6 



In chapter eleventh, the fact that not one of the ele- 

 ments of food by itself possesses any superior nutritive 

 value over the others is further discussed. Nitrogenous 

 food, like all the rest, must be present if a plant is to grow 

 properly, but no excess of this element of food will of 

 itself produce more abundant crops. The analyses of soils 

 show that they abound in nitrogen. Were all other sources 

 of this element wanting, there would still be a continued 

 supply provided for hi rain and dew, and in the many pro- 

 cesses of oxidation going on at the surface of the earth. 

 Probably, wherever we have a generation and circulation 

 of carbonic acid, there is also a provision for the forma- 

 tion of nitrogenous compounds. When Nature thus pro- 

 vides for a supply of nitrogen without the aid of man, it is 

 likely that exhaustion of all other elements of food in the 

 soil will take place by cultivation before this occurs with 

 nitrogen. The inefficacy of the mass of nitrogen in the 

 soil cannot be attributed to its existing in two forms, hi 

 one only of which it is assimilable. This is proved by ex- 

 periments with soils and with farm-yard manure. When 

 the nitrogen of the soil is not available, some other cause 

 must be sought for than its existence in a state in which it 

 is sparingly assimilable. This cause will be found to be 

 the absence of some other elements of food, which, upon 

 being supplied, will at once render the seemingly inopera- 

 tive nitrogen at once energetic. 



The diminution of the amount of available food ele- 

 ments in the arable surface soil, by the cultivation and sale 

 of corn, necessitates the restoration of the removed mineral 

 matters. This is effected to a limited extent by foreign 

 manuring agents, but chiefly by the formation of manure 

 by means of fodder plants. By the system of rotation, 

 green crops which draw their nutriment from the subsoil 

 are introduced between the cereals. By the deep pene- 

 trating roots of the former, the mineral matters of the 



