46 Effect of Zinc Compounds 



Kanda (1904) found that peas were stimulated in dilute solutions of 

 zinc sulphate in the absence of nutrients, the optimum concentration 

 being between "00000287 7, and -000001435 "/, (about 1 in 34,840,000 

 and 1 in 69,700,000), higher concentrations being poisonous when the 

 solutions were changed every four days. Jensen (1907) stated that he 

 obtained no stimulation at all with water cultures, even in a solution as 

 dilute as w/100>000 (about 1 in 1,239,000), but he suggested that it was 

 quite possible that in proper concentration the zinc sulphate might 

 prove to be a stimulant. 



Javillier (1910) grew wheat in nutritive solutions with quantities of 

 zinc salts containing from 1/5,000,000 — 1/250,000 zinc, and found that 

 the dry weight of the plant was increased in so far as the stems and 

 leaves were concerned, though it remained uncertain whether a similar 

 increase occurred in the grain. 



A consideration of the Eothamsted experiments shows that up to 

 the present time there is no conclusive evidence that zinc sulphate acts 

 as a stimulant to barley grown in water cultures. As a general rule 

 the growth of those plants with 1/5,000,000 ZnSOi approximates closely 

 to that of the controls. Beyond this the growth varies in different 

 experiments. In some cases lower concentrations from 1/5,000,000 to 

 1/50,000,000 seem to cause some slight improvement in comparison 

 with the normal, indicating a possible stimulus, but this improvement 

 is not at all well marked. In other cases these great dilutions are 

 apparently indifferent, neither a poisonous nor a stimulative action 

 being exerted on the growth of the plant (Fig. 6). With peas some 

 increase has been obtained with 1/20,000,000, and although the rise is 

 only slight, yet it is possible that it may indicate the setting in of 

 a stimulus which would make itself more strongly felt with still 

 weaker concentrations (Fig. 7). 



(6) Stimulation in sand cultures. 



While Jensen denied stimulation in wheat grown in water cultures 

 even when the solutions were as dilute as n/100,000 zinc sulphate, 

 yet he found increase of growth with the same plant in artificial soil 

 (quartz flour) to which much stronger solutions of zinc sulphate, from 

 5n/10,000— w/10,000, had been added. 



(c) Increased growth in soil. 



Nakamura (1904) dealt with a few plants of agricultural importance, 

 adding -01 gram anhydrous zinc sulphate to 2800 grams air-dried soil. 



