PRINCIPLES OF AGRICULTURAL CHEMISTRY 



phoric acid, being much poorer soils. Soil 6 is fairly productive 

 in good seasons, soil 8 is considered practically worthless, and 

 soil 10 is of inferior quality . 



Iron and Alumina. The percentage of alumina is an imperfect 

 indication of the amount of clay in the soil. Enough 

 silica seldom dissolves to satisfy the requirement for combining 

 with alumina to form kaolinite. The percentage of alumina 

 extracted is always larger. In numerous cases so little silica 

 is present as to raise a question as to the form of alumina in the 

 soil, the hydrate (Gibbsite) being almost the only possible one, 

 aside from zeolitic minerals. 



From 1.5 to 4 per cent, are ordinary percentages of ferric oxide, 

 occurring even in soils but little tinted. Ordinary ferruginous 

 loams vary from 3.5 to 7 per cent. ; highly colored red lands have 

 7 to 12 per cent, ferric oxide and occassionally 20 per cent, or 

 more. Since highly ferruginous soils rarely have a high per cent, 

 of humus, it appears that the iron acts as a carrier of oxygen, and 

 this probably favors oxidation. 



Relative Composition of Soils of Arid and Humid Regions. 

 Hilgard 1 has compiled a great number of analyses of soils of arid 

 and humid regions, made with strong acid, with the result that 

 the soils of the arid region are found to be, on an average, 

 richer in plant food and in lime, than soils of the humid region. 

 Arid soils are prevailingly calcareous, while humid soils are 

 siliceous. This may be in large part due to the fact that the con- 



PERCENTAGE COMPOSITION OF SOILS OF ARID AND HUMID REGIONS. 



1 Bulletin No. 3. U. S. Weather Bureau. 



