4 8 



SOIL CONDITIONS AND PLANT GROWTH 



TABLE XXII. EFFECT OF ACID RAIN-WATER ON THE GROWTH OF TIMOTHY GRASS. 

 CROWTHER AND RUSTON (72). 



Weight of dry matter obtained when plants were regularly watered with : 



Metallic Salts. Complaints are sometimes made by farmers in 

 mining districts that their crops suffer damage from the waste products 

 generally metallic salts turned into the streams from the works, 

 especially where the water is wanted for irrigation, or where, as in 

 Japan, rice is grown in the marshes. The damage done to pastures by 

 the lead mines of Cardiganshire is under investigation at Aberystwyth. 

 A vast number of experiments have shown that copper salts are 

 extraordinarily toxic in water cultures or where they actually come 

 into contact with the plant, even the minute trace sometimes present 

 in distilled water being harmful. This property finds useful applica- 

 tion in removing algse from water and in killing weeds. For example, 

 a 3 per cent, solution of copper sulphate is sprayed over cornfields in 

 early 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 (2) to retard assimilation of 

 carbon 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 dilu- 

 tions, act as a stimulant to plants ; with copper sulphate, however, Miss 

 Brenchley (54) could obtain no evidence of increased growth in water 

 cultures at any dilution, even down to one part in ten millions of water, 

 although the toxic effect was always shown. The pot experiments of 

 Russell and Darbishire lead to the same conclusion (239). 



Zinc salts have often been made the subject of investigation because 



