THE ROOT SYSTEMS OF PLANTS
The great depth and high fertility of the soils of arid and semiarid
regions have made possible the profitable production of agricultural
plants under a rainfall very much lower than that of humid regions.
To make the principles of this system fully understood, it is
necessary to review briefly our knowledge of the root systems of
plants growing under arid conditions.
Functions of roots
The roots serve at least three distinct uses or purposes: First,
they give the plant a foothold in the earth; secondly, they enable
the plant to secure from the soil the large amount of water needed
in plant growth, and, thirdly, they enable the plant to secure the
indispensable mineral foods which can be obtained only from the
soil. So important is the proper supply of water and food in the
growth of a plant that, in a given soil, the crop yield is usually
in direct proportion to the development of the root system. Whenever
the roots are hindered in their development, the growth of the plant
above ground is likewise retarded, and crop failure may result. The
importance of roots is not fully appreciated because they are hidden
from direct view. Successful dry-farming consists, largely in the
adoption of practices that facilitate a full and free development-of
plant roots. Were it not that the nature of arid soils, as explained
in preceding chapters, is such that full root development is
comparatively easy, it would probably be useless to attempt to
establish a system of dry-farming.
Kinds of roots
The root is the part of the plant that is found underground. It has
numerous branches, twigs, and filaments. The root which first forms
when the seed bursts is known as the primary root. From this primary
root other roots develop, which are known as secondary roots. When
the primary root grows more rapidly than the secondary roots, the
so-called taproot, characteristic of lucerne, clover, and similar
plants, is formed. When, on the other hand, the taproot grows slowly
or ceases its growth, and the numerous secondary roots grow long, a
fibrous root system results, which is characteristic of the cereals,
grasses, corn, and other similar plants. With any type of root, the
tendency of growth is downward; though under conditions that are not
favorable for the downward penetration of the roots the lateral
extensions may be very large and near the surface
Extent of roots
A number of investigators have attempted to determine the weight of
the roots as compared with the weight of the plant above ground, hut
the subject, because of its great experimental difficulties, has not
been very accurately explained. Schumacher, experimenting about
1867, found that the roots of a well-established field of clover
weighed as much as the total weight of the stems and leaves of the
year's crop, and that the weight of roots of an oat crop was 43 per
cent of the total weight of seed and straw. Nobbe, a few years
later, found in one of his experiments that the roots of timothy
weighed 31 per cent of the weight of the hay. Hosaeus, investigating
the same subject about the same time, found that the weight of roots
of one of the brome grasses was as great as the weight of the part
above ground; of serradella, 77 per cent; of flax, 34 per cent; of
oats, 14 per cent; of barley, 13 per cent, and of peas, 9 per cent.
Sanborn, working at the Utah Station in 1893, found results very
much the same
Although these results are not concordant, they show that the weight
of the roots is considerable, in many cases far beyond the belief of
those who have given the subject little or no attention. It may be
noted that on the basis of the figures above obtained, it is very
probable that the roots in one acre of an average wheat crop would
weigh in the neighborhood of a thousand pounds--possibly
considerably more. It should be remembered that the investigations
which yielded the preceding results were all conducted in humid
climates and at a time when the methods for the study of the root
systems were poorly developed. The data obtained, therefore,
represent, in all probability, minimum results which would be
materially increased should the work be repeated now.
The relative weights of the roots and the stems and the leaves do
not alone show the large quantity of roots; the total lengths of the
roots are even more striking. The German investigator, Nobbe, in a
laborious experiment conducted about 1867, added the lengths of all
the fine roots from each of various plants. He found that the total
length of roots, that is, the sum of the lengths of all the roots,
of one wheat plant was about 268 feet, and that the total length of
the roots of one plant of rye was about 385 feet. King, of
Wisconsin, estimates that in one of his experiments, one corn plant
produced in the upper 3 feet of soil 1452 feet of roots. These
surprisingly large numbers indicate with emphasis the thoroughness
with which the roots invade the soil.
Depth of root penetration
The earlier root studies did not pretend to determine the depth to
which roots actually penetrate the earth. In recent years, however,
a number of carefully conducted experiments were made by the New
York, Wisconsin, Minnesota, Kansas, Colorado, and especially the
North Dakota stations to obtain accurate information concerning the
depth to which agricultural plants penetrate soils. It is somewhat
regrettable, for the purpose of dry-farming, that these states, with
the exception of Colorado, are all in the humid or sub-humid area of
the United States. Nevertheless, the conclusions drawn from the work
are such that they may be safely applied in the development of the
principles of dry-farming.
There is a general belief among farmers that the roots of all
cultivated crops are very near the surface and that few reach a
greater depth than one or two feet. The first striking result of the
American investigations was that every crop, without exception,
penetrates the soil deeper than was thought possible in earlier
days. For example, it was found that corn roots penetrated fully
four feet into the ground and that they fully occupied all of the
soil to that depth.
