SOIL COMPOSITION 



total potassium, total magnesium, and total calcium in the surface 

 soil of the five most extensive soil types of Illinois. 



TABLE 20.2. CERTAIN PLANT-FOOD ELEMENTS IN ILLINOIS SURFACE SOILS 

 Pounds per Acre in 2 Million of Soil (about 6 Inches) 



It should be kept in mind that these data are based upon the 

 average composition of many soil samples from every type, and 

 that these are widely representative of the most extensive and 

 important soil types in the Central states. They signify determined 

 facts. As a general average of these soils, potassium is better 

 supplied than magnesium for grain farming (in which all coarse 

 products are returned to the land), and potassium is better sup- 

 plied than calcium for a system in which all of the produce is re- 

 moved. In normal soil types the relations existing among these 

 four elements in the subsoil are not essentially different from those 

 in the surface, except in subsoils that are rich in calcium and mag- 

 nesium carbonates. Even in the almost unweathered glacial sub- 

 soils of the principal types of the late Wisconsin glaciation (brown 

 silt loam and yellow-gray silt loam ; see Table 17), 2500 pounds of 

 phosphorus and 158,000 pounds of potassium are the relative and 

 total amounts of those two elements per acre in a 2o-inch stratum. 

 Measured by the average losses from the farm by selling maximum 

 crops of corn and wheat, our two principal grains, the phosphorus 

 in 6 million pounds of this subsoil is sufficient for 173 years, or 

 one half as long as from 1565 to 1911, and the potassium is sufficient 



