374 WEED-KILLEBS 



Plant Poisons. A great many substances act as plant poisons. 

 Some, however, which when in solution are most deadly in their effects, 

 are converted by substances present in soil into insoluble and almost 

 harmless compounds. Others remain in a soluble form in the water 

 of the soil for some time, and therefore are very effective as destroyers 

 of plants. 



Almost any soluble salt, if applied in strong solution, will generally 

 kill plants, probably by producing plasmolysis. 



Soluble sulphides, sulphocyanides and sulphites are extremely 

 powerful poisons and can be used as weed-killers. Even strong brine 

 is effective for this purpose. 



Coupin investigated the poisonous effect of a large number of salts l 

 and determined the minimum strength of a solution which had an in- 

 jurious effect in hindering (not preventing) the growth of the roots of 

 wheat during the first 15 days. 



The following are some of his results, the strengths given being the 

 weakest which had an apparent effect : 



Copper sulphate 1 in 700,000,000 



Mercuric chloride 1 in 30,000,000 



Cadmium chloride 1 in 10,000,000 



Silver nitrate 1 in 1,000,000 



Zinc sulphate 1 in 40,000 



Lithium chloride . . . . 1 in 12,000 



Calcium iodide . . - . . . . 1 in 10,000 



Barium nitrate 1 in 4,200 



Borax 1 in 1,000 



Manganese chloride . . . . . 1 in 1,000 



Calcium bromide . . . . . 1 in 400 



Calcium chloride . . . , . 1 in 260 



In all cases the salts are assumed to be anhydrous. The very 

 minute quantity of copper sulphate which produces a poisonous effect 

 is remarkable a quantity such as no ordinary chemical test would 

 detect. 



The destruction of weeds on his land is an important task for the 

 farmer. As a rule, the most useful and practicable methods are 

 mechanical ones hoeing, ploughing, etc., and only in few cases can 

 chemical methods be employed. The destruction of charlock in barley 

 or oat fields, by the use of differential plant poisons, affords the best 

 example of such methods and has already been described. Appropriate 

 manurial dressings may often serve to discourage the growth of un- 

 desirable plants and foster that of desirable ones. For example, 

 liming " sour " grass land will, in time, change the character of the 

 herbage by hindering the growth of such plants as delight in acid soils, 

 e.g., the sour dock. So, too, clover in lawns or pastures may be 

 encouraged by applications of basic slag, while repeated dressings with 

 nitrate of soda will soon enable the grasses to choke out the legumin- 

 ous plants. 



Since most plant poisons render land sterile for some time, they 

 can, as a rule, only be employed with advantage for the destruction 

 of weeds on waste land, roadways, paths, etc. 



iCompt. Kend., 1901, 645 ; Jour. Chem. Soc., 1901, Abstracts, ii. 355. 





