Research Bulletin (Iowa Agriculture and Home Economics Experiment Station)


Preliminary work with solution cultures inoculated with soil showed that 20.83 mgm. of nitrate nitrogen disappeared in eight days, while the total nitrogen increased. The results obtained with soil cultures were very similar to those obtained with the solution cultures. Treating the soil with straw in the greenhouse brought about a complete utilization of the nitrates, while there was a small accumulation of nitrate in the soil treated with farm manure.

The nitrate-assimilating power of the soil was influenced by a number of factors, and, altho much information was secured by the methods employed, more work on the various factors affecting nitrate assimilation in soils is desirable. It was found that dextrose increased the nitrate-assimilating power of the soil. This material furnished a source of energy for those organisms which utilized both nitrate and ammonia, thus preventing an accumulation of nitrate. Since the nitrate content of the soil at any given time is the resultant of many different forces, some treatment which would eliminate all but nitrate assimilation would facilitate the accurate measurement of this process. The addition of a readily available energy source, especially one poor in nitrogen, tends to bring about this condition.

Straw, partially decomposed straw and farm manure brought about an assimilation of nitrate immediately upon being added to the soil. The straw brought about a larger assimilation of nitrate than the straw manure or the farm manure. Manure, lime and rock phosphate applied over a period of years on Marshall silt loam and Grundy silt loam tended, in general, to lower: the nitrate-assimilating power of the soil. The nitrate-assimilating power of Carrington loam treated in the field with different manures varied thruout the season, showing only slight differences between the different treatments. Very similar results were secured with manure, lime and crop residues on both continuous corn and corn in the three-year rotation.

Five cultures of nitrate-assimilating and nine cultures of denitrifying bacteria were isolated and studied in pure culture. No names have been suggested for these organisms and their systematic relationships have not been determined. A number of nitrate-assimilating and denitrifying tests were carried out with some of the organisms in pure culture.

Nitrate assimilation is common to a large number of soil microorganisms. It has been suggested, however, that the molds are largely responsible for nitrate assimilation when the energy supply of the soil is increased. The results obtained in these experiments indicate that certain molds may be less efficient nitrate assimilators than some bacteria.



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