THE CAUSES OP LOW YIELDS 259 



rate of approximately one bushel per acre for each 

 week's delay in plowing the fallow after the tenth of 

 June. It was quite apparent that the longer the fallow 

 was left unplowed after weed growth commenced the less 

 moisture there was conserved and the less available 

 plant food developed. 



On very weedy soils it is generally advisable to get the 

 weed seeds germinated before plowing the fallow. This 

 often necessitates a delay in plowing. In such cases the 

 bad effects from delayed plowing will be lessened if the 

 weeds and volunteer plants are kept down by some f,orm 

 of surface cultivation such as disking or cultivating. 

 On soils that drift easily late plowing of the fallow may 

 be advisable. (See "Objections to Fallowing" and "De- 

 fense of Fallowing" in Chapter X). 



209. Weeds. Space does not permit of more than a 

 brief reference to this point. The subject is, however, 

 fully discussed in Chapter XII. At this time we shall re- 

 fer only to the principles of weed control, the first of 

 which is that annual weeds like wild oats and the com- 

 mon mustards can be controlled by preventing them 

 from seeding ; the second that biennials can be controlled 

 by plowing the land every year ; the third that perennials 

 can be controlled by preventing them from seeding and 

 by killing the established plants by tillage; and the 

 fourth that eradicating weeds is of little p'ractical value 

 if others are introduced to the farm and sown with the 

 seed, or distributed in any other way. Dozens of sys- 

 tems or combinations of practices are used and recom- 

 mended for weed control. But all control methods trace 

 back to one or more of these fundamental principles. 



210. Insects. -Insects do much less damage to our 

 crops here than they do in milder climates. Yet we are 



