FUNCTION.] EXHAUSTION OF SOIL. 285 



numerous adventitious roots, which had lived in pure water 

 for more than six months, were placed in flasks containing 

 distilled water, which was renewed every five days. When 

 the re-agents did not indicate any foreign salt in this water, 

 I made with these plants precisely the same experiments as 

 Theodore de Saussure had done, and I then found that a 

 vegetable freely immersed by its roots in a very dilute solu- 

 tion of several salts, having no chemical action on its tissues, 

 absorbs all the substances contained in that solution in equal 

 proportions. (Annals of Natural History, xviii. 134.) 



But the following very common experiment seems to me 

 to show conclusively that plants do select the food presented 

 to their roots. If a grain of wheat and a garden pea are 

 sown in the same soil, side by side, are watered with the 

 same water, and treated in all respects alike, it will be found 

 at the end of their growth, that the wheat has taken a large 

 quantity of silica (2870) from the soil, while the pea has 

 taken far less (1000) ; and that for 240 of lime in the wheat, 

 there are 2730 in the pea. The inference seems irresistible 

 that the Pea refuses to take up the silicates on which the 

 Wheat plant feeds, and selects lime which is equally indifferent 

 to the Wheat. 



It must be obvious, that the exhaustion of soil by plants 

 means their having consumed all the nutritive particles that 

 it contains. What are those nutritive particles ? That 

 depends upon the nature of the plants ; when land is exhausted 

 of phosphates it ceases to bear turnips, but will carry corn ; 

 when silica is abstracted it will bear turnips, but not corn. 

 In like manner the absence of potash, soda, lime, magnesia, 

 iron, sulphur, &c., will be injurious to those plants to which 

 these substances are peculiarly necessary. Carbonic acid is 

 indispensable to the formation of vegetable tissue ; but it is 

 only in the presence of nitrogen that it can be decomposed. 

 Therefore the absence of carbonic acid and nitrogen produces 

 exhaustion of the soil. 



Thaer and Boussingault both agree in considering the 

 efficiency of manures dependent in a great measure upon 

 their animalised nature, or their power of adding nitrogen to 



