1864.] CHEMISTRY OF MANURES. 209 



predominates over phosphoric acid in their ash. To quote their 

 own language on this point, as given in the paper already referred 

 to; " Common practice has." they say, '' definitely determined in 

 favor of phosphoric acid rather than of the alkalis, as the special 

 manure to be provided for the turnip from sources external to the 

 farm itself." 



Admitting this case to be a very frequent one (it is certainly 

 not universal), it appears to the reporter susceptible of an explan- 

 ation, by which it falls, quite simple and readily, within the scope 

 of Liebi2:'s law. 



For, in the ordinary course of rotation, cereals and root-crop 

 follow each other, and alternately feed on the soil. Now the 

 cereals, as every one knows, are greedy consumers of silica, partly 

 for the coating of their grain, but principally for that of their 

 straw. The cereals also assimilate phosphoric acid, and divide it 

 in like manner between their grain and straw ; this time 

 however depositing it mostly in the grain. The silica and phos- 

 phates of the grain are, be it remembered, exported from the land. 

 Of potash, the cereals are far less greedy than of phosphoric acid • 

 and of the potash they do assimilate, the larger proportion is 

 deposited in their straw, and returns in the dung to the soil. 

 Keeping these facts in view, and considerinoj also the ori'^nal 

 composition of fair arable soils, containing ordinary proportions of 

 potassic silicates in course of gradual disintegration, it appears to 

 the reporter that the cereals tend to withdraw the acid-in^-redient 

 of these silicates, leaving their alkaline bases as a bequest (so to 

 speak) to the following generation of plants. Thus, when the 

 root-crop enters into possession of the field, it meets with a soi 

 recently drained of available phosphates, but not by any means 

 exhausted of potash. What more natural, under such circum- 

 stances, — what more strictly conformable with Liebig'slaw than 



that soluble phosphates, not potash, should be the cinereal supply 

 required ? 



Upon the whole, therefore, the reporter is constrained to believe 

 that phosphoric acid is no more a "specific" (in any peculiar or 

 mysterious sense) for the root-crops, than potash is for beans and 

 peas, or nitrogen for corn. The more attentively, indeed, the 

 facts are examined, the more strongly do they appear to confirm 

 the grand and simple rule laid down by Justus Liebig, as the prime 

 condition of sound and durable success in husbandry, viz., the 

 Vol. I. No. 3. 



