SOME CONCEPTIONS OF THE SOIL 7 



The members of the kaolinite group are decomposition prod- 

 ucts resulting from the decay of the feldspars and similar 

 minerals. While kaolinite itself shows no nutrients in its 

 formula, it often carries considerable calcium, potassium, 

 magnesium and phosphorus by absorption. Moreover, its 

 close association with other decomposition products such as 

 serpentine, talc, chlorite and epidote tends to accentuate its 

 importance in plant nutrition. The plasticity and cohesion 

 imparted to a soil by the presence of the kaolinite group 

 and its associated minerals are of great practical importance 

 as is also the capacity to hold, either physically or chemically, 

 the bases already mentioned. 



Hematite and limonite are simple iron compounds and 

 usually occur in the soil as a result of the decomposition of 

 certain iron-bearing minerals such as biotite, hornblende and 

 augite. These iron compounds impart the red and yellow 

 colors so characteristic of certain southern soils. Most of the 

 soluble iron of the soil has its source in these minerals. Hema- 

 tite and limonite are produced by the same general processes 

 as are the kaolinite group and are found in very intimate 

 contact with the serpentine, epidote, chlorite and kaolinite. 



5. Soil organic matter. — One of the essential differences 

 between a normal fertile soil and a mass of rock fragments 

 lies in the organic content of the former. The organic matter 

 practically all comes from plants and animals that have in- 

 vested the surface of the soil and the soil material. Through 

 the agency of bacteria and other organisms with which the 

 soil is liberally supplied, this organic tissue quickly loses its 

 original form, and becomes the dark incoherent material so 

 noticeable in fertile soils. The decay is not one of immediate 

 simplification, as might be supposed. The split-off compounds 

 react not only with materials of a similar origin but also 

 with the decomposing mineral fragments. This tendency pro- 

 vides the intimate relationship between the organic and in- 

 organic constituents of the soil already emphasized as an ex- 



