184 DR. DAUBENY ON THE ROTATION OF CROPS, ETC. 



Thus I have myself observed*, that the soil which covers the serpentine rock of 

 Cornwall, a mineral consisting of — • 



SiHca 43-07 



Magnesia 40-37 



Alumina 0-25 



Lime 0-50 



Oxide of iron 1-17 



Water 1245 — Hisinger, 



contains so minute a proportion of magnesia, that in an analysis of a small sample 

 I altogether overlooked its presence, in so great a degree does the mechanical texture 

 of the rock, and the state of combination subsisting between its ingredients, preserve 

 it from the decomposing action of the elements which tend to set loose its treasures. 



Now it seems obvious, that whatever cannot be extracted from a soil by digestion 

 in muriatic acid during four or five successive hours, must be in such a state of com- 

 bination as will render it wholly incapable of imparting anything to a plant, for such 

 a period of time at least as can enter into the calculations of the agriculturist ; and 

 moreover, that all which muriatic acid extracts, but which water impregnated with 

 carbonic acid fails in dissolving, ought to be regarded as at present contributing 

 nothing, although it may ultimately become available to its purposes. 



I have therefore thought proper to distinguish between the actually available re- 

 sources of the soil, and those ultimately applicable to the uses of the plant, designating 

 the former as its dormant, and the latter as its active ingredients. 



The portion dissolved after digestion in muriatic acid will contain both the dor- 

 mant and the active ; that taken up by water impregnated with carbonic acid will 

 consist merely of the latter ; the difference in amount between the two will therefore 

 indicate the dormant portion of its contents. 



The dormant and active portions may both be comprehended under the designation 

 of its available constituents, whilst those which, from their state of combination in 

 the mass, can never be expected to contribute to the growth of plants, may be 

 denominated the passive ones. 



Every soil, which is capable of yielding an abundant crop of any kind of plant after 

 fallowing, must be assumed to possess in itself an adequate supply of all the ingre- 

 dients necessary for its support in an available condition, but it is plain that these 

 could not have existed in an active one, or such an interval of rest would not have 

 been required for rendering them efficient. 



Accordingly it is quite possible, that after ten years cropping, the soil of the expe- 

 rimental garden might still retain plenty of alkaline salts and phosphates, although 

 what was ready to be applied to the uses of the plant had for the most part been 

 absorbed by the crops previously obtained. 



* Lecture on the Application of Science to Agriculture, from the Journal of the Royal Agricultural Society 

 of England, vol. iii. part 1. 