On deeper and somewhat drier soils, corn roots went down as far as
eight feet. The roots of the small grains,--wheat, oats,
barley,--penetrated the soil from four to eight or ten feet. Various
perennial grasses rooted to a depth of four feet the first year; the
next year, five and one half feet; no determinations were made of
the depth of the roots in later years, though it had undoubtedly
increased. Alfalfa was the deepest rooted of all the crops studied
by the American stations. Potato roots filled the soil fully to a
depth of three feet; sugar beets to a depth of nearly four feet.
Sugar Beet Roots
In every case, under conditions prevailing in the experiments, and
which did not have in mind the forcing of the roots down to
extraordinary depths, it seemed that the normal depth of the roots
of ordinary field crops was from three to eight feet. Sub-soiling
and deep plowing enable the roots to go deeper into the soil. This
work has been confirmed in ordinary experience until there can be
little question about the accuracy of the results.
Almost all of these results were obtained in humid climates on humid
soils, somewhat shallow, and underlain by a more or less infertile
subsoil. In fact, they were obtained under conditions really
unfavorable to plant growth. It has been explained in Chapter V that
soils formed under arid or semiarid conditions are uniformly deep
and porous and that the fertility of the subsoil is, in most cases,
practically as great as of the topsoil. There is, therefore, in arid
soils, an excellent opportunity for a comparatively easy penetration
of the roots to great depths and, because of the available
fertility, a chance throughout the whole of the subsoil for ample
root development. Moreover, the porous condition of the soil permits
the entrance of air, which helps to purify the soil atmosphere and
thereby to make the conditions more favorable for root development.
Consequently it is to be expected that, in arid regions, roots will
ordinarily go to a much greater depth than in humid regions.
It is further to be remembered that roots are in constant search of
food and water and are likely to develop in the directions where
there is the greatest abundance of these materials. Under systems of
dry-farming the soil water is stored more or less uniformly to
considerable depths--ten feet or more--and in most cases the
percentage of moisture in the spring and summer is as large or
larger some feet below the surface than in the upper two feet. The
tendency of the root is, then, to move downward to depths where
there is a larger supply of water. Especially is this tendency
increased by the available soil fertility found throughout the whole
depth of the soil mass.
It has been argued that in many of the irrigated sections the roots
do not penetrate the soil to great depths. This is true, because by
the present wasteful methods of irrigation the plant receives so
much water at such untimely seasons that the roots acquire the habit
of feeding very near the surface where the water is so lavishly
applied. This means not only that the plant suffers more greatly in
times of drouth, but that, since the feeding ground of the roots is
smaller, the crop is likely to be small.
These deductions as to the depth to which plant roots will penetrate
the soil in arid regions are fully corroborated by experiments and
general observation. The workers of the Utah Station have repeatedly
observed plant roots on dry-farms to a depth of ten feet. Lucerne
roots from thirty to fifty feet in length are frequently exposed in
the gullies formed by the mountain torrents. Roots of trees,
similarly, go down to great depths. Hilgard observes that he has
found roots of grapevines at a depth of twenty-two feet below the
surface, and quotes Aughey as having found roots of the native
Shepherdia in Nebraska to a depth of fifty feet. Hilgard further
declares that in California fibrous-rooted plants, such as wheat and
barley, may descend in sandy soils from four to seven feet. Orchard
trees in the arid West, grown properly, are similarly observed to
send their roots down to great depths. In fact, it has become a
custom in many arid regions where the soils are easily penetrable to
say that the root system of a tree corresponds in extent and
branching to the part of the tree above ground.
Now, it is to be observed that, generally, plants grown in dry
climates send their roots straight down into the soil; whereas in
humid climates, where the topsoil is quite moist and the subsoil is
hard, roots branch out laterally and fill the upper foot or two of
the soil. A great deal has been said and written about the danger of
deep cultivation, because it tends to injure the roots that feed
near the surface. However true this may be in humid countries, it is
not vital in the districts primarily interested in dry-farming; and
it is doubtful if the objection is as valid in humid countries as is
often declared. True, deep cultivation, especially when performed
near the plant or tree, destroys the surface-feeding roots, but this
only tends to compel the deeper lying roots to make better use of
the subsoil.
When, as in arid regions, the subsoil is fertile and furnishes a
sufficient amount of water, destroying the surface roots is no
handicap whatever. On the contrary, in times of drouth, the
deep-lying roots feed and drink at their leisure far from the hot
sun or withering winds, and the plants survive and arrive at rich
maturity, while the plants with shallow roots wither and die or are
so seriously injured as to produce an inferior crop. Therefore, in
the system of dry-farming as developed in this volume, it must be
understood that so far as the farmer has power, the roots must be
driven downward into the soil, and that no injury needs to be
apprehended from deep and vigorous cultivation.
One of the chief attempts of the dry-farmer must be to see to it
that the plants root deeply. This can be done only by preparing the
right kind of seed-bed and by having the soil in its lower depths
well-stored with moisture, so that the plants may be invited to
descend. For that reason, an excess of moisture in the upper soil
when the young plants are rooting is really an injury to them.