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Determination of the Partial Pressure of Ammonia in Soil Air¹

ABSTRACT

A method for determining partial pressures of NH₃ from 0.01 to 0.50 mm Hg in soil air was studied. Soil samples which had been treated with various amounts of ammonium hydroxide were placed in 1-liter polyethylene bottles and allowed to equilibrate. A known volume of air from the bottle was removed by partially compressing the bottle with a press. The air was bubbled through 0.1N HCl and the ammonium content of the acid determined by the Nessler reaction. The partial pressure of NH₃ (P_NH3) was calculated from the ideal gas law equation.

The method gives a reliable estimate of the partial pressure of NH₃ in soil air. It was shown that the P_NH3 of a sample placed in the polyethylene bottle could be determined after a 1-hour equilibration period.

Samples which were sealed in the polyethylene bottles for 48 hours had lower P_NH3 and higher P_CO2 values than samples placed in the bottles for only 1 hour. Reduced P_NH values in bottles which had been sealed for 1 day or more were attributed to pH reduction due to CO₂ accumulation.

Germination of corn seeds (Zea mays L.) was damaged when the initial P_NH2 value was 0.156 and final values were between 0.077 and 0.104 mm Hg. The germination of corn seeds was not reduced when initial P_NH3 values were 0.091 and final values were 0.067 and 0.048 mm Hg.

¹ Contribution from the Res. and Develop. Div. of Int. Minerals and Chem. Corp., Libertyville, Ill.

² Research Soil Chemist.

Received for publication April 4, 1967. Accepted for publication July 27, 1967.