SOIL. 205 



down as a principle that the mineral substances encountered in vege- 

 tables are obtained in the- soil, and that tlie whole of these substan'ces 

 come from rocks which form the solid crust of our planet. I ought, 

 however, to observe in this place that the phosphates, which are so 

 constantly present in plants that it is to be presumed they are essen- 

 tial to their orijanization, do not figure among the elements of crys- 

 talline rocks ; we only meet with phosphoric acid in the strata of a 

 more recent geological epoch, — strata the formation of which has in- 

 deed followed the appearance of organized beings ; so that it would 

 be quite fair to mamtam that this acid had been introduced intc 

 those new strata by the animated beings which are buried in them. 

 Still the phosphatfes are by no means wanting in the rocks of igne- 

 ous origin. In metalliferous strata, to quote those of more common 

 occurrence only, we find phosphate of lead, of copper, of manganese, 

 and of lime ; it is even difficult to discover a ferruginous mineral 

 which does not contain a larger or smaller dose of phosphoric acid. 

 And I must here add, that if phosphoric acid has been rarely indi- 

 cated as a constituent of mineral substances, this is by no means 

 from its uniform absence there, but because it escaped the researches 

 of the analyst, in the same way as iodine and bromine for a long time 

 escaped notice in all the analyses that were made of sea-water. 

 Chemists, in fact, only discover those bodies readily which exist in 

 some very appreciable quantity in the compounds they examine. 

 The substances whose presence is not foreseen, those which only 

 enter in extremely small quantity into a mineral, are apt to pass the 

 eyes of even the most skilful and conscientious unperceived. 



The ashes of every vegetable examined up to the present time 

 show us phosphates, and yet these salts have never been detected 

 in any of the analyses of saps (not very numerous it is true) which 

 we possess ; it is, nevertheless, all but certain that the sap must 

 contain phosphoric acid in some state of combination or another. 



Thaer compares the soil in husbandry to the raw material upon 

 which the industry of the manufacturer is exercised ; the comparison 

 would, perhaps, be more exact were the soil likened to the mechani- 

 cal agents he uses ; and, in fact, even as the prosperity of manufac- 

 tures and the perfection of their produce depend upon the perfection 

 of the machinery employed, so are the quality and the quantity of 

 crops connected in the most intimate manner with the quality of the 

 soil. The highest skill of the husbandman, even under a favorable 

 climate, and otherwise in the most advantageous circumstances, may 

 all be made nugatory by the incessantly renewed difficulties which 

 meet him in a barren soil. 



To be truly fit for agriculture the earth ought to present several 

 essential qualities ; a soil, for instance, must be sufficiently open, 

 sufficiently loose, to permit the roots of plants to penetrate it, and to 

 prevent the water from stagnating upon it. The matter of which it 

 is composed must, further, be of such a kind that the air may insinu- 

 ate itself into it and be renewed, without, however, too rapid a des- 

 iccation following. 



A great deal has been written since Bergman's time upon th« 



18 



