INTROBTJCTION 



That weeds cause injury to ©rops is a fact about which 

 ther* is no diff®r»nca of opinion. The exact cause of 

 this injury, however, is not so well underetood. Two 

 reasons are usually asBigaed, namely^ th« loss of moistura 

 and plant food. Soma axpariaaanterB amphasiK© the former, 

 while others lay greater strees on the latter. Som© seem 

 to consider the two cause© as equally injurj-ous. That 

 weeds lower the moisture content is the common experience 

 of faj^mere and there is also very conclusive experimental 

 data on this point. The belief that injury results from 

 loss of plant food seems to be based chiefly on reasoning 

 from general principles and from the chemical analysis of 

 weeds. Another cause assigned, based on experimental 

 data, is the lowering of the soil temperature. On© 

 asserts that the lowering of the temperature hinders the 

 efficiency of the roots and lessens the decay of organic 

 raatt^er. Shading of the crops is another cause assigned. 

 Again root interference is proposed ; and the possibility 

 of a toxic effect has betn suggested. 



Thus it will be seen that there are many explanations 

 either singly or m combination, offered for the well-known 



