250 PLANT BIOCHEMISTRY 



Most of the minerals found in rocks are already fully oxidized. 

 Exceptions include minerals containing ferrous iron, for example 

 pyrites, which can be changed to ferric compounds by oxidation. This 

 process requires oxygen and water and takes place more rapidly in 

 humid than in dry regions. Oxidation of elements such as sulfur and 

 nitrogen leads to the formation of mineral acids which have a solvent 

 action on rock constituents. 



2FeS2 + THoO + 15 O -> 2Fe(OH), + 4H.2SO4 



Rocks may be broken by the expansion of plant roots in cracks. 

 Mosses and lichens produce disintegration when they grow in intimate 

 contact with rock and mineral surfaces. Various microorganisms are 

 active in the decomposition of parent soil constituents. Animals such 

 as rodents and earthworms aid in mixing various horizons of soil and 

 in supplying fresh subsoil to surface layers. 



When sufficient soil has been created to supply a substrate for higher 

 plants, soil formation is accelerated. The roots of these plants con- 

 tribute carbon dioxide and, when they die, organic matter which in 

 turn acts as substrate for microorganisms. Products of microorganism 

 metabolism, including carbon dioxide, ammonia, and organic acids, 

 hasten the decomposition of rock material. The intimate relationship 

 between the resulting soil and the animal world is an essential feature 

 of life on this planet. 



Inorganic Matter in Soils 



Most of the chemical elements known to man may be expected to 

 exist in the soil, and a large number of them have been found to be 

 present. However, the greater part of the inorganic matter in soil is 

 composed of relatively few elements. 



The approximate average composition of soils suitable for agricul- 

 ture is as follows: 



