SOILS OF ARID AND HUMID REGIONS (_Continued_).
SOILS OF THE TROPICS.
Within the ordinary limits of atmospheric temperatures, and in
the presence of adequate moisture, chemical processes active
in soil-formation are intensified by high and retarded by low
temperatures, all other conditions being equal. We can usually
artificially imitate, and produce in a short time by the application
of relatively high temperatures, most of the chemical changes that
naturally occur in soil-formation. While it is true that the changes
of temperature are nearly as great in the tropical as in the temperate
climates, these changes all occur at a higher level and within the
limits favoring bacterial and fungous action.
This being true we should expect that the soils of tropical regions
should, broadly speaking, be more highly decomposed than those of
the temperate and frigid zones, and that the intensified processes
continue currently. This fact has not been as fully verified as
might be desirable, by the direct comparative chemical examination
of corresponding soils from the several regions, owing to the want
of uniformity in methods and the fewness of such investigations in
tropical countries. Yet the incomparable luxuriance of the natural as
well as artificial vegetation in the tropics, and the long duration
of productiveness that favors so greatly the proverbial easy-going
ways and slothfulness of the population of tropical countries, offers
at least presumptive evidence of the practical correctness of this
induction.
In other words, the fallowing action, which in temperate regions
takes place with comparative slowness, necessitating the early use of
fertilizers on an extensive scale, is much more rapid and effective
in the hot climates of the equatorial rainy belt; thus rendering
currently available so large a proportion of the soil’s intrinsic
stores of plant-food, that the need of artificial fertilization is
there largely restricted to those soils of which the parent rocks
were exceptionally deficient in the mineral ingredients of special
importance to plants, that ordinarily form the essential material of
fertilizers. Quartzose, magnesian, and other soils resulting from
the decomposition of “simple” rocks will, of necessity, be poor in
plant-food everywhere.
_Humus in Tropical soils._—Another inference from the climatic
conditions of the tropics is that the _properly_ tropical soils are
likely to be rich in humus, as a result of the luxuriant vegetation
which in the decay of its remnants must leave abundant humic residues.
This seems to be generally verified wherever the interval between rainy
seasons is not too long; for otherwise, under the great and constant
heat of the tropics a rapid burning-out of the humus, such as is known
to occur in the arid regions, must also take place. A good example
illustrating the inter-tropical regime as regards humus is given in the
table in chap. 8, p. 137, showing the humus-content of some Hawaiian
soils. Both are of the same order as in the soils of the temperate
humid region, though the nitrogen-content evidently can, consistently
with productiveness, range lower than has thus far been observed in
temperate climates. This again forms a striking contrast with the soils
of the arid regions.
It is greatly to be regretted that not even approximate determinations
of the organic matter, much less of the humus-substance proper, have
been made by any of those who have analyzed tropical soils; excepting
those made of Hawaiian soils at the California Experiment Station.
The “loss by ignition” is of course always very largely water, mostly
referable to ferric hydrate and clay substance, the latter presumably
essentially in the form of kaolinite. When, therefore, ferric oxid and
alumina have been determined, we may approximate to the amount of total
organic matter by making allowance for ferric hydrate at the rate of
about 14% of the ferric oxid, for kaolinite at that of 34.92% of the
alumina found. Deducting these amounts of water from the total “loss by
ignition,” we may obtain at least an approximate idea of the organic
matter, and the probable availability of the nitrogen determined by the
analysis. See chapter 19, p. 357.
While the continuous heat and moisture of the tropics concur toward
rapid rock decomposition, it must be remembered that the copious
rainfall is equally conducive to an _intense leaching effect_.
Striking examples of this action occur in the Hawaiian Islands, in
the highly ferruginous soils resulting from the decomposition of the
black (pyroxenic and hornblendic) lavas that are so characteristic
of the volcanic effusions of that region. The soils formed from
these rocks are sometimes so rich in ferric hydrate (iron rust)
that they might well serve as iron ores elsewhere. But these soils
are very unretentive, and though very productive at first they are
soon exhausted, the abundant rains having sometimes deprived them of
almost every vestige of lime, and of most of the potash contained in
the original rock. At the same time the abundant phosphoric acid of
the original rock has been reduced to almost total unavailability by
combination with ferric oxid, just as in the case of the bog ore of the
temperate climates; so that phosphate fertilization is urgently needed
in these lands, though showing high percentage of phosphoric acid.
(Chap. 19, p. 356.)
Soils highly colored by ferric hydrate occur rather frequently in
the tropics, and have received the general name of “laterite” soils.
Curiously enough, the intense reddish tint mostly shown in these soils,
and which is emphasized in the “terra roxa” of the Brazilians, and
the general “red” aspect of Madagascar, and of the Malabar and Bengal
coasts, is by no means always accompanied by markedly high percentages
of iron oxid; but the latter is very finely diffused, so as to be very
effective in coloration. The plant-food percentages of tropical soils
are generally quite low, so that in the temperate humid regions such
lands would be adjudged to be rather poor. Yet they mostly prove quite
productive and lasting, even without fertilization.
This is doubtless to be explained by the continuous and
rapid rock and soil-decomposition which goes on under
tropical climatic conditions, already alluded to; so as to
supply enough available plant-food for the demands of each
season’s vegetation, analogously to the proverbial “nimble
penny.” This is supplemented also by the rapid decay and
leaching-out of the ash ingredients of the rapidly decaying
and dying vegetation. Nitrification must likewise, of
course, be very active under the continual heat and
moisture, and the humus formed under these circumstances is
likely to be quite poor in nitrogen. On this latter point,
however, definite data are almost wholly wanting.
_Investigations of Tropical Soils._—The most extended chemical
investigations of properly tropical soils have been made by Wohltmann
in his investigations of the soils of India, German Southwest and
Southeast Africa, and Samoa;[150] and by Müntz and Rousseaux of soils
collected under Government auspices in Madagascar. Leather, Bamber
and Mann have also analyzed a large number of soils of India. But we
find in many of these cases a failure to specify distinctly the local
climatic conditions, and even the depth to which the samples have been
taken; so that the investigator is obliged to examine laboriously
the local climates, and especially the amount and distribution of
rainfall, before being enabled to discuss intelligently the data given.
