160 THE PRINCIPLES OF AGRICULTURE 



268. There are four general means of keeping 

 weeds in check : (a) by good tillage (101, 101a) ; 

 (b) by rotation of crops, by means of which any 

 one kind of weed is prevented from becoming 

 thoroughly established ; (c) by complete occupa- 

 tion of the land with crops, — for weeds find op- 

 portunity when the ground is not fully occupied, 

 as in old and thin meadows ; (d) by killing the 

 weeds directly. 



269. Surface tillage should be given as often 

 as the ground becomes hard, or whenever the 

 earth-mulch needs repairing (100). Under gen- 

 eral conditions, tilled crops, as maize and pota- 

 toes, should be cultivated every ten days or two 

 weeks, particularly early in the season. As soon 

 as low crops cover the ground, and thereby afford 

 a mulch, cultivation may cease. 



270. Sowed crops can often be tilled once or 

 twice to advantage very early in the season, by 

 running a fine-toothed harrow over them. Thus, 

 wheat and maize are now often harrowed in early 

 spring. The harrowing destroys but few plants, 

 while it loosens the soil, and conserves moisture 

 before much has been lost by hot weather. Har- 

 rowing meadows and pastures causes the plants 

 to tiller or to stool out, and thereby to cover 

 the gi'ound more completely ; it also breaks the 

 old, hard roots and causes new feeders to appear, 

 thereby re -invigorating the plants. 



