THE FOOD OF PLANTS. 79 



method of mechanical analysis of the soil will be found in my Lehrbuch der 

 Bodenkunde, 1876, and in my treatise on the soil which appears in Bd. 2 of the 

 Handbuch der Landwirthschaft, 1888, edited by Goltz. 



27. The Detection of Certain Food Stuffs in the Soil. 



It is here by no means my intention to give a detailed introduc- 

 tion to the chemical examination of the soil. 



Any one wishing to take up such researches, and intending to 

 make himself exactly acquainted with the value of the results 

 that have been obtained must be referred to the works of E. von 

 Wolff, 1 Knop, 2 and to my Bodenkunde. 3 It is merely intended 

 here to prove that the soil contains certain substances which serve 

 as food for plants. For the experiments we use always the fine 

 earth of the soil, isolated in the manner described in 26. 



To prove the presence of Chlorine in the soil, we pour 200 c.c. of 

 water over 5 gr. of fine earth and allow to stand for some time. 

 We theu filter and determine the Chlorine in the filtrate in the 

 usual way by means of Silver nitrate. 



To determine the presence of Sulphuric acid in the soil, 2-5 gr. 

 of fine earth are mixed with 20-50 gr. of Sodium carbonate free 

 from Sulphuric acid. The mixture is placed in a porcelain dish, 

 treated with water and boiled for some time. In the fluid obtained, 

 Sulphuric acid can readily be detected qualitatively or quantita- 

 tively estimated. 



About 10 gr. of fine earth are mixed with water, and then 

 with constant stirring treated with Hydrochloric acid until the 

 evolution of Carbon dioxide has ceased. After some time we filter 

 off the slightly acid solution, nearly neutralise with ammonia, and 

 warm gently with Sodium acetate. A precipitate is formed, con- 

 sisting chiefly of Ferric oxide, while in the fluid filtered away from 

 the precipitate we can readily detect by means of Ammonium 

 oxalate the presence of Calcium. 



We shall not here consider the somewhat troublesome processes 

 necessary for determining the presence in the soil of other sub- 

 stances, especially of Potassium and Phosphoric acid. As regards 

 these the works cited above must be consulted. 4 



1 See E. von Wolff, Anleitunj zur chem. Untersuchung landwirthschl. wichtiger 

 Stoffe, 1875. 



2 Knop, Die Bonitirung der Ackererde, 1871. 



3 See Detmer, Lehrbuch der Bodenkunde, 1876. 



* See also Konig, Untersuchung landwirthschl. wichtiger Sto/e, 1891. 