Even Wohltmann, in his discussion of North African and Saharan soils,
classes these distinctly arid types among the tropical ones.
[150] Samoa Erkundung, by F. Wohltmann, Kolonial-Wirthsch. Komitee,
Berlin, 1904.
Again, the dry seasons intervening between the tropical rains, varying
in length and from locality to locality, obscure somewhat the relations
of the soils to the climatic conditions. Under the lee of mountains,
even of slight altitude, we find xerophytic (arid-land) vegetation,
as has been noted by many observers in Brazil, even near the Amazon;
in Hawaii, in Jamaica, and in Madagascar. Unless, therefore, a close
discrimination is exercised by field observers, many contradictory
results will appear in analyses of soils of inter-tropical countries.
This is naturally the case in India, where the topographic surface
conformation and seasonal climatic conditions are so complicated and
contrasted. On the whole, the results obtained in Samoa, Kamerun and
Madagascar seem, of those available, to be the most characteristic
of true tropical conditions. In comparing these with the soils
of low plant-food percentages in the temperate humid region (see
chapter 19, p. 352), it must be remembered that those mentioned as
being productive are so by virtue of great depth and relatively
high proportions of lime; while in the tropics, the intense leaching
process prevents lime from reaching any high absolute or relative
percentages, save where limestone formations prevail. Moreover, the
mode of preparation of the soil extracts for analysis by Wohltmann,
and by Müntz and Rousseaux, differ so widely from that forming the
basis of discussion of soil-composition in this volume, that it becomes
necessary to make separate allowances in each case; since some of the
ingredients, phosphoric acid, lime and magnesia, are fully dissolved
by the weaker treatments, while others,—_e. g._, potash—are not, and
are therefore not directly comparable with the data obtained in the
writer’s work. The analyses made in India by Leather and others have
apparently been made substantially in accordance with the author’s
methods and may be considered directly comparable.
SOILS OF SAMOA AND KAMERUN.
Wohltmann has investigated the soils of Samoa, notably those
of the main island of Upolu, under the auspices of the German
“Kolonial-Wirthschaftliche Komitee” in 1903, and gives the results
of his observations and analyses in a report published at Berlin in
1904. The analyses are quite numerous, but unfortunately are made by a
special method which renders them only partly comparable with those of
any other analyst.
Wohltmann’s method is this: “450 grams of fine earth (below
2 millimeters diameter) is treated for 48 hours with 1½
liters of cold chlorhydric acid of 1.15 density. Another
portion, designed for a fuller determination of potash, is
treated for one hour with the same acid, boiling hot. Potash
was determined in both soil extracts; the hot extract gave
from one-third to twice the amount obtained in the cold
extraction.”[151]
Wohltmann justifies this method by the statement that it has
yielded him results more nearly in accord with experience
than any other tried, both with tropical and European soils.
Under these conditions only a few of the determinations in
Wohltmann’s analyses are directly comparable with those upon
which the discussions in this volume have been based. The
figures for nitrogen and phosphoric acid may be assumed to
be fully comparable; that of lime will in general represent
fully only that which is present in the forms of carbonate,
sulfate and humate, and a part of that existing in zeolitic
or hydrous silicate form. Of the two potash determinations
only the one made in hot extraction will be even remotely
comparable, being probably at least 30% lower than would
have been obtained by the writer’s method.
[151] Wohltmann states that the hot extraction sometimes yielded as
much as five times more than the cold; but no such case appears in his
reports on Samoa and Kamerun.
Even thus, however, Wohltmann’s results are highly instructive. He
gives the following summary of his mode of interpreting such analyses:
==================+============+=======+=============
| Very rich. | Good. | Inadequate.
------------------+------------+-------+-------------
Potash | .2 | .1 | .05
Lime and Magnesia | 1.0 | .4 | .07
Phosphoric acid | .2 | .1 | .06
Nitrogen | .2 | .1 | .05
------------------+------------+-------+-------------
It will be observed that the figures of this table differ materially
only in the matter of potash from those given in chapter 19, p. 354;
for the latter substance they would have to be multiplied by from 2 to
4, according to the lime-content and other conditions.
With this understanding a number of Wohltmann’s analyses of soils from
Samoa and Kamerun are given below, the potash determinations made with
hot acid being placed in parentheses after the other.
_Soils of Samoan Islands._—A discussion of these
analyses shows, from the writer’s point of view, a very low
content of potash and lime, with the peculiarity that both
are somewhat higher at the depth of a meter than in the
surface ten-inches. This is probably to be accounted for
from the very high content of organic matter (humus), which
is apparent from the high “loss by ignition,” a very large
proportion of which must be credited to the burning of the
organic matter. That this humus reaches to the lowest depths
examined, is clear from the nitrogen-content given for these
samples. Wohltmann, whose estimate of these soils agrees in
most respects with the writer’s, attributed to them a very
satisfactory nitrogen-content. This would be true of the
total; but as he has not determined either the true humus or
its nitrogen-content, it remains uncertain whether or not a
sufficiency is in an available form, and whether their case
may not be like that of the Hawaiian soil mentioned above
(chapter 19, p. 362), in which despite 10% of humus and
.17% of nitrogen, the land was found to be nitrogen-hungry.
Again, as regards the phosphoric acid, which Wohltmann
considers satisfactory to high, it is questionable to what
extent it is rendered unavailable by the very high content
of ferric hydrate. We are thus left in some uncertainty as
to the real manurial requirements of the Samoan soils, which
doubtless represent very closely also those of Tutuila, the
chief American island of the group.
ANALYSES OF TROPICAL SOILS BY F. WOHLTMANN.
Extraction with cold chlorhydric acid, sp. g. 1.15, for 48 hours.
(A) = Tuanaimato. Virgin Forest Soil, not given.
(B) = Le Utu Sao Vaa Cultivated Soil.
(C) = 0—25 cm. (10 ins.)
(D) = 75—100 cm. (30—40 ins.)
