10 SOILS AND MANURES 



and each constituent brought into touch with the roots in 

 exactly the proportion desired. On the other hand, the 

 experiment cannot be begun with the seed ; the plant must 

 be at least partially developed first. Water does not sup- 

 port the plant in the upright position, and some arrange- 

 ment of corks or wires is necessary for that purpose. 

 Access of light to the roots permits the development of 

 algse and other growths. The small amount of air which 

 can be dissolved in the water is altogether inadequate, and 

 when it is exhausted the roots become covered over with the 

 products of reduction which are poisonous and seriously 

 interfere with the growth of the plants. Of course, all these 

 difficulties can be overcome in one way or another and the 

 plants can be successfully grown. The plan, however, 

 involves so many abnormal conditions that it is scarcely 

 applicable except for special purposes. It is quoted here 

 chiefly because it serves to illustrate certain important 

 points relating to the functions of soils which will be 

 emphasised later on. It confirms the conclusions drawn 

 from the sand culture experiments, viz., that certain ele- 

 ments (vide ante) are indispensable for the growth of 

 plants, and that all of them except carbon, which comes 

 from the air, are taken up by the roots. It can also be 

 shown by experiments of this kind (water cultures and 

 sand cultures) that the elements may be supplied to the 

 plants in several different forms with equally good results ; 

 in fact, if introduced into the soil in almost any form what- 

 ever, they will ultimately benefit the plant, but the action 

 will be quicker or slower according to the solubility of the 

 compounds. 



THE ATMOSPHERE. 



Constituents. The air or atmosphere consists mainly 

 of two gases called respectively oxygen and nitrogen. 

 They are present in the proportion of one to four, or, 



