286 THEORIES OF FERTILISER ACTION [chap. 



phosphoric acid would only displace an almost equivalent 

 amount of the phosphoric acid already in solution, and 

 the concentration of the new solution would only differ 

 from the old in the same degree as the ratio of the 

 phosphoric acid in the soil plus fertiliser (2500+50 lb. 

 of phosphoric acid), bears to the phosphoric acid 

 originally in the soil (z>., 2500 lb. phosphoric acid). In 

 other words, before the fertiliser was added, the soil 

 water was as fully saturated with phosphoric acid as 

 the amount of calcium, iron, aluminium, and other bases 

 would permit, and as these bases are present in enormous 

 excess, the soil water must remain at the same satura- 

 tion point after the fertiliser has been added, just as 

 water will only hold 35 per cent, of common salt in 

 solution with however large a quantity of salt it may 

 be in contact. In the same way the soil contains 

 certain double silicates of which potassium is a con- 

 stituent, and these hydrolise to a slight extent in contact 

 with the soil water to yield a solution containing 

 potassium ions. The addition of a soluble potassium 

 salt, as in a fertiliser, will diminish the dissociation and 

 therefore the solubility of the double silicate, the 

 potassium of which is thrown out of solution ; until, as 

 Whitney and Cameron argue, no more potassium ions 

 remain in solution than were present before the addition 

 of the fertiliser. According to this point of view, the 

 concentration of the soil water for a given plant food, 

 such as phosphoric acid, must be approximately constant 

 for all soils of the same type, however much or little 

 phosphatic fertiliser may have been applied, and since 

 water culture experiments show that this low limit of 

 concentration attained by the soil water is more than 

 sufficient for the needs of the plant, no soil can be 

 regarded as deficient in this or any other element of 

 plant food. It therefore follows that the action, if any, 