======================+===========================================
| SAMOAN ISLANDS.
|
| UPOLU. | SAVAII.
| |
| (A) | (B) | Cleared Land.
| | | | |
Depth. | | (C) | (D) | (C) | (D)
----------------------+----------+-------+-------+--------+---------
Potash[152] | .05 | .048 | .022 | .063 | .036
| (.07) |(.077) |(.043) | (.102) | (.043)
----------------------+----------+-------+-------+--------+---------
Lime | .07 | .113 | .033 | .042 | .023
Magnesia | .37 | .285 | .144 | .067 | .074
Ferric Oxid |21.53 |17.600 |19.653 |15.333 |18.733
Alumina |12.40 | 9.621 |11.217 |18.941 |16.413
----------------------+----------+-------+-------+--------+---------
Silica | .99 | 2.043 | 2.853 | .366 | 1.250
Titanic Acid | | | | |
----------------------+----------+-------+-------+--------+-------
Phosphoric Acid | .30 | .179 | .213 |29.096 | .187
Org. Matter and Water |18.49[153]|17.288 |12.770 |29.146 |16.332
Nitrogen in Soil | .30 | .447 | .186 | .697 | .128
| | | | |
Hygr. Moisture | 6.80 |15.062 |13.242 |15.288 |12.862
----------------------+----------+-------+-------+--------+-------
======================+=================================
| KAMERUN.
|
| ISONGO. | MUNDAME II.
| | | |
Depth. | (C) | (D) | (C) | (D)
----------------------+-------+-------+-------+---------
Potash[154] | .097 | .104 | .076 | .110
|(.101) |(.163) |(.123) | (.168)
----------------------+-------+-------+-------+---------
Lime | .193 | .154 | .150 | .125
Magnesia | .283 | 1.415 | .198 | .099
Ferric Oxid | 7.305 | 7.497 |13.920 |11.707
Alumina |14.298 |14.504 | 5.223 | 5.531
----------------------+-------+-------+-------+---------
Silica | .047 | .108 | .227 | .120
Titanic Acid | | | |
----------------------+-------+-------+-------+---------
Phosphoric Acid | .064 | .224 | .131 | .205
Org. Matter and Water |23.335 |12.299 |10.154 | 9.549
Nitrogen in Soil | .187 | .079 | .164 | .103
| | | |
Hygr. Moisture |20.065 |15.329 |14.811 |16.498
----------------------+-------+-------+-------+----------
[152] The numbers in brackets are determinations made after boiling
with acid for one hour.
[153] Soil air-dry.
[154] The numbers in brackets are determinations made after boiling
with acid for one hour.
It is probable that for crops requiring so much potash as do
the banana and cacao trees, potash is the first need when
they cease to produce well on these soils.
_Soils of Kamerun._—In the soils of Kamerun, also
analyzed by Wohltmann, and of which two are placed alongside
of those of Samoa, it is at once seen that the materials
from which they have been formed are richer in both potash
and lime than the parent rocks of the Samoan, and not quite
so rich in iron. They are also very rich in organic matter,
evidently down to the depth of a meter, as are those of
Samoa. It is probably due to the high humus-content that
these soils, washed as they have been by the second-highest
rainfall in the world (about 35 feet annually) have not been
as thoroughly leached as have been those of the Brahmaputra
valley. The annual rainfall of Samoa is only from nine to
eleven feet on the lower levels, but ranges as high as 18
feet at higher elevations.
It is noticeable that in most of these true tropical soils the content
of magnesia is considerably above that of lime; a fact readily
intelligible from the more ready solubility of lime in carbonated
water. It is hardly doubtful that this disproportion will in many cases
explain a lack of thriftiness, which could be effectually remedied by
a simple application of lime or marl, without resorting to the more
costly fertilizers.
THE SOILS OF MADAGASCAR.
The soils of the island of Madagascar have been analyzed to the
number of about 500 by Müntz and Rousseaux, under the auspices of the
French government.[155] So large a number of analyses should give a
very full understanding of the agricultural capacity and adaptation
of so comparatively limited an area; unfortunately, we are here again
confronted by more or less imperfect data accompanying the samples
collected by government agents, and by the use of an analytical method
different from those of all other nations, and hence incommensurable
except, as in the case of Wohltmann’s method, in regard to certain
ingredients.
[155] Annales de la Science Agronomique, tome 1er, 1901, fasicules 1,
2, 3.
The French chemists use nitric instead of chlorhydric acid; cold for
phosphoric acid and lime, boiling-hot for five hours for potash;
considering the remainder as of no practical importance. Since nitric
acid is in general much less incisive than chlorhydric in its solvent
power, comparison with the analyses made by other nations becomes
difficult. As in the case of Wohltmann, magnesia, lime, and phosphoric
acid may be considered to be quite thoroughly extracted by the
treatment; while extraction of possibly available potash is doubtless
very incomplete. On the whole, however, the estimates of soil-fertility
based on percentages is very nearly the same as those assigned by
Wohltmann in the table given above. Like Wohltmann, they emphasize the
axiom that the same percentage-gauge of fertility cannot be applied in
the tropics as in the temperate zones.
_General Character of the Island._—The island of Madagascar, lying
between the 11th and 25th degrees of south latitude, is quite
mountainous in its central and eastern portion, where the coast falls
off pretty steeply into the sea, leaving only a narrow coast belt of
properly agricultural land in the lower valleys and at the mouth of
the torrential streams. The mountains rise at one point to the height
of nearly 10,000 feet. The western portion of the Island is much less
broken, has much plateau land with low intersecting ranges and streams
of moderate fall, with considerable alluvial lands near the coast.
The rocks are almost throughout gneisses and mica-schists, which, as
heretofore stated (chapter 4, p. 51), form mostly poor soils. There are
a few areas of eruptive rocks and tertiary calcareous deposits, and on
these the lands are much more thrifty. The rocks and red soils of the
central mass, however, extend seaward almost everywhere.
