Effect of Zinc Compounds 39 



Baumann (1885) carried out further experiments and concluded that 

 zinc salts are far more toxic than Freytag suspected, 44 mg. zinc 

 sulphate per litre 1 killing plants of 13 species belonging to 7 families 

 (Coniferae excepted). The various plants withstand the action of the 

 zinc salts in different degrees, the same concentration killing off the 

 species in different times. With the 44 mg. zinc sulphate the following 

 results were obtained : 



Trifolium pratense killed in 16 days 



Spergula arvensis 21 



Hordeum vulgare 30 



Vicia sativa 31 



Polygonum Fagopyrum 60 



Beta vulgaris 76 ,, 



Onobrychis sativa 194 ,, 



With still less poison, 22 mg. zinc sulphate per litre, all the species 

 mentioned were eventually killed with the exception of Onobrychis 

 sativa, while 4'4 mg. zinc sulphate seemed to be harmless for all the 

 plants tested except Raphanus sativus, which is evidently exceptionally 

 sensitive to this toxic substance. 



Jensen (1907) again indicated the poisonous action of zinc salts and 

 also found that a relatively small reduction of toxicity was obtained by 

 the addition of finely divided quartz to the solutions. 



(b) Effect of soluble zinc salts in the presence of nutrients, 



Krauch (1882) grew various plants in the presence of nutrient 

 solutions and quantities of zinc sulphate varying from '1 to '8 gm. per 

 litre (= 1/10,000 to 8/10,000). Barley proved to be very sensitive, even 

 to the weakest strength of the poison, as the plants soon showed reddish 

 flecks, while all were dead within six weeks, the control plants without 

 zinc remaining quite healthy. Certain grasses took longer to kill than 

 barley, those with '4 gm. zinc sulphate per litre dying in about seven 

 weeks, while 13 weeks elapsed before the others were killed. Even 

 after this length of time the plants with *1 gm. zinc sulphate per litre 

 still survived, although in a very sickly condition. With willow, again, 

 even '1 gm. zinc sulphate per litre made the plants very sickly after 

 four weeks, growth being weak, the leaves yellow, and the roots brownish. 

 In this case the solutions were renewed, but the plants treated with 

 zinc compounds were dead within eight weeks from the start, the controls 

 being very healthy. 



1 44 mg. ZnS0 4 .7H 8 = 10mg. Zn = l/22,727 ZnS0 4 .7H 2 approx. 



