In the summer of 1926 certain plots on the Agronomy Farm of the Iowa Agricultural Experiment Station were sampled four times and studies made on the amount of nitrates, moisture, and the reaction of the soils at the time of sampling and the nitrifying power of the soils as determined by three methods, (a) nitrification of the soils' own nitrogen, (b) nitrification of ammonium sulfate and (c) nitrification of ammonium sulfate in the presence of calcium carbonate. The reaction of the soils was also determined after the 30 days of incubation. The following conclusions seem to be justified from this study:
1. The amount of moisture was not affected by the crop rotations used nor by the various soil treatments. Neither did the variations in moisture content which occurred in these soils have any appreciable effect on the crop yields or on their nitrifying power.
2. The crop rotations on the Carrington loam did not seem to affect appreciably the nitrifying power of the soils from plots which were left untreated for 12 years nor those which were manured, manured and limed, or treated with crop residues and lime.
3. The application of manure alone did not increase the nitrifying power of the Carrington loam.
4. The soils from the three-year rotation corn plots showed the largest amount of nitrates present at the various times of sampling and the highest nitrifying power when this was measured by the nitrification of the soils' own nitrogen.
5. Definite correlations were obtained between the crop yields, the nitrifying power and the reaction of the soils after incubation, when the nitrifying power of the soil was tested by measuring the nitrification of ammonium sulfate. The reaction of the soils at the time of sampling also correlated with the nitrifying power of the soils as determined by this method.
6. Perfect correlations were not obtained between the nitrifying power of the soils and crop yields' when the test was made by measuring the nitrification of ammonium sulfate in the presence of CaCO3, altho the soils all showed a high nitrifying power in all cases where the crop yields were high.
7. The nitrifying power of all of the soils was shown to be greater when calcium carbonate was used with the ammonium sulfate in the nitrification test.
8. The soils from plots which had been limed in the field showed the highest nitrifying power, and hence there seemed to be a definite correlation between the reaction of the soils and their nitrifying powers.
Erdman, Lewis W. and Humfeld, Harry
"Studies on nitrification and its relation to crop production on Carrington loam under different treatment,"
Research Bulletin (Iowa Agriculture and Home Economics Experiment Station): Vol. 8
, Article 1.
Available at: https://lib.dr.iastate.edu/researchbulletin/vol8/iss110/1