The rainfall is high on the east side, where the moisture of the
southeast trade winds is first condensed, the precipitation reaching
ten to twelve feet (120 to 144 inches) annually. The western portion
is relatively dry, but rains fall more or less throughout the year;
while in the eastern and central mountainous part there is a distinct
subdivision into a wet and a dry season. Here, while the rivers are
largely torrential, many large fertile valleys have been created by the
heavy denudation of the mountain slopes. This is especially the case in
the Imerina province (in which the capital, Tananarivo, is situated),
and here the valley soils are deep, and rich in humus. The western
portion is but thinly forested. The soils of most of the island are
“red” with ferric hydrate, resembling the laterite soils elsewhere; yet
the iron percentages are not usually very heavy, ranging mostly from
4 to 6, more rarely to 10% and more, of ferric oxid. Most of the red
soils are clayey, crack open in summer and become very hard in drying.
Of the 476 soils analyzed by Müntz and Rousseaux, 156 are from the
province of Imerina, 56 from the adjacent province of Betsileo,
therefore 212 from the central, mountainous part of the island. The
remainder are scattered around the coasts; the most productive being
apparently those of the northern end, Diego Suarez, which is mostly
underlaid by the eruptive rocks forming the mountain mass of Mount
Amber, from which numerous fertile valleys radiate. The valleys of the
west coast also, in the provinces of Bara, Tulear and Betsiriry, have
some very productive soils.
The subjoined table, giving fourteen analyses selected as
representative from the mass of material presented by Müntz and
Rousseaux, gives a fair general idea of the character of the soils
of the great island. It is at once apparent that lime and potash are
extremely deficient in the soils of the mountain slopes of central and
southern Madagascar, these substances having, as elsewhere in the humid
region, been leached down into the valleys; and the materials being
mostly quite clayey, these valley soils have not, as in the case of
the sandy alluvium of the Brahmaputra, themselves been again leached
of their mineral ingredients. Practically these valleys seem to form
the only profitably cultivable area of the central portion; while along
the larger river courses, such as the Mangoky, Ikopa, Mahajamba and
others, good alluvial “bottoms” and deltas form available lands. It
seems to the writer that, in view of their own expressed opinion that
tropical soils are not to be gauged on the same percentage-basis of
soil-ingredients as those of temperate regions, Müntz and Rousseaux
rather underestimate the productive value of many of these lands;
regarding which the field notes report good production, and the crops
of which are certainly not the first that they have borne in the course
of Malagassy history. It is as though their anxiety to forestall
overestimates of agricultural prospects by intending settlers, had led
them to somewhat overshoot the mark.
ANALYSES OF MADAGASCAR SOILS BY MUNTZ AND ROUSSEAUX.
======================+=================================+===========
| IMERINA. | BETSILEO
----------------------+---------------------------------+-----------
| CENTRAL MOUNTAIN REGION.
----------------------+----------+------------+---------+-----------
Number of Soil. | 4--4 | 115 | 71--4 | 272
----------------------+----------+------------+---------+-----------
Locality | Ambohitr-| Ankazobe | Ambohib-|Fandrandava
| omby. | North. | zaka. | Valley.
----------------------+----------+------------+---------+-----------
| Red Soil.| Ochreous | |
| | Soil. | |
----------------------+----------+------------+---------+-----------
Potash (K₂O) | .020 | .006 | .071 | .017
Lime (CaO.) | .350 | .060 | traces | trace
Magnesia (MgO.) | .022 | | |
Ferric Oxid (Fe₂O₃) | 9.333 | | |
Phosphoric Acid (P₂O₅)| .050 | .032 | .061 | .267
Nitrogen (N) | .096 | .027 | .030 | .020
----------------------+----------+------------+---------+-----------
Remarks | Small |Deficient in|No great | Only
| cultural | plant-food |cultural | moderately
|resource. |ingredients.|resource.| fertile.
| |May maintain| |
| |vegetation | |
| |on account | |
| |of humidity.| |
----------------------+----------+------------+---------+-----------
======================+==========+=======================+========
| TULEAR. | MAINTIRANO. |MAJUNGA.
----------------------+----------+-----------------------+--------
| West Coast.
----------------------+----------+------------+----------+--------
Number of Soil. | 253 | 261 | No. 267 | 343
----------------------+----------+------------+----------+--------
Locality |R. bank of| Plateau of |Village of|
| Sakondry |Antsoamena. |Anhozorabe|
| north of | | South of |
|Tongobory | |Mandroso. |
| Valley. | | |
----------------------+----------+------------+----------+--------
| | Sandy | | Hill
| | soil. | |summit.
----------------------+----------+------------+----------+--------
Potash (K₂O) | .086 | .015 | .014 | .012
Lime (CaO.) | 4.180 | trace | trace | trace
Magnesia (MgO.) | | | |
Ferric Oxid (Fe₂O₃) | | | |
Phosphoric Acid (P₂O₅)| .083 | .043 | .011 | .017
Nitrogen (N) | .075 | .051 | .039 | .043
----------------------+----------+------------+----------+--------
Remarks | Potatoes,| Manioc, |Remarkable|Grasses,
| manioc, | maize, | crops, | tall
|represents| beans, | manioc, | herbs.
| soil of | peanuts, | rice. |
| whole |coco-trees, | |
| region. | mangoes. | |
----------------------+----------+------------+----------+--------
======================+==============================
| DIEGO SUAREZ.
----------------------+------------------------------
| N. COAST.
----------------------+--------------+---------------
Number of Soil. | 3 | 331
----------------------+--------------+---------------
Locality | Anamakia. | Montagne
| | d’Ambre.
----------------------+--------------+---------------
| Ochr’us | Ochr’us
| earth. | earth.
----------------------+--------------+---------------
| |
Potash (K₂O) | .161 | .031
Lime (CaO.) | .620 | trace
Magnesia (MgO.) | |
Ferric Oxid (Fe₂O₃) | |
Phosphoric Acid (P₂O₅)| .380 | .124
Nitrogen (N) | .124 | .177
----------------------+--------------+---------------
Remarks | Promises | Crops, coffee.
| considerable | Good cultural
| fertility. | resources.
----------------------+--------------+---------------
======================+========================+===================
| ANDEVORANTE. | AMBATONDRAZAKA.
----------------------+------------------------+-------------------
| EAST COAST.
