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 
