1. In the different soil types, as well as in the same soil under different rotations, the greatest number of organisms occurred at a depth of 4 inches.
2. Bacteria were found in considerable numbers at much lower depths in the loess soil than in the drift soil.
3. There was a more or less gradual decrease in numbers to a depth of 3, 5 and in one case of fifteen feet. No sudden increases were observed even where gains in moisture occurred.
4. The greatest decrease in numbers of organisms occurred within the first 12 inches and in some cases within the first 8 inches.
5. The rotation of crops increased the number of organisms beyond continuous cropping.
6. At 4 inches from the surface, the soil under the three year rotation showed larger numbers than that under any two year rotation, but at 8 inches fewer organisms than the soils under the two year rotation with clover or cowpeas turned under.
7. Rye turned under in the two-year rotation decreased the number of bacteria.
8. Fewer bacteria occurred in the soil under continuous clover than in that under continuous corn, due to the difference in treatment for the crop. Little differences were shown below 12 inches depth.
9. The soil under the four year rotation showed smaller numbers than in any of the plots except those under continuous clover and corn and the two-year rotation with rye turned under, due probably largely to the crop grown.
10. The humus content of the soils in all the plots, except two, and the nitrogen content of all the soils, decreased more or less regularly down to three feet. In the plots under the two-year rotation with clover or cowpeas turned under, there was more humus at eight than at four inches from the surface.
11. While in some cases there seemed to be some relation between numbers and the humus or nitrogen content of the soils, in general the variations observed in these latter were insufficient to account for the differences in numbers. The variations in moisture content of the soils were also insufficient to account for the results.
12. Aeration may be the governing factor, or possibly the effect of toxic substances produced in the growth of plants may be the cause of the variations in the bacterial content of the different plots.
Brown, Percy Edgar
"Bacteria at different depths in some typical Iowa soils,"
Research Bulletin (Iowa Agriculture and Home Economics Experiment Station): Vol. 1
, Article 1.
Available at: http://lib.dr.iastate.edu/researchbulletin/vol1/iss8/1