156 ROTATION OF CROPS. 



of the plant only in quantity corresponding to that of the phos- 

 phates, however abundantly ammonia or carbonic acid may be 

 supplied. The production of the constituents of the juice con- 

 taining sulphur and nitrogen is inseparably connected with the 

 presence of these salts. 



Every soil upon which a weed attains maturity is fitted for 

 culture if that weed yields, on incineration, alkaline ashes. 



The alkalies of these ashes arise from silicates, so that in addi- 

 tion to the alkalies, soluble silica must exist in the same soil. 

 Such a soil may contain a quantity of phosphates of lime and 

 magnesia sufficient for potatoes and turnips, without on that ac- 

 count being rich enough for crops of wheat. 



These considerations must show the great importance which 

 onght to be attached to phosphates in the practice of agriculture. 

 These salts are present in the soil only in small quantity, and 

 therefore the greater attention should be paid to prevent its ex- 

 haustion. 



In the limited but enormous extent of the ocean, whole worlds 

 of plants and animals succeed each other. A generation of these 

 animals obtain all their elements from plants, and the constituents 

 of the organs of the animal after death assume their original form, 

 and serve for the nourishment of a new generation of animals. 



The oxygen employed by the marine animals in the process of 

 respiration, and removed from the air, dissolved in the water 

 (this air contains from 32 to 33 volumes per cent, of oxygen, while 

 atmospheric air contains only 21 per cent.), is restored again to 

 the water, by the vital processes of marine plants. In the pro- 

 ducts of the putrefaction of the carcases of the dead animals, 

 their carbon is converted into carbonic acid, their hydrogen into 

 water, and their nitrogen assumes again the form of ammonia. 



Thus, we observe that in the sea, a perpetual circulation takes 

 place, without the accession or removal of an element, and this 

 circulation is unlimited in its duration, al.hoirrh it may be in its 

 extent, by the finite quantity of nour*nkrncn1 contained in plants 

 in a limited space. 



With respect to manr.e plastts, the^e eannci u- any discussion 

 as to their receiving food by their roots in the form of humus. 

 What nourishment indeed can the. thick ioots of the giant sea- 



