84 SOIL CONDITIONS AND PLANT GRO WTH 



spring at the rate of fifty gallons per acre to destroy charlock 

 {Brassica sinapis), one of the most troublesome weeds on light 

 soils. The solution adheres to the rough horizontal leaves of 

 the charlock and kills the plant, but runs off the smooth vertical 

 leaves of the wheat without doing much damage. Even the 

 insoluble complex copper salt present in Bordeaux mixture 

 and sprayed on to fruit trees to kill fungoid pests, was found 

 by Amos^ to retard assimilation of carbc||^ dioxide by the 

 leaf. 



Copper salts do not appear to be anything like so toxic in 

 the soil as in water culture. 



It is often asserted that any toxic substance must, at 

 proper dilutions, act as a stimulant to plants ; with copper 

 sulphate, however. Dr. Brenchley (53<a:) could obtain no evi- 

 dence of increased growth in water cultures at any dilution, 

 even down to i part in 10,000,000 of water, although the 

 toxic effect was always shown. The pot experiments of 

 Russell and Darbishire lead to the same conclusion (241^). 



Ferrous salts are toxic and are commonly regarded as one 

 cause of the sterility of badly aerated soils. 



Aluminium salts have received attention, to them having 

 been attributed much of the injury formerly ascribed to acidity 

 in acid soils (see p. 1 1 2). While in low concentrations they 

 are apparently beneficial (p. 75), at higher concentrations they 

 are harmful, especially in acid soils. 



Salts of arsenious acid were found by Dr. Brenchley to be 

 much more toxic than those of arsenic acid ; they are used 

 as weed-killers. Greaves, however, claims to have obtained 

 evidence that arsenic compounds stimulate the nitrogen fixing 

 organisms in the soil ; ^ while Green has isolated bacteria 

 which are capable of converting arsenites into arsenates, and 

 vice versA? 



Most metallic salts appear to be toxic, except those of the 



^Joum. Ag. Science, 1907, ii., 257-266. 

 ' yourn. Ag. Research, 1916, 6, 389-416. 



*H. H. Green, 5th and 6th Report, Veterinary Research, South Africa, 1918, 

 593-624. 