----------------------+------------+-----------+----------+--------
Number of Soil. | 107 | 370 | 81 | 105
----------------------+------------+-----------+----------+--------
Locality | Dist. of | Ambaniman-| Imeriman-|Sabotsy.
| Vatomandry.| hovinana. | droso. |
|Ampitamafana| | |
----------------------+------------+-----------+----------+--------
| Plain | | |
| valley, | Valley. | |Alt 1000
| sandy | | | M.
| soil. | | |
----------------------+------------+-----------+----------+--------
| | | |
Potash (K₂O) | .069 | .012 | .490 | .005
Lime (CaO.) | .070 | trace | 1.200 | .070
Magnesia (MgO.) | | | |
Ferric Oxid (Fe₂O₃) | | | |
Phosphoric Acid (P₂O₅)| .047 | .161 | 1.566 | .369
Nitrogen (N) | .016 | .254 | .249 | .164
----------------------+------------+-----------+----------+--------
Remarks | Coffee, | Very | Amply |Fertile
| vanilla, | fertile | rich in |soil fit
| rice, &c., | soil. |plant food|for all
| rich red | |suited for|cultures.
| soil. Rep. | | intensive|
| whole | | cultures.|
| valley. | | |
----------------------+------------+-----------+----------+--------
Be that as it may, the influence of the tropical climate and rainfall
upon the composition of these soils is certainly very marked. While
gneiss is not credited with producing first-class soils, its usual
content of orthoclase feldspar should at least insure a respectable
average content of potash; but this, it will be seen, is mostly not the
case; and that of lime seems even worse, aside from the case where,
as in some regions near the coast (especially in the west and south),
calcareous formations, probably of tertiary age, have contributed to
soil-formation. At some points there seem to exist phosphate deposits,
well known elsewhere to occur in such rocks, which impart to the soils
exceptionally high percentages of phosphoric acid, even exceeding one
per cent. The phosphates of course remain practically untouched by the
leaching processes, and appear to be somewhat widely diffused; so that
the soils of Madagascar may be said to be, on the whole, well supplied
with this important plant-food.
In the central province of Imerina the valleys and lower slopes show a
fair content of both lime and potash; but in the province of Betsileo,
adjoining it on the south, nearly every one of the soils analyzed is
reported as containing only “traces” of lime, together with very small
amounts of potash in most cases. The ultimate analyses of ignited red
earths, of which an average is here given, are of interest in this
connection.
ULTIMATE ANALYSIS OF IMERINA RED SOILS,
IGNITED; AVERAGE OF THREE.
Silica 55.2
Potash .3
Lime trace
Magnesia 1.1
Ferric oxid 10.6
It is quite obvious that only leaching-down and concentration of the
feeble resources of such material in the valleys can produce soils
worthy of permanent cultivation.
One point, however, is strikingly illustrated in several of the
analyses given in the subjoined table. We find in the original quite a
number of cases in which the field notes report considerable fertility,
while the chemists’ judgment is very unfavorable. Thus we find recorded
for the soil No. 267, taken near the village of Anjozorabe, in the
Maintirano region, “luxuriant vegetation and remarkable crops,” with
such minute proportions of potash, lime and phosphoric acid that
the authors are compelled to say that the land offers “no cultural
resources.” The same occurs in the cases of soils Nos. 370, 261,
and several others having either “good crops” or “abundant natural
vegetation.” Unless we assume that in these cases the samples were
not properly taken, we are obliged to conclude that under the local
climatic conditions, such minute amounts of plant-food are developed
with sufficient rapidity to supply good growth. This would be quite
parallel to the case of the tea soils of Assam, whose production lasted
30 years before showing exhaustion, on plant-food percentages only
slightly greater than those here noted, and determined by a much more
incisive method.
It is thus quite obvious that a different standard of interpretation
must be applied to tropical soils as compared with either the temperate
humid, or the arid regions; and that uniform methods of analysis are
needed to evolve a definite rule from the present uncertainties.
THE SOILS OF INDIA.
The soils of India have been investigated to some extent by the
geological survey of India; by Voelcker, who went there on a special
mission to investigate agricultural conditions; and since, more
especially by Leather, Bamber and Mann; and by Moreland. Leather’s
account is the most complete on the general subject and can best serve
as the basis for a review of the entire peninsula.[156]
[156] On the Composition of Indian Soils. Agr. Ledger, 1898, No. 2.
According to Dr. Leather, “the four main types of soils to be dealt
with, and which certainly occupy by far the larger of the Indian
cultivated area,” are: _The Indo-Gangetic alluvium_, covering the chief
cultivable areas of the Indo-Gangetic plain; the _black cotton soils or
regur_, occupying the main body of the plateau of the Central provinces
(the Deccan) from the Vindhya range south; the _red soils_ lying on the
metamorphic rocks of Madras; and the “_laterite_” _soils_ which are met
with in many parts of India. To these should be added the _alluvial
soils of the Brahmaputra valley_, in Assam. It is hardly to be
expected that so large an area as that of India, with its diversified
topography, and a climate ranging from about four inches of rainfall in
the northern Panjab to the world’s maximum in Assam, and southward to
typical tropical conditions, could be even thus briefly characterized.
The observers have rarely given for the several soils analyzed, special
local and climatic data, which cannot always be obtained from the
official publications; so that it is not easy to discuss them from the
points of view of aridity and humidity.
_The Indo-Gangetic Plain._—The general rain-map of India shows the
Panjab and Rajputana to be arid throughout; thence eastward the
rainfall increases to 25 and 30 inches on the Ganges; notwithstanding
which, alkali (reh) is abundant about Aligarh, Meerut and Agra. Thence
toward Calcutta there is a steady increase of rainfall until, at the
head of the Bay of Bengal, 70 inches is reached.
If under these conditions the Indo-Gangetic plain admits of any
generalizations as regards soil composition, it must be attributed in
the main to its predominantly alluvial character. It should therefore
be relatively rich in lime, magnesia and potash. So far as the first
is concerned, Leather remarks that the only rocky particles larger
than sand to be found in all this large belt of land is the nodular
limestone called kankar, formed by the deposition of calcium carbonate
within the soil, at the depth of a few feet. It occurs very generally
in India, and as stated above (chapters 9 and 19), this occurrence
of calcareous hardpan, of varying hardness, is almost universal
in the arid regions. The analysis given in the table, selected as
representative from those given by Leather, show that the general
forecast is realized in them, as soils of an arid region.
