DR. DAUBENY ON THE ROTATION OF CROPS, ETC. 245 



Here also we have reason to believe, that the soil of the experimental garden was 

 richer than that upon which our calculations are founded, so that the falling-off of 

 the crop cannot be attributed to any actual deficiency either of alkali or of phos- 

 phoric acid in the soil. 



The amount of magnesia in the soil was also very small, not exceeding 3*8 lbs. to 

 the 100 square feet. 



This however would have been sufficient for thirty-four crops of barley, according 

 to the estimate given (in p. 230) of the quantity taken up by the crop in ten years. 



When, however, we proceed to inquire into the quantity of these ingredients, which 

 are at the particular moment in a condition to be taken up by the spongioles of the 

 roots, we find the case very different. 



I have already pointed out, that, with a view of imitating nature as nearly as pos- 

 sible, water impregnated with carbonic acid is a preferable solvent to muriatic acid, 

 since it may be presumed, that what is not extracted from the soil by a sufficiently 

 large amount of the former, is not in a condition to be readily assimilated by the 

 plants that grow in it. 



I therefore took sifted portions, each weighing 5 lbs., of the soil, from the part of 

 the garden contiguous to the scene of my experiments, as well as from several of the 

 plots which had grown either the same or different crops during ten years without 

 the addition of manure, and having introduced them into earthen pots, with a hole at 

 the bottom covered over with a piece of wire gauze fine enough to prevent the earth 

 from falling through, I added to each a known quantity of distilled water which had 

 been saturated with carbonic acid gas. 



After a certain amount of the water had passed through, generally two quarts were 

 taken and evaporated to dryness, after which the residuum was treated, in the first 

 place with water, which took up the alkaline salts together with a little calcareous 

 matter, and afterwards with muriatic acid, which dissolved the rest of the lime, 

 whether in combination with carbonic or with phosphoric acid. 



Having got rid of the earthy matter from the aqueous solution by means of oxalate 

 of ammonia, the alkalies remaining were converted into sulphates, heated and 

 weighed, after which the nature of the alkali, combined with the sulphuric acid, was 

 determined by the usual method. 



The acid solution was then treated with ammonia, and the precipitate, when well- 

 washed and dried, was set down as phosphate of lime, iron being rarely present, and 

 never except in minute proportions. 



The following results were obtained, by operating in this manner on the soils enu- 

 merated below, and, granting that objections may be raised against the precision of 

 the method adopted, they at least suffice to show, that the ten years' cropping had 

 reduced very materially the amount of matter immediately available for the purposes 

 of vegetation, however little it appear to have trenched upon the latent resources of 

 the soil. 



2 K 2 



