312 The Woburn Pot- Culture Experiments^ 1914. 



Putting together the general conclusions, it may be said to 

 have been demonstrated : — 



1. — That copper in the form of sulphate of copper has an 

 injurious effect when used in quantity supplying '05 per cent, 

 of copper or more, but that "02 per cent, of copper, or less than 

 this, can safely be used in this form and has a slightly 

 stimulating effect. 



2. — That copper in the form of phosphate of copper has a 

 generally stimulating influence, and can be used in quantities 

 supplying up to -10 per cent, of copper without producing any 

 toxic effect on the plant. 



3. — That copper in the form of carbonate of copper is nearly 

 as harmful as sulphate of copper, when used in quantities 

 approaching -10 per cent, of copper. With '05 per cent, the 

 effect is doubtful, but '02 per cent., or less than this, has, when 

 used in the form of carbonate, a stimulating influence. 



4. — That copper in the form of nitrate of copper when 

 supplying -02 per cent, of copper or more is distinctly harmful, 

 but when used in less amounts has a stimulating influence. 



5. — That copper in the form of arsenite of copper is very 

 harmful, and that even so small a quantity as "05 per cent, of 

 copper in this form may be toxic in its effects. 



(5) The Influence of Lead Salts on Wheat. 



In 1912 the first set of experiments on this subject was 

 conducted, the results being reported in Journal R.A.S.E., 1912, 

 Vol. 73, page 324. Lead phosphate, lead nitrate, and lead 

 carbonate were the salts then tried, the quantities of the metal 

 employed with each salt being respectively -03, "02, and '01 

 per cent. When used in these quantities as phosphate and 

 carbonate, lead was found to be peculiarly inert. With the 

 nitrate, however, a stimulating influence was found, though 

 this was, in all likelihood, due to the nitrogen supplied rather 

 than to the lead. Used as the nitrate, no injurious effect was 

 found even when '03 per cent, of lead was added, the results 

 being thus very different to those obtained with zinc when the 

 same quantity of that metal was used, and different again to 

 those with lithium when used to only one-tenth of that amount. 

 It having been found that '03 per cent, of lead was without 

 harmful effect, it was now resolved to try lead in higher 

 amounts. Accordingly, the new experiments embraced the 

 use of lead in the following amounts : '10, "05, "03 per cent., 

 and the salts employed were the phosphate, carbonate, nitrate, 

 sulphate, 'and chloride. These experiments were conducted 

 simultaneously with those on copper salts just described, the 

 pots used being rather smaller earthenware ones, holding 34 lb. 



