252 PLANT BIOCHEMISTRY 



most fertile soils in the world. The black prairie grass soils belong 

 lo this group. 



A balanced distribution ot organic matter, sescpiioxides, alkaline- 

 earth compounds, and silica is found in humid climate soils of central 

 Europe and central-eastern United States. Soils of these regions have 

 a tendency to be slightly acid. 



South temperate, subtropical, and tropical soils in humid climates 

 are high in iron and aluminum. They contain less organic matter and 

 less silica than soils in cooler climates, and they are usually low in 

 calcium and magnesium. In such soils, called laterites, compounds 

 of iron and aluminum are dominant. In moist tropical climates 

 organic matter decomposes rapidly in the soil, producing CO2. The 

 carbonates which are formed attack silica, forming more soluble com- 

 pounds which are removed by leaching. Since alkali and alkaline-earth 

 compounds also are rapidly lost, the predominant compounds become 

 the iron and aluminum group. 



Organic Matter in Soils 



Soil organic matter is composed of the bodies of dead organisms and 

 the excretions of living organisms deposited on or in the soil. The- 

 oretically soil organic matter consists of only the non-living remains 

 of organisms and the products of their decomposition. It is impossible 

 to separate such material from the living microorganisms that inhabit 

 the soil and are the most important agents in the decomposition of 

 dead material. The bodies of living microorganisms are therefore com- 

 monly included as part of the soil organic matter. Thus our soil 

 organic matter consists of dead roots, leaves, fruits, and stems of plants; 

 carcasses of worms, insects, and animals; bacteria, fungi, and protozoa; 

 and decomposition products of dead organisms. 



Since the soil organic matter is a mixture of plant and animal 

 material, the chemical compounds present include carbohydrates, pro- 

 teins, lipides, and other compounds found in plants and animals. The 

 gradual decomposition of these compounds ultimately leads to the 

 formation of such products as water, carbon dioxide, ammonia, 

 methane, and simple inorganic salts. But before these simple end 

 products are formed, many intermediate compounds have been pro- 

 duced, some of them of less complexity than the original compounds, 

 others of greater complexity. 



The mixture of compounds resistant to decay formed during the 

 decomposition of organic compounds in the soil is called humus. Soil 

 organic matter is called humus when decomposition has proceeded to 

 the stage where the specific structure of the original organized tissue 



