The Nature of the Clay-Humus Complex 129 



ganese, and many others. The utihzation of some of these elements 

 held in the inorganic fraction of the soil is also inflnenced by the 

 soil humus, through its chemical interaction with the inorganic com- 

 plexes. One should consider further the colloidal effects of humus 

 on the soil; its buffering properties, which modify the soil reaction; 

 its combining power with bases; its influence upon the oxidation- 

 reduction potential of the soil; its adsorption of certain toxic ma- 

 terials injurious to plant growth; its ability to supply certain catalytic 

 agents and small quantities of various trace elements essential for 

 plant growth; its influence upon soil structure, upon the moisture- 

 holding capacity of the soil, and upon soil temperatiue. Humus also 

 brings about in the soil numerous other reactions which are of direct 

 or indirect importance to plant growth. 



The functions of humus in the soil may thus be considered three- 

 fold: (1) physical, thereby modifying the color, texture, and struc- 

 ture of the soil, as well as its moisture-holding capacity and aera- 

 tion; (2) chemical, influencing the solubility of certain soil minerals, 

 forming compounds with some of the elements, such as iron, which 

 thus become more readily available for plant growth, and increasing 

 the buffering properties of the sofl; (3) biological, by serving as a 

 source of energy for the development of microorganisms, by making 

 the soil a better medium for the growth of higher plants, and by 

 supplying certain essential nutrient elements and compounds re- 

 quired by higher plants. 



The Nature of the Clay-Humus Complex 



Among the important aspects of humus in the soil, the most sig- 

 nificant is its interaction with the clay constituents, which gives rise 

 to clay-humus. In this respect, two types of soil are frequently recog- 

 nized: in one the clay and humus particles are held together by 

 calcium ions; in the other, iron, and possibly also manganese and 

 aluminum, may replace calcium, although aluminum has not yet been 

 demonstrated to play this part. Tiulin classified the sofl colloids into 

 two groups: in one (group H colloids) the iron acts as a cement, 

 holding the clay and humic particles together; iron, and possibly 

 also aluminum, appear to be responsible for the binding of the humus 

 to the sand particles in the B horizon of a podzol. The organic mat- 

 ter dispersed from the B horizon is rich in iron and possibly also in 

 aluminum; it is comparable to the ^-humus of Waksman. When dis- 

 persed in acid solutions, these substances may become negatively 



