364 inohctAxic elements of manures and crops. 



additional effect, depends especially on an influence exerted on the 

 5uil by the crops which leave them. Had these crops been power- 

 fully exhausting, we should expect that their refuse or residue, how- 

 ever considerable in quantity, could do no more than lessen the 

 amount of exhaustion produced ; in which case, its useful influence, 

 however real, would pass unnoticed, were it estimated by the produce 

 of the succeeding crop. If, on the contrary, a crop has been but 

 slightly scourging, whether in consequence of the smallness of its 

 quantity, or because it may have derived from the air the major part 

 of its constituent elements, the useful influence of the residue will 

 not fail to be conspicuous. When the relative value of different sys- 

 tems of rotation is discussed in the way we have done, we in fact 

 estimate the value of the elementary matter derived from the atmo 

 sphere by an aggregate of crops; but the procedure generally fol- 

 lowed is silent when the question is to assign to each crop in 

 particular the special share which it has had in the total profit. To 

 reply to this question, of which a knowledge of the various residues 

 is one of tiie elements, we must first ascertain the quantity of ele- 

 mentary matter supplied by the soil and the atmosphere, with refer 

 ence to each of the crops which enter into the rotation ; in other 

 words, the same investigations must be undertaken in reference to 

 each plant considered by itself, that have been made in reference to 

 the series collectively. There is untpiestionable room, in this direc- 

 tion, for an important scries of experiments. 



^ 3. OK THE INORGANIC SUBSTANCES OF MANURES AND CROPS. 



We have but just considered the organic matter developed in a 

 series of successive harvests. To complete the study of rotations, 

 to the extent at least that this can be done in the present state of 

 science, we have still to examine the relations that may exist between 

 the mineral substances which enter into the constitution of the pro- 

 duce, and those that make part of the manure given. 



We have already shown in a general way that certain mineral 

 salts, certain saline matters or salifiable base?, are essential to the 

 constitution of vegetables. To the best of my knowledge, no seed 

 has yet been met with that is without a phosphate ; and it is now 

 known that the alkaline salts powerfully promote vegetation. 



Such is their ascertained mfluence, indeed, that tobacco, barley, 

 and buckwheat sown in soils absolutely without organic matter, but 

 containing saline substances, and only moistened with distilled water, 

 produced perfect plants, which flowered and fruited, and yielded ripe 

 seeds.* Whence it follows, that the presence of saline matter fa- 

 vors remarkably the assimilation of the azote of the atmosphere 

 during the act of vegetation. 



The importance of considering rotations in connection with the 

 inorganic substances that arc assimilated by plants was perfectly 

 well known to Davy. " The ex[ rtalion of grain from a country 

 which rec«nves nothing in exchange that can be turned into manure, 

 must exhaust the soil m the long run," says the illustrious chemist j 



* Liebig, in Jnurn. dc Phnriniirio, vol. Iv., M ^vnct, p. 94. 



