THE THEORETICAL BASIS OF DRY-FARMING
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?"