ANALYSES OF SOILS OF INDIA.
NORTHERN INDIA.
============================+=================================
| Indo-Gangetic Alluvium.
+----------------+----------------
| Panjab. | Lower Ganges.
| |
+--------+-------+-------+--------
| Sotar |Changa | Ison, | Sibpur,
| Valley.|Manga. |Ganges,|Calcutta
| Clay |Loamy | Doab. | Clay
| Loam. |soils. | Sandy | soils.
| [157] | | Loams.|
----------------------------+--------+-------+-------+-------
Insoluble matter | 81.57 | 81.54 | 88.08 | 73.58
Soluble silica | | | |
----------------------------+--------+-------+-------+--------
Potash (K₂O) | .74 | .54 | .64 |
Soda (Na₂O) | .08 | .25 | .09 | 1.82
----------------------------+--------+-------+-------+--------
Lime (CaO) | 1.44 | .98 | .47 | 1.01
Magnesia (MgO) | 1.97 | 1.72 | .32 | 1.64
Br. ox. of Manganese (Mn₃O₄)| | .11 | | 7.19
Peroxid of iron (Fe₂O₃) | 4.32 | 5.11 | 3.10 | 7.58
Alumina (Al₂O₃) | 5.85 | 4.36 | 4.38 | 9.89
Phosphoric acid (P₂O₅) | .23 | .14 | .08 | .07
Sulfuric acid (SO₃) | ? | .02 | .05 | .00
Carbonic acid (CO₂) | 1.13 | .45 | .37 | .28
Water and org. matter | 2.67 | 4.78 | 2.42 | 5.93
+--------+-------+-------+--------
Total | 100.00 |100.00 |100.00 |100.00
| | | |
Nitrogen | .02 | .082| .027| .051
----------------------------+--------+-------+-------+--------
| Brahmaputra Alluvium.
+---------------------------------------
| Assam.[158]
+---------+---------+----------+--------
| Fezpur |Lakhimpur|Golaghat. |Sipsagar
| bank. | New |Nigriling.| Teela
| Old |Alluvium.| New | land.
|Alluvium.| |Alluvium. | New
| | | |Alluvium
----------------------------+---------+---------+----------+--------
Insoluble matter | 85.18 | 84.60 | 85.88 | 91.52
Soluble silica | | | |
----------------------------+---------+---------+----------+--------
Potash (K₂O) | .35 | .24 | .26 | .14
Soda (Na₂O) | .30 | .12 | .23 | .16
----------------------------+---------+---------+----------+--------
Lime (CaO) | .04 | .11 | .03 | .06
Magnesia (MgO) | .46 | .33 | .36 | .20
Br. ox. of Manganese (Mn₃O₄)| | | |
Peroxid of iron (Fe₂O₃) | 2.08 | 2.78 | 2.74 | 1.52
Alumina (Al₂O₃) | 5.03 | 5.63 | 5.10 | 3.30
Phosphoric acid (P₂O₅) | .05 | .06 | .06 | .06
Sulfuric acid (SO₃) | .02 | .02 | .02 | .02
Carbonic acid (CO₂) | | | |
Water and org. matter | 5.59 | 6.11 | 5.32 | .18
+---------+---------+----------+--------
Total | | | |
| | | |
Nitrogen | .14 | .20 | .18 | .08
----------------------------+---------+---------+----------+--------
| Laterite Soils.
+-------------------------------
| Bengal.
+----------+---------+----------
|Lohardaga.| Chota |Hazaribagh
| | Nagpur. |District.
| |Singhbhum|
| |District.|
----------------------------+----------+---------+----------
Insoluble matter | 29.67 | 59.06 | 80.46
Soluble silica | | |
----------------------------+----------+---------+----------
Potash (K₂O) | .10 | .27 | .38
Soda (Na₂O) | .04 | | .32
----------------------------+----------+---------+----------
Lime (CaO) | .38 | .28 | 1.72
Magnesia (MgO) | .21 | .33 | .38
Br. ox. of Manganese (Mn₃O₄)| .07 | .48 | .50
Peroxid of iron (Fe₂O₃) | 48.71 | 26.64 | 6.12
Alumina (Al₂O₃) | 8.81 | 7.27 | 7.19
Phosphoric acid (P₂O₅) | .64 | .08 | Trace.
Sulfuric acid (SO₃) | | | Trace.
Carbonic acid (CO₂) | .06 | .16 | .12
Water and org. matter | 11.31 | 5.43 | 2.81
+----------+---------+----------
Total | 100.00 | 100.00 |
| | |
Nitrogen | .010 | .024 | .03
----------------------------+----------+---------+----------
[157] Analysis by Voelcker.
[158] Analyses by Mann.
SOUTHERN INDIA.
=============================================================
Madras Presidency.
----------------------------+--------+-----------------------
|Laterite| Upland.
| Soil. | Red Soils.
+--------+---------+-------------
|Sidapet.| Madura. |Trichinopoli.
| |Dindigal.| Parambalur.
| | |
| | |
----------------------------+--------+---------+-------------
Insoluble matter | 76.86 | 90.47 | 86.74
Soluble silica | | |
----------------------------+--------+---------+-------------
Potash (K₂O) | .09 | .24 | .05
Soda (Na₂O) | .17 | .12 | .15
----------------------------+--------+---------+-------------
Lime (CaO) | Trace.| .56 | .48
Magnesia (MgO) | .77 | .70 | .70
Br. ox. of Manganese (Mn₃O₄)| .19 | .08 | .10
Peroxid of iron (Fe₂O₃) | 10.09 | 3.51 | 5.70
Alumina (Al₂O₃) | 8.84 | 2.92 | 5.68
Phosphoric acid (P₂O₅) | Trace.| .09 | .05
Sulfuric acid (SO₃) | | |
Carbonic acid (CO₂) | .12 | .30 | .11
Water and org. matter | 2.87 | 1.01 | .24
+--------+---------+-------------
Total | 100.00 | 100.00 | 100.00
| | |
Nitrogen | .015| .006 | .021
----------------------------+--------+----+----+-------------
====================================================================
Madras Presidency.
