70 SCIENCE AND SOIL 



These analyses show the general range in composition of the 

 mineral material constituting the bulk of our most common, most 

 extensive, and most valuable soils, in the central part of the United 

 States, both north and south (along the Mississippi Valley), where, 

 as a very general rule, the surface is covered by a blanket of loess 

 two feet or more in depth. 



In the fresh condition, as in the deeper strata, loess usually 

 contains considerable amounts of calcium carbonate and more or 

 less magnesium carbonate, as is the case with the samples from 

 Galena and Vicksburg, both of which are known to represent strata 

 of considerable depth. The Dubuque sample was evidently taken 

 from the surface, and this may be the case with the Kansas City 

 sample, in both of which the carbonates have evidently been greatly 

 reduced by leaching. 



As an average, the phosphorus content amounts to 1150 pounds 

 per acre for a stratum of 6f inches (2 million pounds) , including the 

 Galena sample, which is decidedly low, and the Dubuque sample, 

 which is abnormally high. The average of nine different composite 

 samples of subsoils from different places in the deep loess areas in 

 Illinois shows mo pounds of total phosphorus in 2 million pounds 

 of loess, the extreme variation being from 740 to 1540 pounds. 

 (See Illinois Experiment Station Bulletin 123, pages 288-289.) 



While phosphorus is, in a sense, an incidental substance in loess 

 deposits, the element potassium is an important constituent of the 

 most common original minerals, and any marked variation in 

 potassium content must be accounted for largely by decomposition 

 and loss by weathering of the particles, the chief losses having 

 occurred probably before the accumulation into the present loessial 

 deposits. 



The average of the two northern samples (Galena and Dubuque) 

 shows 35,000 pounds of potassium, while the sample from the south- 

 west (Kansas City) shows 30,600 pounds, and the southern loess 

 contains only 18,000 pounds of potassium in 2 million. If loess is 

 derived chiefly from glacial drift, as viewed by the United States 

 Geological Survey (38th Monograph, page 159), then it would be 

 expected that the southern loess, transported far from glacial de- 

 posits, would be lower in potassium than the northern loess, which 

 has been less exposed to weathering. As an average of the nine 



