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The confidence with which scientific investigators, familiar with the arid regions, have attacked the problems of dry-farming rests largely on the known relationship of the water requirements of plants to the natural precipitation of rain and snow. It is a most elementary fact of plant physiology that no plant can live and grow unless it has at its disposal a sufficient amount of water.

The water used by plants is almost entirely taken from the soil by the minute root-hairs radiating from the roots. The water thus taken into the plants is passed upward through the stem to the leaves, where it is finally evaporated. There is, therefore, a more or less constant stream of water passing through the plant from the roots to the leaves.

By various methods it is possible to measure the water thus taken from the soil. While this process of taking water from the soil is going on within the plant, a certain amount of soil-moisture is also lost by direct evaporation from the soil surface. In dry-farm sections, soil-moisture is lost only by these two methods; for wherever the rainfall is sufficient to cause drainage from deep soils, humid conditions prevail.

Water for one pound dry matter

Many experiments have been conducted to determine the amount of water used in the production of one pound of dry plant substance. Generally, the method of the experiments has been to grow plants in large pots containing weighed quantities of soil. As needed, weighed amounts of water were added to the pots. To determine the loss of water, the pots were weighed at regular intervals of three days to one week. At harvest time, the weight of dry matter was carefully determined for each pot. Since the water lost by the pots was also known, the pounds of water used for the production of every pound of dry matter were readily calculated.

The first reliable experiments of the kind were undertaken under humid conditions in Germany and other European countries. From the mass of results, some have been selected and presented in the following table. The work was done by the famous German investigators, Wollny, Hellriegel, and Sorauer, in the early eighties of the last century. In every case, the numbers in the table represent the number of pounds of water used for the production of one pound of ripened dry substance:

Pounds Of Water For One Pound Of Dry Matter

Wollny Hellreigel Sorauer

Wheat 338 459

Oats 665 376 569

Barley 310 431

Rye 774 353 236

Corn 233

Buckwheat 646 363

Peas 416 273

Horsebeans 282

Red clover 310

Sunflowers 490

Millet 447

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.

The water in the large bottle would be required to produce the grain in the small bottle.

In view of these findings concerning the water requirements of crops, it cannot be far from the truth to say that, under average cultural conditions, approximately 750 pounds of water are required in an arid district for the production of one pound of dry matter. Where the aridity is intense, this figure may be somewhat low, and in localities of sub-humid conditions, it will undoubtedly be too high. As a maximum average, however, for districts interested in dry-farming, it can be used with safety.

Crop-producing power of rainfall

If this conclusion, that not more than 750 pounds of water are required under ordinary dry-farm conditions for the production of one pound of dry matter, be accepted, certain interesting calculations can be made respecting the possibilities of dry-farming. For example, the production of one bushel of wheat will require 60 times 750, or 45,000 pounds of water. The wheat kernels, however, cannot be produced without a certain amount of straw, which under conditions of dry-farming seldom forms quite one half of the weight of the whole plant. Let us say, however, that the weights of straw and kernels are equal. Then, to produce one bushel of wheat, with the corresponding quantity of straw, would require 2 times 45,000, or 90,000 pounds of water. This is equal to 45 tons of water for each bushel of wheat. While this is a large figure, yet, in many localities, it is undoubtedly well within the truth. In comparison with the amounts of water that fall upon the land as rain, it does not seem extraordinarily large.

One inch of water over one acre of land weighs approximately 226,875 pounds. or over 113 tons. If this quantity of water could be stored in the soil and used wholly for plant production, it would produce, at the rate of 45 tons of water for each bushel, about 2-1/2 bushels of wheat. With 10 inches of rainfall, which up to the present seems to be the lower limit of successful dry-farming, there is a maximum possibility of producing 25 bushels of wheat annually.

In the subjoined table, constructed on the basis of the discussion of this chapter, the wheat-producing powers of various degrees of annual precipitation are shown:—

One acre inch of water will produce 2-1/2 bushels of wheat.

Ten acre inches of water will produce 25 bushels of wheat.

Fifteen acre inches of water will produce 37-1/2 bushels of wheat.

Twenty acre inches of water will produce 50 bushels of wheat.

It must be distinctly remembered, however, that under no known system of tillage can all the water that falls upon a soil be brought into the soil and stored there for plant use. Neither is it possible to treat a soil so that all the stored soil-moisture may be used for plant production. Some moisture, of necessity, will evaporate directly from the soil, and some may be lost in many other ways. Yet, even under a rainfall of 12 inches, if only one half of the water can be conserved, which experiments have shown to be very feasible, there is a possibility of producing 30 bushels of wheat per acre every other year, which insures an excellent interest on the money and labor invested in the production of the crop.

It is on the grounds outlined in this chapter that students of the subject believe that ultimately large areas of the "desert" may be reclaimed by means of dry-farming. The real question before the dry-farmer is not, "Is the rainfall sufficient?" but rather, "Is it possible so to conserve and use the rainfall as to make it available for the production of profitable crops?"