ARTIFICIAL MANURING. 135 



taken up by tlie roots. There are fields wLicli are rich in 

 these elements, without being- fertile in an equal pro})ortion : 

 in the latter case, they are united with other elements into 

 chemical compounds, which counteract the dissolving* power 

 of water. By the contemporaneous action of water and air — • 

 of the oxyg"en and carbonic acid of the atmosphere — these 

 compounds are decomposed, and those of their constituent 

 elements, which are soluble in water, but which had been 

 insoluble by the chemical affinity of the other mineral sub- 

 stances, re-obtain the property of being- absorbed by the roots 

 of the plants. 



The duration of the fertility of a field depends on the 

 amount of the mineral aliments of plants contained in it ; 

 and its productive power for a given time is in a direct pro- 

 portion to that part of its composition which possesses the 

 capacit}' of being- taken up by the plant. A number of the 

 most important ag-ricultural operations, especially the me- 

 chanical, exercise an iniluence on the fertility of the fields 

 only thus far, that they remove the impediments which are 

 opposed to the assimilation of the mineral food into the veg-e- 

 table org-anism. By ploug-hing-, for exam[)le, the surface of 

 the fields is renewed and made accessible to air and moisture. 

 The nutritious elements contained in the soil in a latent state 

 acquire, by these operations, the properties necessary for their 

 transmission into tlie plants. It is easy to conceive the 

 useful influence which, in this respect, is exercised on the 

 produce of the fields by the care and industry of the farmer. 

 But all these labours and etibrts do not increase the amount 

 of mineral elements in the field ; in rendering- soluble in a 

 given time a larg-er quantity of the insoluble substances, and 

 obtaining- by these means a richer crop^ the time is merely 

 hastened in which the soil becomes exhausted. 



The experience of centuries has shown that, with the help 

 of manure, of the excrements of animals and man, with 

 which we supply those fields which have ceased to })roduca 

 crops of g-rain, &c., serving- as food for man and animals, in a 

 sufficient quantity, the orig-inal fertility can again be re- 

 stored ; an exhausted field which scarcely yielded back the 

 seed is made to produce a twenty and more fold crop, 

 according- to the proportion of the manure provided. 



Regarding- the mode of action of the manure, it has been 

 observed, that all excrements do not exercise an equal influ- 

 ence on plants. The excrements of sheep and cattle, for 

 instance, increase in most fields the crop of roots and herba- 



