EXHAUSTION OF SOILS BY CERTAIN PLANTS. 151 



the degree in which it is exhausted of these by certain species 

 of plants belonging to the potash, lime, or siliceous plants. 

 This will be rendered obvious by the following examples. 



A soil, consisting of four Hessian acres, has removed from it 

 by a crop of — 



Salts of potash Salts of lime, magnesia, 



and soda. and \mroxide of iron. Silica. 



lbs. lbs. lbs. lbs. lbs. 



260 05 



„ r .„ + (In straw 95'31 ) 1on . r , 34*75 > a -,. r r 



* heat I In corn 35'20 $ 13 ° dl 32-&0 \ 67 5d 



d^„ ( In straw 150'40 ) in0 . f0 354'SO ) „_. .„ tR an 



PeaS \ In corn 44 02 \ 19S 42 16-68 \ 3 ' L4G 46 60 



C In straw 4073 ) L, q 36 00) . 



R > e j In corn 42-05 $ 21'82 $ 5? 82 139 77 



Beet-root, without leaves - - 361 '00 37*84 



Helianthus tuberosus - - - 556'00 104'00 



The same surface is deprived by these crops of the following 

 quantity of phosphates*— 



* In the above numbers we have not an exact, but an approximate, pro- 

 portion of the ingredients of the soil removed by the various crops. The 

 analyses of the ashes have been used, as far as they are already made and 

 known. The analysis of the ashes of the seeds and of the straw of wheat 

 is by Saussure ; that of pea-straw by Hertwig ; that of peas by Dr. Will ; 

 that of the ashes of the straw and seeds of rye by Dr. Fresenius, of the beet- 

 root by Hruschauer, and of Helianthus tuberosus by Braconnot. Exact 

 and trustworthy results can only be obtained by estimating the ashes of 

 the crops grown on a given surface, and by subjecting these ashes to 

 analysis ; and not as in the above cases, in which the analyses were made 

 upon the ashes of plants grown under different circumstances and in differ- 

 ent situations. Boussingault, for example, obtained from pea-straw (from 

 a crop heavily manured) 11*2 per cent, of ashes ; Saussure obtained only 8 

 per cent, (in straw with seeds), and Hertwig only 5 per cent. These 

 numbers change the absolute quantities, but have little or no influence on 

 the relative proportions. 



The analyses by Sprengel of the ashes of vai*ous plants are quite inexact, 

 and do not merit the slightest confidence. The ashes of the seeds of 

 wheat, of peas, of beans, rye, &c, consist of phosphates, without any mix- 

 ture of carbonic acid ; these ashes do not contain silica. But Sprengel 

 finds in peas 18 per cent, and in rye 15 per cent, of silica. The ashes of 

 the seeds of rye contain 48 per cent., those of peas 34'23 per cent., of an- 

 hydrous phosphoric acid; but Sprengel finds in peas only 4 per cent., in 

 rye 8 per cent., of phosphoric acid. It is worthy of observation, that all 

 the bases in the ashes of peas are present as tribasic phosphates, while in 

 the ashes of rye and of wheat, they exist as bibasic salts. 



