Proceedings of the British Association. 353 



* 



likely to be useful, and to the general treatment which the land 

 may require. It is also obvious, that the same importance at- 

 taches to a knowledge of the constitution of the subsoil, since 

 the advantages of exposing to atmospheric influences, and thus 

 disintegrating the portions underneath by deep ploughing, and 

 other methods of bringing the subsoil to the surface, will in a 



great degree depend upon its containing ingredients which the 



crop requires for its subsistence, and of which the superficial soil 

 has been already in a great degree exhausted. Thus, for exam- 

 ple, it will often become a question with the farmer, whether it 

 will be more economical to mix with the soil a given quantity of 

 phosphate of lime, or to incur the labor of so breaking up a por- 

 tion of the subjacent rock, as to unlock, as it were, for the use 

 of the crop, that quantity which it contains in close union with 

 its other constituents. This inquiry, however, presupposes a 

 knowledge on his part of the existence of phosphate of lime in 

 the soil, and of the relative proportion it bears to the other ingre- 

 dients, — data, which can be obtained only through the assistance 

 of refined chemical analysis. A few simple and easy calculations 

 may show how very small a proportion of this ingredient might 

 suffice during a long period of time, for the demands even of those 

 crops which require the largest amount of it for their nutrition. 

 Suppose the subsoil of a single acre of ground, turned up to the 

 depth of a foot, to weigh one thousand tons : now if this rock 

 should be found to contain only a thousandth part of phosphate 

 of lime, it will follow that no less than a ton of this substance 

 might be extracted from the uppermost foot of the subjacent 

 rock, by the action of the elements, or by chemical means. Now 

 one ton of phosphate of lime would be adequate to supply one 

 hundred and twenty five tons of wheat, or six hundred and eighty 

 tons of turnips. And if we reckon the average crop obtained 

 from an acre of land to be, of wheat one ton, and of turnips, 

 fifteen, it is evident that we have at hand as much phosphate of 

 lime as would be necessary for one hundred and twenty five crops 

 of the former, or for forty five crops of the latter. Dr. Daubeuy 

 said he had great reason to believe, that many of our secondary 

 rocks, those especially which contain organic remains, and which 

 appear in a great measure to be made up of shells, would be 

 found, if examined, to contain as large a quantity of phosphate 

 of lime as that mentioned. Though the soil of Great Britain be 



Vol. xnv, No. 2— Jan.-March, 1843. 45 



