RAW MATERIALS OF FOOD 123 



dioxide of the air, hydrogen and oxygen largely from 

 water, nitrogen, sulphur and phosphorus from nitrates, 

 sulphates and phosphates dissolved in the water of the 

 soil. The remaining elements essential to the nutrition 

 of plants are potassium, magnesium, calcium and iron, 

 and these are obtained from the same mineral salts. 

 Other elements, such for instance as sodium, siUcon, 

 etc., are habitually absorbed by the plant, but they are 

 not essential as materials of food. This can be proved 

 by growing the plant not in soil but in a mixed solution 

 of various salts. Such water cultures, as they are called, 

 show that a plant can grow and flourish when it is pro- 

 vided with a selection of soluble salts in dilute watery 

 solution containing only the elements named. A good 

 " complete " water culture solution is the following : — 



This is a stock solution. 2 or 3 c.c. should be used 

 for I litre of culture solution, distilled water being 

 added to make up the litre, and a drop of iron chloride 

 added. Other soluble salts can be substituted provided 

 they contain the same elements, for instance potassium 

 nitrate, calcium phosphate, calcium sulphate, magnesium 

 chloride. 



But if one of the elements represented in the salts 

 is omitted from the solution the plant does not flourish, 

 and eventually dies. The most rapid failure results 

 from the omission of nitrogen, and this is clearly because 

 nitrogen is an essential constituent of the protein 

 molecules. When the store of combined nitrogen in 

 the seed has been used up by the growing plant, no 



