The Wohurn Pot-Culture Experiments, 11)10-11-12. 325 



The general conclusion can be drawn that lead, when 

 present to the extent of *03 per cent., does not possess any toxic 

 influence upon vegetation, and that the nitrate, with that 

 amount present, would seem to possess stimulative properties. 

 Further experiments are needed to determine the amounts that 

 can be safely or advantageously used. 



General Conclusions. 



The foregoing account cannot be read without the conclu- 

 sion being come to that the experiments have established 

 clearly that the presence in the soil of qviite small quantities of 

 what are termed the " rarer constituents '' has a far greater 

 influence on vegetation than has hitherto been supposed. We 

 need only take the single case of lithium salts to show that 

 Mr. Hills' suppositions have been more than justified, and that 

 useful knowledge has been obtained through pursuing the 

 enquiries which he proposed. 



Further, it has been shown how very differently such 

 metals as lithium, zinc and lead behave in respect of their 

 action on vegetation, for, while lithium cannot be used to an 

 extent above •002 per cent, in a soil without producing harm, 

 zinc may be employed in ten. times this amount with impunity, 

 and lead probably to an even greater extent. Hence a large 

 field has been opened up for further enquiry, and it is necessary 

 that the enquiry be now extended to other crops than wheat, 

 for it does not, by any means, follow that all crops respond 

 similarly to stimulus. Considerable advance has, however, 

 now been made in showing that certain metallic salts have 

 either a toxic or a stimulating effect upon vegetation, the 

 particular effect depending upon the quantity of the metal 

 present. 



Next, there come important considerations of another kind 

 — at what stage of the plant's life are these influences exerted .'' 

 The evidence so far adduced leads strongly to the belief that 

 it is during the germination of the seed rather than at the 

 later stages of the plant's growth, that these influences are 

 exercised. 



In conclusion, these experiments have witliout doubt shown 

 that minute quantities of certain metallic compounds exercise 

 upon the wheat plant an action which is capable of altering its 

 entire character and bringing al)out changes in its development, 

 root formation, and even in the nature of the grain produced. 



II. The Belation of Lime to Magnesia in Soils, 

 (a) The addition of Magnesia. 



Frequent references have been made in past accounts of the 

 Wob.urn Pot-culture Station to work done in regard to lime and 



