LESSONS FROM EXPERIENCE 299 



been practised at some of the stations for twelve years. 

 The evidence so far as to its practicability is negative. 



246. Destruction of Weeds. The destruction of weeds 

 is nearly always desirable, as undter dry-farming condi- 

 tions weeds are one of the most serious obstacles to suc- 

 cessful crop production. When summer tillage is prac- 

 tised on a bare fallow during the entire season, cultiva- 

 tion should be frequent and thorough enough to destroy 

 all weeds before they attain sufficient size to transpire 

 appreciable quantities of water or to reseed themselves. 

 This tillage will also keep the surface in a condition suf- 

 ficiently loose 'and open to allow the rain that falls to 

 penetrate it. When the soil becomes well filled with 

 water early in the season and additional rains can reason- 

 ably be expected, it may sometimes be desirable to allow 

 the weed's to attain a larger growth and then plow them 

 under in order to provide additional organic matter in 

 the soil, but it must be borne in mind that this gain in 

 organic matter is made at the expense of the soil 

 moisture. 



Our investigations show that summer tillage is, with 

 the exception of green manuring, the most expensive and 

 least profitable method under trial. Exceptions are to be 

 noted in the case of kafir and milo at Dalhart, corn at 

 Scottsbuff, and winter wheat at North Platte and 

 Huntley. 



The purpose of summer tillage is accomplished by the 

 prevention of vegetative growth rather than by the main- 

 tenance of a mulch. Numerous experiments made in con- 

 nection with this work have furnished an abundance of 

 evidence that when vegetative growth is restrained the 

 loss of water from a mulched surface is practically the 

 same as from an unmulched one. 



