196 THE SOIL: INORGANIC MATTER 



surfaces of the solids exposed and on the composition of the 

 solids and of the dissolved substances. Adsorbed materials 

 are available to plant roots, whereas chemically absorbed 

 substances are not. 



Plant foods move up and down for the most part in the 

 soil, due to the force of gravity which pulls water down and 

 to the force of surface tension which pulls water up. Diffusion 

 is a phenomenon which affects dissolved substances in soil 

 to a very slight extent. 



Soil water may be divided into two classes : Film water, 

 or the liquid which surrounds the soil grains and furnishes 

 plants with food; and waste water, or water which drains 

 away from soils. The composition of film water is not easily 

 obtained, but to give some idea of its strength it may be 

 stated that it. varies from 3 to 33 parts per million of plant 

 food material, containing more potash and lime than it does 

 phosphoric acid and nitrogen. Drainage water is con- 

 siderably higher in calcium and magnesium bicarbonates 

 and sulphates, sodium chloride, and nitrates, than in any 

 other constituents. The presence of bicarbonates and sul- 

 phates of calcium and magnesium make water "hard," the 

 former imparting what is called "temporary" hardness 

 because the bicarbonates are precipitated as carbonates on 

 boiling. The latter give water what is called "permanent" 

 hardness because the sulphates are not readily precipitated. 



To obtain some idea of the minerals in a soil it is advan- 

 tageous to know something of the rocks from which they 

 are derived. Granite is one of the most common igneous 

 rocks and is a source of many soils. It is composed of quartz, 

 feldspar, mica, and hornblende. On decomposition it results 

 in a soil of good texture and excellent chemical composition, 

 containing all the plant food elements, including phosphorus 

 which is derived from apatite disseminated in small particles 

 throughout the granite. Limestone rocks yield light clay 

 soils of excellent composition, but they are lacking in calcium 

 carbonate. Sandstones and shales form sandy and clayey 

 soils of various compositions. 



Considering the different kinds of soils it may be briefly 

 mentioned that the difference between arid and humid soils 



