Effect of Copper Compounds 25 



to decrease as the ratio of the quartz sand to the poisonous solution 

 increased, provided that a water content suitable for growth was present. 

 Jensen states that the fatal concentration of copper sulphate in solution 

 cultures is approximately j^th that of the fatal concentration in his 

 artificial soil. 



When copper salts are added to soil a complication at once sets in 

 due to the double decomposition which is always likely to occur when 

 any soluble salt is added to soil. The reaction may be graphically 

 expressed as follows, in a much simplified form 



AB + CD = AG+BD. 



Haselhoff (1892) extracted several lots of 25 kgm. soil, each with 

 25 litres of water in which quantities of mixed copper salts varying from 

 200 mg. had been dissolved, the mixture consisting of three parts 

 copper sulphate and one part copper nitrate. This operation was repeated 

 15 times, the soils being allowed to drain thoroughly after each treat- 

 ment, so that altogether each 25 kgm. soil was extracted with 375 litres 

 water. The drainage waters were analysed, so that the amount of copper 

 absorbed by the soils could be estimated. It was found that by ex- 

 tracting with water containing such soluble copper salts as sulphate and 

 nitrate, the food salts of the soil, especially those of calcium and potas- 

 sium, were dissolved and washed out, copper oxide being retained by the 

 soil. In this way a double action was manifest, whereby the fertility 

 of the soil was reduced by the loss of plant food, while its toxicity was 

 increased by the accumulation of copper oxide. So long as the soil 

 contained a good supply of undissolved calcium carbonate the harmful 

 action of the copper-containing water was diminished, but as soon as 

 the store was exhausted by solution and leaching, the toxic influence 

 became far more evident. 



(e) Mode of action of copper on plants. 



Quite early in the investigations on the effect of copper on plants 

 the question arose as to its mode of activity whether the toxicity was 

 merely due to some mechanical action on the root from outside, whereby 

 the absorptive power of the root was impaired, or whether the poisonous 

 substance was absorbed into the plant, so acting directly on the internal 

 tissues. Gorup-Besanez made definite experiments towards ascertaining 

 the truth of these theories as far back as 1863, endeavouring first 

 of all to see whether the plants take up any appreciable quantity of 

 poisons which exist in the soil as mixtures or combinations and which 



