130 Humus: Nature and Fomiation 



charged, act as cations, and move toward the cathode during the 

 electrodialysis of soils. They are usually not considered to confer a 

 favorable structure on the soil. 



The group I colloids of Tiulin comprise the clay-humus complex 

 which is held together by calcium ions. Agriculturally, it is believed 

 to be the most important type. This complex is largely responsible 

 for the favorable physical conditions found in various soils and com- 

 posts. The clay appears to have an appreciable base-exchange ca- 

 pacity. The presence of calcium is essential if only for ensuring the 

 formation of the correct type of humus. 



Increasing the amount of soil organic matter in this combination is 

 of great importance. The addition of clay to sandy soil may be as 

 important as the addition of organic matter itself. A certain quantity 

 of stable manure or compost or mass of plant residues can be con- 

 verted into this type of complex only when it is composted with a 

 certain type of clay before it is applied to the soil. 



A third organic-inorganic colloid has been prepared, although it 

 has not been isolated from field soil. Ensminger and Gieseking 

 found that proteins can be strongly adsorbed by bentonites through 

 their basic groups acting as an exchangeable base; this process of 

 adsorption renders the protein more resistant to proteolytic enzymes. 

 In view of the fact that only little is known about the nitrogenous 

 constituents of the soil organic matter, it is difficult to postulate the 

 formation of protein complexes and the reasons for their stability. 



How Humus Is Formed 



The various plant and animal residues on the farm and in the 

 home, the crop wastes, and those crops which are specially grown 

 as a source of humus, all vary considerably in chemical composition 

 and in the rapidity of decomposition. The rate of liberation of the 

 chemical elements, notably the carbon and the nitrogen, in forms 

 available for crop growth, and the nature and amount of humus pro- 

 duced from these residues will, therefore, also vary. Among the 

 sources of humus, plant stubble, stable manures, green manures, and 

 artificial composts occupy a leading place. 



Plant stubble includes the root systems of the plants, as well as 

 the stems, leaves, and other residues left after the crop has been 

 harvested. There is considerable variation in the amount and chem- 

 ical composition of the stubble, depending on the nature and abun- 

 dance of the crop, method of soil cultivation, and fertilizer treat- 



