IRRIGATION EXPERIMENTS IN IDAHO. 



21 



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. 



Description of plots. 



Number 

 of plots. 



Average vol- 

 ume of water 

 applied per 

 acre. 



Average yield 

 per acre. 



Average yield 



per acre^foot 



of water. 



Alfalfa: 



Plots making maximum yield in each 

 experiment. 

 Grain and cultivated crops: 



All plots included in investigation 



Plots making maximum yield in each 

 experiment. 



79 

 26 



221 

 60 



Acre-feet. 



2.40 

 2.73 



1.33 

 1.74 



4.91 tons 



5.47 tons 



36.38 bushels . . 

 44.92 bushels . . 



2.04 tons. 

 2 tons. 



27.39 bushels. 

 25.79 bushels. 



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 f ertilized 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 an average 

 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- 

 ciency 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 



