272 REPORTS ON THE STATE OF SCIENCE.—1914. 
from year to year. An argument often advanced in favour of deep 
ploughing is that the depth of root penetration is thereby increased. 
The futility of this argument so far as dry-farm soils are concerned 
becomes evident when it is realised that the normal penetration of roots 
in the Intermountain and Great Plains soils is far below any depth 
that could possibly be reached with the plough. Deep ploughing may 
possibly increase the absorption-rate of rainfall when the precipitation- 
rate is so high as to saturate the surface soil temporarily, but this effect 
can also be secured by leaving the surface rough and corrugated when 
cultivating. Many of the field tests of the Office of Dry-Land Agricul- 
ture have failed to show any increase in yield from deep ploughing, 
an operation which means an added expense to an industry in which 
economy in labour must be rigidly exercised to show a reasonable 
profit. 
Loss of Water from Weeds. 
A relatively small proportion of the total annual rainfall is con- 
served in the fallow. The maximum quantity of stored moisture 
available for the crop seldom exceeds 4 inches of rainfall in section’ 
where the annual rainfall ranges from 13 to 18 inches. 
This low efficiency is due in part to loss from run-off, but mainly to 
surface evaporation and to loss through the transpiration of weeds. 
Numerous measurements have shown that a rainfall of less than one- 
half-inch does not contribute to the permanent store of moisture in 
the soil unless the surface soil is already wet from previous rains. 
If the rainfall penetrates the soil below a depth of 6 inches, its rate 
of loss due to evaporation is low. But if the fallow is weedy, the 
stored water is lost through the transpiration of the plants almost as 
rapidly as if the moist subsoil were freely exposed to the air. The 
water-requirement of weeds is fully as high as some of our most 
valuable crop plants. For example, pigweed (Amaranthus retroflexus), 
tumble-weed (Amaranthus grecizans), and Russian thistle (Salsola 
pestifer) have a water requirement as high as the millets and sorghums, 
while sunflower (Helianthus petiolarus) and lamb’s quarters (Chene- 
podium album) rank higher than many of the legumes.1* The dry- 
farmer can, therefore, produce a valuable forage or grain crop with 
no greater expenditure of water per pound of dry matter than is lost 
through the weeds on his fallow. 
Determinations by W. W. Burr? in Nebraska, R. W. Edwards 1° 
and J. G. Lill!® in Kansas, and C. B. Burmeister 1° in Texas, all 
unite in showing that the evaporation loss from land from which the 
weeds are sliced off with a hoe is but little greater than from culti- 
vated plants. In other words, cultivation is effective in conserving 
water mainly through the destruction of weeds rather than in the re- 
duction of surface evaporation. This is well illustrated by Lill’s 
measurements at Garden City, Kansas, as shown in fig 3. The 
18 Briggs and Shantz, Jour. Agricultural Research, U.S. Department of Agri- 
culture, 8, 60, 1914. 
14 Research Bulletin No. 5, Nebraska Experiment Station, p. 61, 1914. In co- 
operation with the Office of Dry-Land Agriculture and Biophysica] Investigations. 
1 Office of Dry-Land Agriculture in co operation with the Office of Biophysical 
Investigations. 
