ON THE CHEMICAL CONSTITUENTS OF SOILS. 215 



These have been found to form excellent manures 

 for heavy clayey soils, particularly for such as disin- 

 tegrate with difficulty. They are most efficacious 

 when burnt, but can only be applied in this state 

 after harvest, and ought to be ploughed into the soil 

 as quickly as possible. By the action of lime upon 

 clay, the potash contained in the latter is rendered 

 soluble. This may easily be shown by mixing one 

 part of marl with half its weight of burned lime, 

 adding water, and setting aside the mixture to re- 

 pose for some time. Even after a space of 24 hours, 

 an appreciable quantity of potash may be detected 

 in the water."* 



A most striking proof of the influence of potash 

 upon vegetation has been furnished by the investi- 

 gations of the " administration " of tobacco in Paris. 

 For many years accurate analyses of the ashes of 

 various sorts of tobacco have been executed, by the 

 orders of the " administration " ; and it has been 

 found, as the result of these, that the value of the 

 tobacco stands in a certain relation to the quantity 

 of potash contained in the ashes. By this means a 

 mode was furnished of distinguishing the different 

 soils upon which the tobacco under examination had 

 been cultivated, as well as the peculiar class to 

 which it belonged. Another striking fact was also 

 disclosed through these analyses. Certain cele- 

 brated kinds of American tobacco were found gradu- 

 ally to yield a smaller quantity of ashes, and their 

 value diminished in the same proportion. For this 

 information I am indebted to M. Pelouze, professor 

 of the Polytechnic School in Paris. 



* One of the causes of the advantages produced by subsoil ploughing 

 is, that it exposes the soil to the disintegrating influences of the atmo- 

 sphere. Hence it is that the subsoil plough is so beneficial in siliceous 

 soils, and exerts no apparent effect upon those which contain much 

 clay. The former disintegrate and liberate their potash both with fa- 

 cility and rapidity ; whilst the disintegration of the latter proceeds with 

 slowness, and no appreciable effects are produced. (See Journal of the 

 Agricultural Society, Vol. II. p. 27.) It is probable, however, that if 

 the land received a dressing of lime afler subsoil ploughing, the effects 

 would be produced more rapidly. — Ed. 



