yg PHYSIOLOGY OP NUTRITION 



monium chloride (NH4CI) and ammonium sulphate [(NH4)2S04] are physio- 

 logically acid. The injurious effects of these salts are prevented by certain 

 reactions in complex agricultural soUs, but in saind or water cultures account 

 must be taken of these phenomena. 



§2. Importance of. the Essential Ash-constituents.^— Not much is known 

 concerning the importance of the single ash-constituents."* Of some it can be 



said only that their absence results in re- 

 tardation of plant development. Two 

 buckwheat plants are shown in Fig. 50, 

 one of which has been grown in a solution 

 containing all the essential elements and 

 exhibits an entirely healthy appearance,^ 

 while the other, cultivated in a nutrient 

 solution lacking potassium, has hardly 

 developed at all. The difference in 

 growth is very great, although the dry 

 substance of the normally grown buck- 

 wheat plant contains only about 2.5 per 

 cent, of potassium. 



Sulphur is a necessary element because 

 it is essential to the formation of proteins, 

 which are so important in plants. It must 

 be supplied as the sulphate of one of the 

 essential metals; all other compounds of 

 sulphur are injurious. It cannot be re- 

 placed by any other element. 



Phosphorus also is necessary. It is a 

 constituent of nucleins (a special group of 

 proteins), and of phosphatides. It may 

 be introduced in tbfe solu'tion only as one 

 of the phosphates of the tribasic acid 

 (H3PO4), since other phosphorus com- 

 pounds have been found to be harmful. 

 It cannot be replaced by any other 

 element. 



Potassium is also absolutely essential. 

 It accompanies carbohydrates and is sup- 

 posed to promote their formation. 



Paris, 1899. Tome IV.* Mayer, A,, XQOi— 1902. [See 



-Buckwheat plants in water- 

 culture. A, with potassium; B, without 

 potassium. 



> Beithelot, M., Chimie vegetale et agricole. 

 note 1, p. 33.1 



'' For modern studies on the relation between plant growth and the salt proportions and 

 total concentration of the nutrient solution see : Xottingham, W. E., A quantitative chemical 

 and physiological study of nutrient solutions for plant cultures. Physiol, res. i : 133-245. 

 1914. (This includes a very thorough study of Knop's solution and a review of the literature.) 

 Shive, 191S, 1, S- [See note a, p. 77.] The whole subject of the necessity of the various 

 elements for plant growth is well discussed by Russell, 1913. [See note i, p. 69.] — Ed. 



