538 University of California Publications in Agricultural Sciences [Vol. 1 



Kruger,^^ MacDougal,-^** Chuard and Porchet,^^' Demoussy,^^ 

 Prandi,^" 01ive,*° and Molinari and Ligot.^^ In addition to all 

 these results, wliii-h show either no toxicity or decidedly stimu- 

 lating effects on plants from the use of copper (usually CuSO^) 

 in soil, there are extant a number which testif}^ to the high 

 resistance of plants in soil to extremely large amounts of copper 

 (Such, for example, as from 2 per cent to 5 per cent of tlie 

 dry weight of the soil). Among these may be mentioned the 

 observations of Van Sh^ke*" and Pammel.'*^ 



All of these findings render it extremely improbable that 

 copper in soil, can at any time be considered definitely toxic 

 in relatively small quantities (say, below 0.10 per cent of the 

 dry weight of the soil). On the contrary, the evidence seems 

 very well established that positive stimulation of plants may be 

 induced through the use of small quantities of copper (say, from 

 0.01 to 0.05 per cent of the dry weight of the soil), in the form 

 of CuSO^ particularly, and possibly also in other forms. Our 

 investigations as discussed would seem to confirm and be con- 

 firmed by earlier investigations of the senior author and F. H. 

 Wilson and by numerous other experiments carried out in differ- 

 ent parts of the American and European continents and in Eng- 

 land. These observations would appear therefore to refute the 

 conclusion of Dr. Brenchley which is above quoted and to point 

 clearlj', through the added data which we have submitted, to 

 the conclusion that copper in the form of CuSO^ is to be re- 

 garded, at some concentrations, as being decidedly stimulating 

 to some plants grown in soils, and, what is perhaps more im- 

 portant, relatively innocuous in large amounts. The mechanism 

 of the stimulation obtained does not involve one single effect, 

 liut probably several. We know, for example, through experi- 



35 Ber. deutsch. bot. Gesell., 12, p. 8 ; E. S. E., 5, p. 926. 

 36Bot. Gaz., 27, p. 68; E. S. E., 11, p. 24. 



37 Bull. Soc. Vaud. Sci. Nat., 4tli series, vol. 36, p. 71; Biill. Miirith. 

 Soc, Valais Sci. Nat., no. .33, p. 204. 



38 Ann. Agron., 27, p. 257 ; E. S. E., 13, p. 657. 



39Staz. Sper. Agr. Ital., 40, p. 531; E. S. E., 19, p. 755. 



40 S. Dak. Agr. Exp. Sta., Bull. 112; E. S. E., 21, p. 436. 



41 Ann. Gembloux, 18, p. 609 ; E. S. E., 20, p. 873. 



42 N. Y. Agr. Exp. Sta., Bull. 41. 



43 Iowa Agr. Exp. Sta., Bull. 16. 



