10 THE SOIL AND THE PLANT 



material, large partially disintegrated aggregates of the parent 

 rock are found superimposed upon the solid bedrock. 



The organic substances are important in determining the 

 physical properties of soils. They are largely colloidal in nature 

 and thus have pronounced absorptive properties. This charac- 

 teristic makes their presence particularly desirable in the coarse, 

 open, sandy soils which are so readily leached by percolating 

 water. To soils containing large amounts of fine particles, as 

 clay, the organic residues give a more open granular structure, 

 so necessary for proper cultural treatments, for the penetration 

 of gases and for the movement of water. 



The soil which has undergone changes through numerous 

 generations is thus found to consist of an inorganic framework, 

 of coarse and small particles, some existing separately but most 

 occurring as aggregates, surrounded with a colloidal jelly-like 

 layer, made up of very fine inorganic materials and substances of 

 organic origin. This colloidal material is extremely fine, being 

 smaller than 0.00004 of an inch in diameter. The average size 

 of the colloidal soil particles appears to be close to 0.000004 of 

 an inch. Such particles are only visible with the most powerful 

 microscopes. It is these particles which determine to a large 

 degree the physical properties of the soil. The tendency of 

 soU particles to adhere in a plastic mass is largely determined 

 by them. By reason of the large surface exposed per unit 

 weight they have great absorptive capacity for gases, liquids 

 and dissolved substances, and to them we owe the characteristic 

 of soils to retain water and basic substances. The spaces between 

 the solid particles are filled with water and gases. Diffusion 

 tends to bring these gases to the same composition as the normal 

 atmosphere, but there are always marked differences between the 

 two. The speeds of decomposition of organic materials and per- 

 meability of the soils largely determine their differences. Since 

 carbon dioxide is formed in large amounts in soils by the decom- 

 position of organic materials, and since oxygen is consumed in 

 the process, there are larger amounts of carbon dioxide and smaller 

 amounts of oxygen in the soil air than in the normal atmosphere. 

 Where penetration of air is greatly retarded, other gases such as 

 methane and hydrogen, may appear in considerable quantities. 



Movement of Water in Soil. — The soil air may be partly 

 replaced by water, which forms films around the solid par- 



