1917] Forbes: Irrigation Effects of Copper Compounds Upon Crops 471 



000,000, caused increased growth of root tips growing in these 

 solutions. This observation accords with those of some other 

 experimenters, not only with copper solutions but with solutions 

 of various other metals, and bears a certain analogy to stimulat- 

 ing effects upon animals observed with very small amounts of 

 poisons, such as arsenic and strychnine. Stimulation was also 

 observed in the case of certain pot cultures watered with dilute 

 copper solution in sucli a way that these solutions were filtered 

 through a thin layer of soil before they reached the plant roots. 

 Under these conditions a portion of the root systems must come 

 in contact with extremely dilute copper solutions residual from 

 the reactions of copper salts with the soil. As in the case of 

 water cultures, these extremely dilute solutions must have ex- 

 erted the stimulating effects which were apparent in several 

 cultures made in this manner. 



In the case of pot cultures also, in which stated amounts of 

 copper were uniformly mixed throughout the soil, apparent stim- 

 ulation of growth was occasionally observed ; for instance, with 

 0.01 per cent of copper in the form of precipitated carbonate in 

 a culture of corn. 



A satisfactory explanation of stimulation effects is not avail- 

 able. It is to be supposed that stimulation in a soil culture 

 in which copper sulphate is used may be explained by the action 

 of the SO4 ion upon the soil in releasing plant food for the use 

 of the plant. However, such stimulation is seen in water cultures 

 where this does not occur. Lipman-' has observed that under 

 certain conditions the nitrifying flora of soils is stimulated by 

 salts of copper, zinc, iron and lead. Such stimulation, through 

 increased elaboration of nitrates, may account for the behavior 

 of cultures showing increased growth. Stimulation effects, there- 

 fore, which undoubtedly occur both in water and in soil cultures, 

 are perhaps due to more than one different cause — to chemical 

 and bacterial agencies in soils, and to a pathological disturbance 

 in water cultures.^^ 



Taking into account the very minute amounts of copper salts 

 with which stimulated growth is associated, and the very gradual 



27 Lipman, C. B., and Burgess, P. S., Univ. Calif. Publ. Agr. Sci., vol. 1, 

 no. 6, pp. 127-139, 1914. 



28 See Bibliography, pp. 487-488, references 2, 4, 27, 33, 53, 45. 



