74 SCIENCE AND SOIL 



carbonates of calcium and magnesium, which may have con- 

 stituted 75 to 90 per cent or more of the original rock, having been 

 nearly or completely dissolved out (see Table 5). 



Two striking facts are revealed by the analyses of these ten soils 

 from ten different geological formations: 



1. The amount of phosphorus is very small compared with the 

 requirements of large crops for many years, the amount varying 

 from 720 pounds in the Gabbro soil to 1500 pounds in the Helder- 

 berg limestone soil. Counting 17 pounds of phosphorus for 100 

 bushels of corn, the 720 pounds would be sufficient for only 43 

 such crops; or, if both grain and stalks are removed from the land 

 and if one pound of phosphorus per acre is the yearly loss in drain- 

 age water, the 720 pounds is sufficient for only 30 such crops; 

 while the best soil contains sufficient total phosphorus in a 61- 

 inch stratum for only 63 such crops. The average of the ten soils 

 shows noo pounds of phosphorus in two million pounds of soil, 

 or about one half as much as in the average crust of the earth. 



2. The amount of potassium is very large, varying from 20 to 

 50 times as much as the phosphorus. The 15,400 pounds of po- 

 tassium in 6f acre inches of the poorest soil would be sufficient 

 for 100 bushels of corn every year for 800 years, while the 57,400 

 pounds in the best soil would suffice for 3000 years, if it could be 

 made available as needed and if only the grain were removed. 

 If both grain and stalks were removed, these supplies are sufficient 

 for 200 and 800 crops, respectively, counting 19 pounds of potas- 

 sium for 100 bushels of corn and 52 pounds for the stalks for such 

 a crop, not including the loss in drainage, which, however, would be 

 somewhat greater than for phosphorus. Six of these soils average 

 nearly as rich in potassium as the earth's crust, while the poorest 

 soil is about one third as rich. 



Several of these soils are less abundantly supplied with magne- 

 sium and calcium than with potassium, not only in total amounts, 

 but also in comparison with the requirements of some general farm 

 crops. In some cases the soils contain less than one third as much 

 magnesium, and less than one fifth as much calcium, as potassium; 

 while corn contains more than one third as much magnesium as 

 potassium, and clover hay contains almost as much calcium as 

 potassium, and one fourth as much magnesium (see Table 13). 



