PREVENTION AND ERADICATION OF WEEDS 55 



which stand out at an angle from the stem, retain moisture 

 long enough to allow any poisonous agent to become effective. 

 This is particularly the case if the leaves are rough or crinkled, 

 as the irregularities hold the moisture for a longer time. It 

 is more difficult, and often impossible, to apply chemical 

 methods for the eradication of weeds among leguminous crops, 

 as the broader leaves of clover, trefoil, sainfoin, serradella and 

 others behave like the leaves of weeds, and hold the chemicals 

 long enough to cause serious injury to the plant. 



With root crops the constant cultivation carried on is 

 usually sufficient to keep weeds in check, so that as a rule 

 it is only cereals that are dealt with by chemical means. 



The success of the chemical eradication of weeds is to 

 some extent dependent upon the weather. It is essential 

 that the substance employed should remain upon the leaves 

 long enough to come into action, and, therefore, if heavy rain 

 falls immediately after an application much of the benefit is 

 lost. Consequently it is necessary to watch the weather 

 carefully, and if unfortunately rain supervenes too soon, the 

 application should be repeated. Again, if a dry substance is 

 employed it would easily be blown away if applied when the 

 leaves were dry, so in this case it is usual to carry out the 

 work when the plants are wet with dew, as by that means the 

 chemical is fastened to the leaves and does not tend to blow 

 away when they dry. 



The chemical substances used as weed killers can be 

 divided into two groups : 



(1) Chemicals that merely destroy the weeds and have no 

 direct beneficial action upon the growth of the crops. These 

 substances are usually applied in the liquid form as sprays. 



(2) Compounds that not only destroy the weeds but also 

 exercise a manurial action, thus directly benefiting the crop at 

 a later date. These substances are usually very finely ground 

 manures, and are applied as dry powders when the leaves are 

 damp. 



(i) Sprays. Most of these are corrosive in nature, and 

 burn up the delicate tissues of the leaves and stems, either 

 killing the weeds outright or so crippling them that the crop 

 is able to get well ahead before the weeds can again start into 

 growth to any extent. These chemicals are always applied 

 in solutions of various strengths. Copper sulphate, iron sul- 

 phate and sulphuric acid are the most general sprays, but 

 various other soluble substances have been tested, 



