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Predicting Nitrogen Fertilizer Needs of Iowa Soils: II. A Simplified Technique for Determining Relative Nitrate Production in Soils¹

ABSTRACT

A technique for determining the amount of nitrate nitrogen released in soils upon incubation has been developed and adapted for routine use in the Iowa State College Soil Testing Laboratory. A 10-gram sample of soil, mixed with vermiculite, is placed in a filter tube and leached free of nitrate with distilled water. Suction is then applied to remove excess water. In this way, nitrate removal, as well as adjustment of moisture content to near optimum for microbial activity, is effectively accomplished with soils possessing a wide range in texture and content of organic matter. After incubation for 2 weeks in a humid chamber at 35°C., the sample is again leached, and nitrate produced during the period is determined by the phenoldisulphonic acid method. A unique feature of the method is that only one nitrate analysis is required in determining the quantity of nitrate released on incubation. Nitrate initially leached from the sample may be discarded. The mixing of vermiculite with the soil facilitates moisture control due to the high water-holding capacity of this material. Moreover, it permits free exchange of air in the sample during incubation, even though suction is applied in the initial adjustment of moisture content prior to incubation.

Comparison of nitrate released during 2 weeks using the leaching method and another technique involving incubation and extraction in bottles revealed that nitrate production was about 56% greater using the former technique. A higher coefficient of variation was found with the bottle method than with the leaching method. The greater precision obtained with the leaching procedure may be attributed to the improved method of controlling moisture and aeration. The method described has been adapted to large-scale use in the soil testing laboratory.

¹ Journal paper No. J-2453 of the Iowa Agr. Exp. Sta., Ames, Iowa. Project No. 1190. The assistance of Helen Heidel and Robert Munson in the laboratory is gratefully acknowledged. Presented before Div. IV, Soil Science Society of America, Dallas, Tex., Nov. 19, 1953.

² Professor and Assistant Professor in soils, respectively.

Received for publication December 29, 1953.