From a bacteriological study of soil plots under both continuous cropping and various crop rotations, these general conclusions were drawn:
1. The rotation of crops caused the development of greater numbers of organisms in the soil and of greater ammonifying, nitrifying, and nitrogen-fixing power by the soil, than continuous cropping either to corn or to clover.
2. Greater numbers of organisms, greater ammonifying, nitrifying, and nitrogen-fixing powers were found in a soil under a three year rotation of corn, oats, and clover, than in a soil under a two year rotation of corn and oats, or in a soil under a two-year rotation with clover, cowpeas or oats, turn en unner as green manure.
3. The use of a green manure in a two-year rotation did not always increase the number of bacteria or the ammonifying, nitrifying, or nitrogen-fixing power of the soil, and it is suggested that the explanation may be sought in the moisture factor or it may be found in the introduction of such large amounts of organic matter.
4. There was an indication that the crop present on the soil was of more importance from the bacterial standpoint than the previous cropping of the soil.
5. The ammonification of dried blood and of cottonseed meal did not always run parallel.
6. The nitrification of dried blood and of ammonium sulfate proceeded almost parallel.
7. Nitrification and ammonification proceeded in the same direction.
8. Evidence is supplied that bacterial activities and crop production are very closely related.
Brown, Percy Edgar
"Bacteriological studies of field soils, II. The effects of continuous cropping and various rotations,"
Research Bulletin (Iowa Agriculture and Home Economics Experiment Station): Vol. 1
, Article 1.
Available at: http://lib.dr.iastate.edu/researchbulletin/vol1/iss6/1