CH. II J Phosphates and Potassium Compounds inSoil 29 



a little hydrochloric acid and treated with potassium 

 ferrocyanide solution, or by the very deep red colour 

 obtained when some of the hydrochloric acid solution 

 is almost neutralised with ammonia and then treated 

 with potassium sulphocyanide solution. 



Another constituent of the hydrochloric acid extract 

 of the soil is potassium which occurred in the complex 

 sihcates of the soil. Unfortunately there is no very 

 simple way of demonstrating its presence, but a method 

 for laboratory use is given on p. 229. Both phosphorus 

 and potassium salts are essential plant foods and among 

 the most important constituents of the soil from the 

 farmer's point of view. Yet they do not form any very 

 great proportion of the whole and even in a fertile soil 

 there is often not more than three or four lbs. of either 

 in a ton of soil whilst the amount that plants can get 

 hold of may only be a few ounces. The plant, however, 

 does not want a great deal; one ton of mangolds only 

 contains some 10 lbs. of potassium and 1| lbs. of phos- 

 phorus^ so that the quantities present are not as in- 

 adequate as they appear. 



We have now come to an end of the importa.nt mineral 

 constituents of the soil. When such a soil is supplied 

 with water, is properly aerated, and receives a sufficient 

 amount of heat from the sun, it speedily becomes the 

 abode of many plants and animals. As these die their 

 remains mingle with the soil, and so a fresh constituent 

 appears, known as organic matter, which has the dis- 

 tinguishing characteristic that it got there through the 

 agency of hving organisms and has the chemical dis- 

 tinction of being easily and completely burnt away. 

 The presence of this organic matter is easily shown by 



^ In accordance with British custom these amounts are stated as the 

 oxides KjO and P2O5. 



