16 Theory of Agriculture. [Jan., 



cropping for 25 or 30 years. This results from the fact that the 

 sedimentary rocks contain some vegetable matter and iron in a state 

 of protoxide, which, by its farther oxidation, forms ammonia. The 

 most important inorganic bodies required for the growth of crops, 

 exist in abundance in the soil, and the underlying rock too, fur- 

 nishes a fresh supply as its decomposition goes on. 



We have now brought before our readers the most important 

 facts which lie at the foundation of a consistent theory of agri- 

 culture. We may now sum up some of the results; first, that 

 vegetables derive their sustenance from the organic matter incor- 

 porated with the soil, or the debris of rocks, and that this organic 

 matter exists in the form of acids, in combination with the alka- 

 lies and alkaline earths. These matters are continually forming. 

 The organic matter or humus is oxidated slowly, and when oxi- 

 dated, combines with the inorganic matter of the soil and forms 

 with it a soluble salt. These soluble salts constitute the food of 

 plants. The practice in all countries, and in all times, favors this 

 view. Any course of cultivation which dissipates the organic 

 matter, tends to produce barrenness. Thus deep ploughing in 

 warm climates exposes too much organic matter at one time to 

 the action of air and other agents, and hence is found to be inju- 

 rious, or to exhaust rapidly. On the contrary, deep ploughing is 

 required in cold and temperate climates, for the chemical action 

 upon the organic matter is slower, and is required in order to 

 create abundance of food, by exposing a greater surface to the 

 action of oxidating agents. 



In the second place, crops take some portion of their food from 

 the atmosphere. Without doubt it will be found that there is a 

 great difference among plants in the quantity which is derived 

 from this source. 



In the third place, plants differ among themselves as to the kind 

 and quantity of the vegetable and mineral salts which they re- 

 quire; and yet it is undoubtedly true, that the losses of saline 

 matters may be substituted for each other; or in other words, 

 may replace each other in the same crop under different circum- 

 stances. 



