24 BULLETIN 268, U. S. DEPARTMENT OF AGRICULTURE, 



ACCESSORY TILLAGE. 



Under the heading of accessory tillage may be included three dis- 

 tinct practices: (1) Summer tillage, or the tillage of an uncropped 

 fallow field during an entire season; (2) shallow tillage after the crop 

 is removed at harvest and before seeding in the following spring; and 

 (3) the cultivation of an intertilled crop, like corn or potatoes, during 

 the earlier part of the growing season. Each of the three practices 

 above mentioned can again be subdivided into two groups, namely, 

 tillage for the destruction of weeds and tillage for improving the 

 physical condition of the soil. 



DESTRUCTION OF WEEDS. 



The destruction of weeds is nearly always desirable, as under dry- 

 farming conditions weeds are one of the most serious obstacles to suc- 

 cessful crop production. When summer tillage is practiced on a 

 bare fallow during the entire season, cultivation should be frequent 

 and thorough enough to destroy all weeds before they attain sufficient 

 size to transpire appreciable quantities of water or to reseed them- 

 selves. This tillage will also keep the surface in a condition suffi- 

 ciently 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 reasonably be expected, it may sometimes be 

 desirable to allow the weeds 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. 



Tables II to VII 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 Scottsbluff, 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 maintenance of a mulch. 

 Numerous experiments made in connection with this work have fur- 

 nished 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. 



The cheapest and most efficient methods of weed destruction 

 necessarily form a soil mulch. The results accruing from the pre- 

 vention of weed growth have been very generally attributed to the 

 mulch itself when the mulch is, in fact, only incidental. 



Tillage for the purpose of destroying weeds after harvest is war- 

 ranted only in those exceptional cases when sufficient water remains 

 in the soil to start weed growth after harvest or when heavy rains 

 come soon after. In such cases early fall plowing is the most effective 



