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.