eo ee ee Ee Te i ee ee ee 
TRRIGATION EXPERIMENTS IN IDAHO. o1 
TaBLE XI.—Comparison of volumes of water used on plots giving maximum yields and 
the average volumes used on all plots on clay loam soils during 4 years, 1910 to 1913, 
inclusive. 
88 SSS 
Average vol- 
Number | ume of water | Average yiela | “VeT8° yield 
Description of plots. per acre-foot 
of plots. | applied per per acre. 
ero of water. 
Alfalfa: Acre-feet. 
All plots included in investigation........ 79 2.40 | 4.91 tons....-- 2.04 tons. 
Plots making maximum yield in each 26 Daou Os 4p bONSas ar 2 tons. 
experiment. 
- Grain and cultivated crops: 
All plots included in investigation........ 221 1.33 | 36.38 bushels. -} 27.39 bushels. 
Plots making maximum yield in eac 60 1.74 | 44.92 bushels. .|} 25.79 bushels, - 
experiment. 
Table XI shows that the average quantity required to produce the 
maximum yield of crops was greater than the average quantity 
applied to all tracts included in the investigation. All the results 
from the experiments on clay loam soils have been plotted as curves, 
the volume of water applied and the yield produced being the two fac- 
tors considered. | 
Figure 3 shows both the individual and the average yields secured, 
and the quantities of water applied to some 207 fields of spring and 
winter grains. The yields upon soils that have been fertilized either 
with manure or by the plowing under of alfalfa sod have been plotted 
as triangles, while those upon ordinary or infertile soils have been 
plotted as small circles. The points representing both fertile and 
infertile soils are widely scattered, indicating that the results secured 
from any one plot can not be depended upon and that only anaverage 
of the results from a large number of experiments should be taken 
into consideration. The points representing each experiment have 
been connected by straight lines. The curves showing the average 
of results in both fertile and infertile soils have been constructed 
by averaging the points at which these lines crossed the vertical 
lines of the diagram. Owing to the many factors which may affect 
the size of the crop produced with any given quantity of water, as is 
indicated by the variation in the points upon which the curves are 
based, these curves will not definitely fix the size of the crop that can 
be produced on any particular tract. Each is based upon a large 
number of determinations, however, and shows within reasonable 
limits the results which may be expected from clay loam soils. 
A comparison of the two curves shows that a much higher effi- 
Giency is almost invariably secured from the water when applied to 
fertile soil, and they both show that with either the yields may be 
expected to increase as the water applied is increased until as much 
as 1} feet per acre have been applied, after which the yields will 
tend to decrease with greater applications. These curves show a 
striking agreement with the results given in Table XI and would 
