This bulletin presents the results of studies made on the soils from a number of field plots which are located on Carrington loam and have been under definite soil treatments and cropping systems for a period of 12 years. The following conclusions seem justified:

1. The highest numbers of bacteria were found in the soils in March, 1926. In 1925 the highest numbers of bacteria were found in July and November. Great fluctuations in numbers of bacteria occurred from one sampling to another.

2. The greatest number of fungi was found in the soils in November, 1924. The numbers gradually decreased during the winter months and reached a minimum in June, 1925. Another maximum was reached in January, 1926. This increase was followed by a decrease in numbers which reached a low point in April. From then on great fluctuations occurred which can not be explained from the data secured.

3. The numbers of actinomycetes varied with the number of bacteria. An increase in bacteria was followed by an increase in numbers of actinomycetes. The relative proportion of these two groups of organisms was between 1 to 10 and 1 to 20.

4. No correlation existed between the numbers of microorganisms and the nitrate accumulation in the soils.

5. The amount of moisture varied in the soils at the different samplings, but this factor did not have any appreciable influence on the numbers of microorganisms.

6. Applications of manure alone, or of manure and lime, did not affect the numbers of fungi in this soil.

7. Superphosphate and rock phosphate, when added with lime and manure, caused a slight increase in the numbers of fungi present.

8. All of the soil treatments studied increased the numbers of bacteria in this soil. Manure and lime increased the number more than did the manure alone; manure, lime and rock phosphate increased the number more than did the manure and lime; and manure, lime and superphosphate brought about the greatest increase in numbers of bacteria.

9. The various soil treatments did not seem to affect the number of actinomycetes in this soil.

10. All of the soil treatments increased the crop yields. There was a direct correlation between the number of bacteria in the different plots and crop yields. Where superphosphate or rock phosphate was used a correlation was noted between numbers of fungi and actinomycetes and crop yields.

11. The data were not complete enough to permit of definite conclusions regarding the effect of crop rotations on the numbers of microorganisms in this soil. Studies along this line must be carried on over a long period of years.