----------------------------+-------------+-------------------------
| Alluvium. | Regur.
| Averages. |
+------+------+------+----------+-------
|Loamy |Sandy |Regur.| Trichin- |Kistina
|Soils.|Soils.| Avg. | opoli. |
| | | 18 |Parambalur|Narsa-
| | |Soils.| Taluk. | opet.
----------------------------+------+------+------+----------+-------
Insoluble matter |71.79 |93.09 | 68.41| 65.16 | 68.29
Soluble silica | | | | |
----------------------------+------+------+------+----------+-------
Potash (K₂O) | .22 | .04 | .41| .14 | 1.14
Soda (Na₂O) | .17 | .07 | .31| .01 | 1.30
----------------------------+------+------+------+----------+-------
Lime (CaO) | .54 | .13 | 2.90| 2.18 | 3.43
Magnesia (MgO) | 1.29 | .33 | 2.27| 2.47 | 1.94
Br. ox. of Manganese (Mn₃O₄)| .13 | .04 | .17| .25 | .09
Peroxid of iron (Fe₂O₃) | 9.59 | 2.46 | 7.13| 9.27 | 6.96
Alumina (Al₂O₃) | 9.98 | 1.74 | 10.14| 13.76 | 10.28
Phosphoric acid (P₂O₅) | .11 | .05 | .06| Trace. | .00
Sulfuric acid (SO₃) | | | | Trace. | Trace.
Carbonic acid (CO₂) | .09 | .11 | 1.62| .91 | 1.88
Water and org. matter | 6.09 | 1.94 | 6.58| 5.85 | 3.96
+------+------+------+----------+-------
Total |100.00|100.00|100.00| 100.00 | 99.27
| | | | |
Nitrogen | .037| .015| .03| .024 | .012
----------------------------+------+------+------+----------+-------
_The Brahmaputra Alluvium in Assam._—Aside from the immediate
alluvium of the Indus, of which no definite data are available, the
Indo-Gangetic plain represents the drainage of the _southern_ slope of
the Himalaya chain. That of most of the _northern_ slope is represented
by the Brahmaputra, which not only originates in a region of heavy
precipitation—Thibet—but continues in the same throughout its course,
and rounding the easternmost spur of the Himalaya range enters, in
southern Assam, upon the region of the maximum rainfall known. Its
alluvial deposits should therefore show the reverse characteristics of
those of the Ganges; they should, as thoroughly leached soils, be poor
in lime, magnesia and potash. We have fortunately on this subject the
excellent work done by Mr. H. H. Mann for the Indian Tea Association,
the report of which was published in 1901, and contains, besides a
large number of analyses, good descriptions of the general soil and
cultural conditions of the Assam tea districts, with suggestions for
their improvement.
The tea plantations of Assam are located almost wholly on the new and
old alluvium of the Brahmaputra river, bordered on the north by the
eastern spur of the Himalayas, on the south by the low ranges of the
Khasia hills. The soil is mostly quite sandy, the late alluvium gray in
color, the older reddish and more loamy. Of the four analyses given in
the table and fairly representing the average character of these soils,
the two first are from the north side, the latter two from the south
side of the river.
It will be noted that the prominent feature of all these soils is an
extremely low percentage of lime, the general average being about .08%
as against nearly 1.0% in the average Indo-Gangetic soils. In the
latter, potash ranges between .65 and .70%; in the Assam soils between
.25 and .35. Magnesia averages nearly 1.3 in the Indo-Gangetic, against
about .50 in the Assam tea soils. It is thus apparent that the same
general facts as regards the leaching-out of soil ingredients already
shown for eastern and western North America are strikingly verified in
northern India; but reversed as regards the points of the compass. The
preferential leaching-out of lime as compared with magnesia and potash,
is here again well exemplified. It would be interesting to have an
analysis of the Brahmaputra water to compare with that of the Ganges.
That tea should flourish for twenty to thirty years in such soils,
is a good indication of one cause at least of the total failure of
tea culture in California, where tea plants are difficult to maintain
alive, and after 25 years form rounded, scrubby bushes not over four
feet high. Similar failures of tea on calcareous soils are on record
from India. The low lime-content of the Assam soils, then, does not
necessarily imply that these soils should be limed to maintain tea
production. According to Mann, the main deficiency is in nitrogen, as
the figures imply; but whether his recommendation of green-manuring
with leguminous crops to increase the nitrogen-supply is practicable
without first supplying more lime to the Assam soils, is questionable.
Since phosphoric acid is also low, his recommendation to use freely the
basic or Thomas slag is doubtless a good one, since lime would thus
also be moderately increased.
Bamber gives a number of analyses of tea soils from low ground in
Assam, which are very rich in vegetable matter and quite acid. Like
those reported by Leather, these “bhil” soils are very poor in lime and
nitrogen, but fairly supplied with potash and phosphoric acid.
_The Regur or Black Cotton Soils of Southern India._—The
second-greatest reasonably uniform soil-area of India is that covered
by the regur, or black cotton soils, in south central India, notably
the Deccan, where these soils are said to have been cultivated without
fertilization for 2000 years and are still fairly productive.[159]
Both in their physical character, chemical composition, and cultural
characteristics, these regur soils are very similar to the “prairie
soils” of the Cotton states and especially to the “black adobe” of
California. Like the latter they are of unusual depth without change of
tint; they crack wide open during the dry season on account of their
high clay content; and the soil is thus partly inverted by the surface
soil falling into the cracks. To the latter fact Leather ascribes, in
part, the long duration of fertility in the regur lands. The regur also
contains fragments of calcareous hardpan (here called guvarayi), just
as in the Great Valley of California. The eighteen analyses of regur
given by Leather agree so nearly in their essential points that it is
admissible to average them; two other examples are however also given
in the table.
[159] That is to say, they now produce about 600 pounds, or 10 bushels
of wheat per acre, as do the Rothamstead soils after fifty years’
exhaustive cultivation. Probably both have come down to the permanent
level of production corresponding to the amount of plant-food made
currently available each year by the fallowing process in originally
very rich soils. The present product of cotton on the regur lands does
not seem to be on record; judging by the wheat product it should not be
over one hundred pounds of lint per acre.
