The determination of the numbers of bacteria, actinomycetes and molds in various horizons of typical profiles of some Iowa soils was made during two seasons and the results compared with careful descriptions of the soil profiles by horizons. The Carrington loam was sampled in typical profiles several times thru the season of 1926 in Buchanan County. The Lindley sandy loam was also sampled at different dates. In 1927 the Carrington loam, the Clyde silty clay loam and the Dickinson fine sandy loam were sampled in the same county at several dates during the early part of the season. In the same season the Marshall silt loam and the Knox silt loam were sampled at several different dates in typical profiles in Crawford county.

It is apparent from the results as a whole that the content of microorganisms decreased from the surface to the lower horizons in typical soil profiles of the various soil types. The largest numbers of bacteria appeared in either the A₁ or the A₂ horizon, and the largest numbers of actinomycetes likewise appeared in one or the other of these horizons. In the case of the molds, the largest numbers sometimes occurred in the third surface horizon. In all the soils, however, the most striking decreases occurred from the A₃ to the B horizon. Further decreases occurred from the B₁ to the B₂ horizon, from the B₂ to the C₁ horizon and from the C₁ to the C₂ horizon, but these decreases were relatively smaller. In the case of the molds the decreases in the lower soil horizons were usually relatively insignificant. The total decrease in numbers of molds from the surface soils to the lowest depths was relatively smaller than in the case of the bacteria and actinomycetes.

In some instances the variations in numbers of microorganisms in the three surface, or A horizons, were relatively small. In other cases, considerable decreases occurred from the A₁ to the A₂ or from the A₂ to the A₃ horizon.

The variations in the moisture content of the different soil layers in the samples studied seemed to have no definite relationship to the numbers of bacteria, actinomycetes or molds. In some cases, decreases in numbers of microorganisms occurred between layers in which the moisture content was very much the same. Increases in numbers of microorganisms frequently occurred along with decreases in moisture. Hence, while in a number of cases decreases in numbers corresponded with decreases in moisture, apparently the decreases were occasioned largely by other conditions in the soil.

The variations in moisture content seemed to be of the least significance in the lower soil layers, the numbers decreasing without regard to the variations in moisture content. In the surface soils there was sometimes an indication of a relation between the moisture content and the numbers of bacteria and actinomycetes.

With the higher moisture content there was sometimes a higher content of bacteria. This was not uniform, however, and the general conclusion must be drawn that the variations in numbers occur with little or no regard to slight variation in moisture conditions. It should be emphasized that the differences in moisture conditions in the various soils compared were not very large and probably other factors were of greater significance. With extremes of moisture, either a high content or a low content, the effect would undoubtedly have been more significant.

There were some indications of seasonal effect upon the numbers of microorganisms in the different soils studied, especially in Series I. The largest numbers of microorganisms, in general, seemed to occur in the early spring samplings and again in the early fall samplings. Low counts ordinarily appeared in the samples late in the fall. Indications of similar seasonal effects were noted in the soils studied in Series II and Series III, altho the samplings in these two cases were not carried over the entire season. The results, in general, do seem to confirm previous observations which have indicated larger numbers of microorganisms in the June and September" samplings than in the samplings taken during the summer.

It should be noted further that variations in numbers of organisms, as affected by seasonal conditions, occurred only in the surface layers and were chiefly confined to the A₁, A₂ and A₃ horizons. No definite effect could be noted in the B or C horizon from seasonal variations. The seasonal effect appeared to be similar on the bacteria and actinomycetes, but there was little influence upon the mold content. It is quite possible that more extreme seasonal conditions than those which prevailed while these experiments were under way would have had a much larger influence on the numbers of microorganisms.

The soil type differences were reflected in some cases in the content of microorganisms in the various soil layers. Apparently the characteristic of the most significance was the content of organic matter or the color of the soil. Obviously, however, the texture of the soil was also of very large importance. Topography, the character of the subsoil and all other characteristics which serve to distinguish the soil type may play a part. The most striking relationship between the numbers of organisms present and the soil types studied was found in Series II. The Dickinson fine sandy loam, which is lighter in color than the Carrington loam and the Clyde silty clay loam, showed a lower content of microorganisms, especially bacteria and actinomycetes. Undoubtedly this was due in large part to the lower organic matter content of the soil. In Series III, the Knox silt loam was, in general, lower in numbers of bacteria and actinomycetes than the Marshall silt loam, again a difference which is probably due in large part to the variation in content of organic matter. Other factors, however, may be of quite as large significance in individual cases. Heavy-textured types, such as the Clyde silty clay loam, may actually be lower in content of microorganisms even tho they are blacker in color than soils which are not so heavy in texture. This was noted in the case of some of the comparisons between the Clyde silty clay loam and the Carrington loam. When the texture is very heavy, aeration undoubtedly is limited. The development of microorganisms may be hampered because of a lack of air, while the content of organic matter and the color of the soil may be such that a large number of microorganisms would be expected to be present.

