84 



PHYSIOLOGY OF NUTRITION 



rapidly than are their kations, thereby rendering the culture solution alkaline. 

 Potassium nitrate (KN0 3 ) is an example of these. To the second group 

 belong those salts whose, kations are absorbed more rapidly than are the 

 anions, thus giving the nutrient medium an acid reaction. Ammonium 

 chloride (NH 4 C1) and ammonium sulphate [(NH 4 ) 2 S0 4 ] are physiologically 

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

 in complex agricultural soils, but in sand or water cultures account must be 

 taken of these phenomena. 



§2. Importance of the Essential Ash-constituents. 1 — Not much is known 

 concerning the importance of the single ash-constituents. d 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. 



1 Berthelot, M., Chimie vegetale et agricole. Paris, 

 1899. Tome IV.* Mayer, A., 1901-1902. [See note 1, 

 P- 33- 



d For modern studies on the relation between 



plant growth and the salt proportions and total 

 concentration of the nutrient solution see: Totting - 

 ham, W. E., A quantitative chemical and physio- 

 logical study of nutrient solutions for plant cultures. 

 Physiol, res. 1 : 133-245. 1914. (This includes a 

 very thorough study of Knop's solution and a re- 

 view of the literature.) Shive, 1915, 1, 2. [See 

 note a, p. 83.] The whole subject of the necessity 

 of the various elements for plant growth is well 

 discussed by Russell, 1915. [See note i, p. 73.] 



The relations between plant growth and the 

 supply of mineral salts may be studied also by 

 using the solution-culture method and three or 

 more single-salt solutions supplied separately, in 

 rotation. This had been attempted, without suc- 

 cess, at the Laboratory of Plant Physiology of the 

 Johns Hopkins University, and it remained for 

 Gericke to succeed at the University of California. 

 (See Gericke, W. F., Water culture experimentation. Science n.s. 56:421-422. 1922.) 

 Gericke obtained good growth of wheat with 0.0 1 volume-molecular solutions for KNO3, 

 CaS0 4 , and MgHP0 4 , the solution rotation being four days for the first solution and one 

 day for each of the other two. A very small amount of iron was supplied in the otherwise 

 single-salt solutions. This method deserves further attention. 



B A 



Fig. 50. — Buckwheat plants in water- 

 culture. A, with potassium; B, without 

 potassium. 



