PHYSICS OF THE SOIL, 



CHAPTEK I. 



NATURE, ORIGIN AND WASTE OF SOIL. 



64. Nature of the Soil The great bulk of most soils is 

 made up of small fragments of rock of various kinds, but 

 nearly always there is associated with these varying 

 amounts of organic matter derived from the breaking down 

 of plant and animal tissue. 



On the surface of the soil grains, too, there is always ad- 

 hering more or less of substances which have been dis- 

 solved in the soil-water but which have been deposited again 

 when the water was evaporated. 



In most soils, but chiefly in the clayey types, there oc- 

 curs some aluminium silicate having water combined with 

 it, which is regarded as giving to them their sticky, plastic 

 quality when wet. The amount of this material in a good 

 soil is always small, seldom reaching more than 1.5 per 

 cent, but the particles are so extremely minute that very 

 little by weight has a marked effect upon its character. 



65. Soils and Sub-soils In climates where the rainfall is 

 sufficient for large crops it is common to speak of the sur- 

 face few inches of rock fragments as the soil while that 

 below is known as the sub-soil. The fundamental reason 

 for making this distinction is found in the fact that the 

 latter is less productive than the surface soil. So general 

 is this difference in fertility that the term "dead-furrow" 

 has been universally applied to the finishing of a land 

 in plowing where the two furrows are thrown in opposite 



