THE SOIL AND SOIL FORMATION 63 



phur and phosphorus. Hydrogen and oxygen are taken 

 up through the roots of plants in the form of water, 

 while carbon is taken from the air through the leaves 

 in the form of carbonic acid gas. The atmosphere con- 

 tains only about four-hundredths of I per cent of car- 

 bonic acid gas. There is, nevertheless, a great abun- 

 dance in the air for crop-producing purposes. 



Mechanical classification of soils. Clay may be sep- 

 arated from the sand by stirring the soil with water, 

 allowing the sand to settle, then taking it out, and 

 evaporating the water from the clay which is left in sus- 

 pension. The silt, sand and other coarser particles may 

 be separated by apparatus devised for that purpose. 



The following table shows how Professor F. H. King, 

 in his book, " The Soil," classifies soils as to their me- 

 chanical make-up, showing the general proportions of 

 sand, clay and decaying vegetable matter in the several 

 classes of soil: 



Sand Clay Humus 



per cent per cent per cent 



1. Sandy soil, containing. .. 80 to 90 8 to 10 1 to 3 



2. Sandy loam, containing. , 60 to 80 10 to 25 3 to 6 



3. Loam, containing 25 to 60 60 to 25 3 to 8 



4. Clayey loam, containing. 10 to 25 60 to 80 3 to 8 



5. Clay soil, containing 8 to 15 70 to 80 3 to 6 



The so-called heavy soils are those made up too largely 

 of clay, and are described as " heavy," because they are 

 difficult to handle with the plow, cultivator or hoe. But 

 the air space being greater, these soils, bulk for bulk, are 

 lighter than the soils of coarser texture. They are soft 

 when wet, tough when partially dry, and when dried 

 they become very hard. Soils composed principally of 

 fine clay will clog on the plow, the particles adhering 

 very closely to the smoothest steel implement. Some of 

 the densest clays are called gumbo soils. They are not 

 only handled with difficulty, but they are too dense to 

 allow an excess of water to drain out ; they do not admit 

 sufficient air ; plant roots cannot readily penetrate them, 



