THEORIES OF THE OPERATION OF THE SOIL. 393 



are taken up in the organism of plants and are essential in 

 composing its structure, and has proceeded to calculate the actual 

 amount in pounds weight abstracted by the growth of crops 

 of a particular quantity, it has never yet, by an analysis of the 

 soil before the planting, and as exact an examination after 

 the crop has been removed, determined the loss in such case. 

 Why this has not been done, or whether it be beyond the present 

 power of chemical analysis to accomplish, extraordinary as is the 

 degree of perfection to which the science has been advanced, 

 must be left to others to answer. I am perfectly aware, of course, 

 that the same identical soil cannot be subjected to the process 

 of analysis, and then employed for the purposes of vegetation, 

 with a view of ascertaining what has been lost or abstracted ; 

 but an equal weight taken from the same place with that em 

 ployed for growing the plants might be examined, and after 

 wards that in which the plants were grown, so that, by this kind 

 of comparison, the truth might be to a degree approximated. I 

 am quite aware that it may be said, in this case, that the amount 

 of mineral ingredients found in the produce would show the 

 exact amount abstracted j but it would be extremely interesting 

 to know, by an examination of the soil, that these results exactly 

 or nearly corresponded. But it is found that land left to itself 

 for a length of time recovers its fertility, and, after a lapse of two, 

 three, or more years, the same crop, which failed when grown in 

 immediate succession to another of the same kind, can be advan 

 tageously cultivated again. It would be highly curious, then, by 

 retaining a portion of the land in which the plant had been 

 grown, and leaving it exposed to the ordinary influences of light 

 and heat, and rain and frost, to ascertain in what length of time 

 the soil would recover its exhausted elements of fertility. This 

 has not, within my knowledge, been attempted. 



The ingenious theory of Decandolle, that the exudations or 

 excrementitious matter from one kind of crop unfitted the ground 

 for an immediate repetition of the same species of plant, seems 

 now to be generally abandoned. It is a well-established prin 

 ciple, which practical men understand quite as well as the scien 

 tific, that a rotation of crops is indispensable to a successful 

 agriculture ; and the theory is altogether probable that a par 

 ticular crop exhausts the soil of certain elements essential to its 

 production, which must be somehow supplied before a second 



