MICRO-NUTRIENT PROBLEMS 49 



With the spark the best aluminium line for measurement is, 

 according to Lundegardh (1934), 3961-5 A., but if the sample 

 under examination has a high calcium content, which may 

 frequently be so with plant material, the calcium line 3968-5 A. 

 may interfere with the aluminium line. 



Aluminium may be determined polarographically with the 

 use of lithium chloride, barium chloride or magnesium chloride 

 as supporting electrolyte, but, owing to the fact that the 

 aluminium wave occurs at a rather high negative potential as 

 well as to difficulties resulting from the presence of phosphates, 

 it is unlikely that the polarograph will afford a simple means 

 for the determination of aluminium in plant material. 



Two colorimetric methods, suitable for use with the absorptio- 

 meter, depend on the formation of lakes, fairly stable in the 

 presence of acetic acid, when alizarin and the ammonium salt 

 of aurin tricarboxylic acid, respectively, are added to a solution 

 of an aluminium salt. Both appear to be adaptable to the 

 determination of aluminium in plant tissues. The first method 

 appears to have been described first by Atack (1915). The 

 modification of it described by Underhill and Peterman (1929) 

 has been used in my laboratory on fairly pure solutions with 

 marked success. The second method was described by Hammett 

 and Sottery (1925) and was adapted for the determination of 

 aluminium in animal tissues by Myers, Mull and Morrison (1928). 

 Amounts of aluminium of the order of 5/xg. can be measured by 

 both methods. 



Cobalt and Nickel. Although evidence has occasionally been 

 adduced to indicate that small quantities of cobalt and nickel 

 may bring about an increase in the rate of growth of plants, 

 there has up to now been no definite proof provided that 

 either of these elements is essential for the growth of any plant. 

 There is, however, very definite evidence that cobalt is essential 

 for sheep and cattle, and as the deficiency of this element in the 

 animal must arise from the low content of cobalt in the plants 

 on which the animal feeds, the determination of small quantities 

 of cobalt, at any rate, in plants may be necessary for investiga- 

 tions on cobalt deficiency in animals. There does not seem so far 

 to be any very definite indication that nickel is essential either 

 for any plant or any animal, but since the determination of 



