WHAT THE SOIL SUPPLIES 315 



sandy soil to rest upon a subsoil of clay, which holds a body of 

 water within reach of the soil above. Without such a sub- 

 soil a sandy soil would be in danger of becoming too dry. 



,23. Movement of soil salts. It is a very common mis- 

 take to suppose that the only soil salts available for plants 

 are those with which the roots come in contact. For this 

 reason, the different useful salts occurring in what is called 

 " plough depth " of soil (6 to 9 inches) have been estimated, 

 and the conclusion reached that when this amount of any 

 necessary salt has been used up, the soil will be impoverished. 

 What has been said concerning the capillary movement of 

 water through the soil should correct this impression. The 

 moving water contains the useful salts in solution, and there- 

 fore the salts are also moving towards the surface continually. 

 It must be remembered that this movement is not only 

 towards the points where the water and salts are entering the 

 plants, but also towards the general surface from which the 

 water is being evaporated. Quite apart from the use of salts 

 by plants, therefore, they are continually moving towards 

 the surface in solution and being deposited in the surface 

 layers by the evaporation of the water. 



This means that the salt available for plants is not only 

 that which happens to be within the surface soil at a given 

 time, but also all that is within reach of the water movement. 

 This usually multiplies many times the amount in the surface 

 soil, for the movement of water may extend to a great depth, 

 and when it reaches the underlying rocks, the supply of 

 certain salts may be indefinite. 



24. Soil analysis. There is a general impression that if a 

 sample of soil be sent to a chemist to analyze, he can dis- 

 cover what it lacks and prescribe a suitable fertilizer. This 

 kind of work is sometimes provided for upon an extensive 

 scale. If the statements of the preceding paragraphs are 

 true, it is evident that no such chemical analysis can tell what 

 the soil of any farm needs. In the first place, the appropriate 