The differences in content of microorganisms in the surface soil, as influenced by the type characteristics mayor may not continue into the lower soil layers. In general there seemed to be little relationship between the variations in conditions in the subsoil and the content of microorganisms. The coarse textured subsoil of the Dickinson fine sandy loam had a larger content of microorganisms than did the Clyde silty clay loam or the Carrington loam, which have much larger numbers in the surface soils. Larger numbers of microorganisms were noted in some cases in the lower layers of the Knox silt loam, while the surface soil was lower in content of microorganisms than the surface soil of the Marshall silt loam.

Comparing the numbers of organisms in the subsoils of the loess types, the Marshall silt loam and the Knox silt loam, there seemed to 'be many more bacteria and actinomycetes than in the case of the soils of drift origin, the Carrington loam, the Clyde silty clay loam, the Dickinson fine sandy loam and the Lindley sandy loam. The earlier studies on numbers of bacteria in the Missouri loess soils, compared with those in Wisconsin drift soils, are largely confirmed by these data. The difference is undoubtedly due to differences in the physical conditions of the subsoils of the loess types and of the drift types, which are ordinarily very heavy and even impervious in texture There is a possibility that the difference in the lime content of the drift soils may also be of significance, altho no definite evidence along this line is indicated by these results. This may be one reason for the higher content of microorganisms in the lower layers of the Knox silt loam as compared with the Marshall silt loam.

Only one comparison gives any information regarding the effect of treatment on the numbers of microorganisms in the different soil layers. In Series I it appears that the cultivation of the Carrington loam had increased the numbers of microorganisms in the different layers to a considerable extent. The soil in virgin sod was found to be much lower in numbers of microorganisms thru the A horizon, and even down into the Band C horizons. At the lower depths, however, the effect of cultivation was not shown very distinctly. It is quite probable that differences in the characteristics of the lower soil layers would have more effect upon the content of microorganisms than the treatment of the surface soil. Extremes of soil treatment, such as applications of large amounts of farm manure, would undoubtedly very materially affect the content of microorganisms in the surface layers, and this difference might extend even down into the B horizon. Probably the texture of the soil would be of particular significance in determining the rapidity with which the lower soil layers are influenced by the treatment of the surface soil.

The relationships between the numbers of bacteria and the numbers of actinomycetes and molds were usually very similar in the different soils. The numbers of actinomycetes seemed to be affected in a very similar way to the numbers of bacteria, but in many instances the numbers of molds were affected in quite the opposite direction. Large increases in numbers of bacteria and actinomycetes occurred when decreases in the number of molds were found. The mold content of the soils was very low and the differences in mold count were often hardly large enough to be significant. Large numbers of actinomycetes were found in all the soils, but the total numbers were very much smaller than the total numbers of bacteria. In general, the decreases in the lower layers from the surface layers, were relatively less in the case of the actinomycetes and much less in the case of the molds than was true of the bacteria, which confirms earlier results.

In general, the results show striking decreases in the numbers of bacteria in the lower soil layers over those present in the surface soils. Variations in the decreases occurring between different horizons were often quite large, and they generally occurred without reference to differences in moisture. There is some evidence of seasonal effects, and there is some indication of an influence from long continuous soil treatment, but the chief factor which is probably responsible for these variations is the natural difference in the physical and chemical characteristics of the lower soil layers. Probably the most important is the difference in the organic matter content, which is reflected in the color of the soil. The texture, however, is also undoubtedly of large significance, particularly when fine textured types are compared with coarse textured soils. Probably all the characteristics which serve to distinguish the individual soil types have an influence on the content of microorganisms in the various soil layers. At least in two cases definite relationships between the soil types studied and the content of microorganisms are shown.