1. Data concerning the phosphorus, nitrogen and carbon content of Iowa soils, as published in the various Iowa Soil Survey reports, have been assembled, summarized and analyzed statistically for the purpose of characterizing the various soil types for the state as a whole.

2. The mean phosphorus, nitrogen and carbon content of each soil type is recorded, and the variability within the types is pointed out.

3. In spite of the variability within soil types, the differences among types, in most cases, were found to be significant or highly significant. Certain soil types were found to be similar in their phosphorus, nitrogen and carbon content, but significantly different from other soils.

4. In general, surface soils contained larger quantities of phosphorus, nitrogen and carbon than subsoils.

5. Dark colored loess soils contained larger quantities of phosphorus, nitrogen and carbon than light colored loess soils.

6. Fine textured soil types contained larger quantities of phosphorus, nitrogen and carbon than the coarser textured types of the same series or of different series. A high or low phosphorus, nitrogen or carbon content, however, seems to be a series characteristic entirely apart from the textural influence. This appears to be related to the topography and native vegetation factors.

7. The loess soils as a group do not differ from the drift soils either in their phosphorus, nitrogen or carbon content nor in their carbon-nitrogen ratio. The bottomland soils, however, contain significantly larger amounts of phosphorus, nitrogen and carbon than the terrace soils, but the two groups do not differ in their carbon-nitrogen ratio. The terrace and bottomland soils as a group contain significantly larger quantities of phosphorus and nitrogen than the loess and drift soils, but the difference in carbon is hardly large enough to be significant. There is little, if any, difference in the carbon-nitrogen ratio.

8. The mean carbon-nitrogen ratio for all soils of Iowa vvas found to be 12.15:1, but the largest number of soils had a ratio slightly lower, the mode of the frequency distribution curve being at 12:1.

9. The close relation between the nitrogen and carbon content of Iowa soils is shown by the high correlation coefficients, which were found to be 0.95 for the drift soils and 0.93 for the loess soils.

10. Although the temperature and humidity factors are of primary importance in determining the nitrogen and carbon content of soils, it appears that within the comparatively narrow range of variation of these factors within the state of Iowa, the factors of topography, soil texture and type of vegetation have been of greatest importance in the differentiation of soil types.

11. In general, the results of this study support the soil type concept now in use in soil classification.