PLANT NUTRITION 251 



Tlic composition of crop-producing soils may vary widely from 

 the average gi\en above. Thus American agricultural soils have been 

 found to contain as low as 44 per cent and as high as 97 per cent 

 silica. Aluminum oxide has been found to vary from below 2 per cent 

 to 27 per cent. All soils contain small quantities of many elements 

 not listed above, including manganese, phosphorus, and sulfur. 



The composition of soils is usually expressed in terms of the oxides 

 of the elements. However, the assumption should not be made that 

 all elements exist as their oxides in soil. The only common oxide 

 present in large amounts is the oxide of silicon, SiO^. Most elements 

 are present as parts of complex compounds such as iron or aluminum 

 silicates. 



A study of soils and their composition has shown that there are four 

 main groups of constituents important in determining the nature of a 

 soil. The predominance of one or more of these four constituents 

 will establish certain characteristics of a soil. Organic matter is one 

 important constituent; the other three are groups of inorganic com- 

 pounds: 



1. Compounds of silicon, usually called silica. 



2. Compounds of alkali and alkaline-earth metals. The important 

 elements are Na, K, Mg, and Ca. 



3. Compounds of iron and aluminum, referred to as the sesquioxides 

 of iron and alumininn, for example, FcoOg and AI2O3. 



Soils high in silica but low in the other two groups of inorganic 

 constituents are usually acid. Such acid soils occur in cool, humid 

 climates and are generally open-textured, sandy, gravelly, or stony. 

 These cold-climate soils may contain large amounts of organic matter, 

 often only semidecomposed. The soils of cold and moist climates, 

 called "gray earths," "forest soils," or "podzols," belong in the high- 

 silica group. 



Arid climates, whether hot or cold, allow the accumulation of alkali 

 and alkaline-earth compounds in soils. If evaporation of water equals 

 or exceeds rainfall, there is little percolation of water through the 

 soils and very little leaching of soluble compounds. Under these con- 

 ditions alkali and alkaline-earth metals accumulate as carbonates and 

 other basic compounds. As a rule such alkaline soils are low in organic 

 matter. Irrigation of many arid-climate soils results in the production 

 of crops with very large yields. 



Under cool, semiarid conditions organic matter and calcium ac- 

 cumulate, but the alkali metals, sodium and potassium, are not present 

 in large amounts as basic compounds. Such soils, found in the wheat- 

 growing areas of western United States and Canada, are among the 



