20 Effect of Copper Compounds 



(6) Masking effect caused hy addition of soluble substances to solutions 

 of copper salts. 



Experiments were carried on with barley, in which the plants were 

 grown in the various grades of distilled water indicated above, both 

 with and without the addition of nutrient salts. It was found that the 

 presence of the nutrients exercises a very definite masking effect upon 

 the action of the poisonous substance, so that the deleterious properties 

 of the toxic substance are materially reduced. Later work, in which 

 known quantities of such toxic salts as copper sulphate were added to 

 pure distilled water showed that in the presence of nutrient salts a 

 plant is able to withstand the action of a much greater concentration of 

 poison. For instance, a concentration of 1 : 1,000,000 copper sulphate 

 alone stops all growth in barley, but, if nutrient salts are present, a 

 strength of 1 : 250,000 (at least four times as great) does not prevent 

 growth, though the retarding action is very considerable (Figs. 2 and 3). 



These later Rothamsted results fit in very well with those obtained 

 ten years ago (1903) by True and Gies in their experiments on the 

 physiological action of some of the heavy metals in mixed solutions. 

 Plants of Lwpinus albus were tested for 24 — 48 hours with different 

 solutions in which the roots were immersed. Given the same strength 

 of the same poison, the addition of different salts yielded varying 

 results. For instance, with copper chloride as the toxic agent, the 

 addition of magnesium chloride did not affect the toxicity, calcium 

 chloride decreased it, while sodium chloride slightly increased the 

 poisobous action. Calcium sulphate with copper sulphate enabled a 

 plant to withstand four times as much copper as when the latter was 

 used in pure solution. Calcium salts in conjunction with those of 

 copper proved generally to accelerate but not to increase growth, but 

 with silver salts they did not cause any improvement. Perhaps this 

 amelioration is in inverse proportipn to the activity of the heavy metals. 

 With a complex mixture consisting of five salts — copper sulphate and 

 salts of sodium, magnesium, calcium and potassium, all except calcium ; 

 being present in concentrations strong enough to interfere with growth 

 if used alone — it was shown that " as a result of their presence together 

 not only is there no addition of poisonous effects, but a neutralisation 

 of toxicity to such degree as to permit in the mixed solutions a 

 growth-rate equal to or greater than that seen in the check 

 culture." If the concentration of the copper salts was increased the 



