It is clear from the above results, obtained in Germany, that the amount of water required to produce a pound of dry matter is not the same for all plants, nor is it the same under all conditions for the same plant.  In fact, as will be shown in a later chapter, the water requirements of any crop depend upon numerous factors, more or less controllable.  The range of the above German results is from 233 to 774 pounds, with an average of about 419 pounds of water for each pound of dry matter produced.

During the late eighties and early nineties, King conducted experiments similar to the earlier German experiments, to determine the water requirements of crops under Wisconsin conditions.  A summary of the results of these extensive and carefully conducted experiments is as follows:—­

Oats 385
Barley 464
Corn 271
Peas 477
Clover 576
Potatoes 385

The figures in the above table, averaging about 446 pounds, indicate that very nearly the same quantity of water is required for the production of crops in Wisconsin as in Germany.  The Wisconsin results tend to be somewhat higher than those obtained in Europe, but the difference is small.

It is a settled principle of science, as will be more fully discussed later, that the amount of water evaporated from the soil and transpired by plant leaves increases materially with an increase in the average temperature during the growing season, and is much higher under a clear sky and in districts where the atmosphere is dry.  Wherever dry-farming is likely to be practiced, a moderately high temperature, a cloudless sky, and a dry atmosphere are the prevailing conditions.  It appeared probable therefore, that in arid countries the amount of water required for the production of one pound of dry matter would be higher than in the humid regions of Germany and Wisconsin.  To secure information on this subject, Widtsoe and Merrill undertook, in 1900, a series of experiments in Utah, which were conducted upon the plan of the earlier experimenters.  An average statement of the results of six years’ experimentation is given in the subjoined table, showing the number of pounds of water required for one pound of dry matter on fertile soils:—­

Wheat 1048
Corn 589
Peas 1118
Sugar Beets 630

These Utah findings support strongly the doctrine that the amount of water required for the production of each pound of dry matter is very much larger under arid conditions, as in Utah, than under humid conditions, as in Germany or Wisconsin.  It must be observed, however, that in all of these experiments the plants were supplied with water in a somewhat wasteful manner; that is, they were given an abundance of water, and used the largest quantity possible under the prevailing conditions.  No attempt of any kind was made to economize water.  The results, therefore, represent maximum results and can be safely used as such.  Moreover, the methods of dry-farming, involving the storage of water in deep soils and systematic cultivation, were not employed.  The experiments, both in Europe and America, rather represent irrigated conditions.  There are good reasons for believing that in Germany, Wisconsin, and Utah the amounts above given can be materially reduced by the employment of proper cultural methods.