffer system used for nitrate prevents possible carryover from EPA’s ammonium chloride buffer to the ammonia test.