It will be noted that while the contents of lime, magnesia and alumina
are uniformly high, the content of potash has a wide range; it rises
very high (1.14%) in the maximum, while the average is fair.
One conspicuous defect of these soils is their extremely low
content of nitrogen, in view of which their lasting productiveness
is difficult to understand; unless it be that, as in California,
their high lime-content causes a copious crop of leguminous weeds,
constantly replacing the nitrogen supply.[160] Unfortunately we have no
determinations of humus or of its nitrogen-content. Leather attributes
the black color of the regur to some mineral substance rather than
to humus; but his arguments are not quite convincing, so long as the
Grandeau test has not been made. In view of the low rainfall and the
closeness of the texture of regur, it is probable that little if any
nitrates are currently washed out of the black cotton lands.
[160] See Voelcker, Report on the Improvement of Indian Agriculture,
1892, p. 46, par. 60.
The regur soil-sheet seems to be underlaid over the greater part of its
area by a basaltic eruptive sheet (not by metamorphic rocks, as stated
by Leather), and it is not easy to conceive how such a soil stratum can
have been formed from such rocks as a sedentary formation. Elsewhere
such soils are usually rather light and porous, as is the case in the
Hawaiian and Samoan islands; and very high in iron-content. The regur
has the character of an alluvial backwater or lake deposit; but how
such a formation can have occurred on the Deccan plateau, is a question
not easily answered.
_Red Soils of the Madras Region._—Interspersed with and to seaward
of the regur lands there are in the Madras presidency considerable
bodies of “red” lands, which appear to be sedentary soils formed from
underlying dark-colored, mostly eruptive rocks. Some of these are very
rich in lime and potash, others very poor, and it seems impossible
to classify them under any definite category either from the chemical
or physical point of view, except as to their red tint. Even this
tint, however, is not always found associated with exceptionally high
contents of iron oxid, but due rather to its fine diffusion in the
soil mass. As compared with the regur, with which the “red ” areas are
interspersed, these soils contain, on the average, less lime, potash
and ferric oxid; and phosphoric acid is uniformly low. The alluvial
(brown and black) soils from the same region, exemplified in the table,
are doubtless derived partly from the regur, and their color and
composition varies accordingly.
“_Laterite Soils._”—These are defined by Wohltmann (Tropische
Agricultur, 1892) as being “the characteristic sedentary soils
(Verwitterungsböden) of the tropics, formed under the influence of
heavy precipitation, high temperatures and drought.” This definition
does not indicate their derivation from any particular rock, such as
laterite is supposed to be; but its definition puzzles even geologists,
and so, as Leather observes, the definition of laterite soils will
naturally puzzle agricultural chemists. Accordingly it is difficult to
deduce from the analyses given any definite common characters. Leather
describes those analyzed by him as red or reddish, sandy and gravelly,
the gravel or cobbles often incrusted with a dark-smooth crust of
limonite, which to the uninitiated looks as though the rock itself had
been fused and vitrified. The samples from Lohardaga and Singhbhum show
the effects of these limonite crusts upon the composition of the soils,
which resembles that of the Hawaiian soils mentioned above; but in the
latter the iron oxid is wholly pulverulent. But it is probable that, as
in the case of the latter, the high content of phosphoric acid shown in
the statement (.64 for the Lohardaga soil) is tightly locked up in the
insoluble form of ferric phosphate. Wohltmann’s definition of laterite
soils seems best represented by the “terra roxa” of Brazil, which as he
states has .02 to.08% of potash, .02 to.10% of lime, and .045 to .10%
of phosphoric acid. Humus and nitrogen are very deficient in all these
soils.
While most prominent in the coast region of Bengal, they also occur
not only near Madras (Saidapet) but also in the belt of high rainfall
on the Malabar (western) coast of the Indian peninsula.
The productiveness of the laterite soils seems throughout to be only
moderate, yet much higher than would be expected of soils of similar
composition in the temperate zones, where the rate of soil-formation is
so much slower than in the tropics.
From the analyses of “coffee soils” from Yarcand in the Sheveroy hills,
north of Madras, we learn that coffee does well with a fairly liberal
supply of lime (.30 to.44%) and phosphoric acid, but is satisfied with
a much smaller amount of potash than is found in the tea soils of Assam.
A farther systematic investigation of the soils of India, with
simultaneous accurate observations on their depth, subsoil, geological
derivation, topographical location and relations to rainfall, could
not fail to yield very important practical results. The examination of
samples collected and sent in by persons unfamiliar with the proper
mode of taking soil specimens, and the information which should
accompany them, always involves a great deal of uncertainty and waste
of labor, and indefiniteness of results.
INFLUENCE OF ARIDITY UPON CIVILIZATION.
In connection with the facts given and discussed above, as to the
relative productive capacity of lands of the humid and arid regions,
it becomes of interest to consider what influence, if any, these
differences may have had in determining the choice of the majority of
the ancient civilizations in favor of countries where nature imposes
upon the husbandman, who supplies the prime necessaries of life, the
onerous condition of artificial irrigation.[161]
[161] Verhandlungen der Deutschen Physiologischen Gesellschaft in
Berlin, December, 1892; North American Review, September, 1902.
_Preference of Ancient Civilizations for Arid Countries._—A brief
review suffices to establish the fact of such choice. Aside from Egypt,
the permanent fertility of which is ascribed to the inundations of the
Nile, we find to westward the oases of the Libyan and Sahara deserts,
the high fertility of which has become proverbial and has caused them
to be cultivated from ancient times to the present. Similarly, on both
sides of the Mediterranean Sea, we find that, instead of the humid
forest country, it was in the arid but irrigable coast countries, such
as the vegas of Valencia, Alicante, Granada, Malaga, and the even
more arid domain of which Carthage was the metropolis; and farther
east, in the Graeco-Syrian archipelago and the adjacent coasts, that
noted centers of civilization were developed and maintained. Thence
the arid belt requiring irrigation extends from Egypt and Arabia to
Palestine, Syria, Assyria, Mesopotamia and Persia, and across the Indus
through the anciently recognized regions of Indian civilization—Sindh,
the Panjab, Rajputana and the Northwestern provinces—to the Ganges,
embracing such well-known centers as Lahore, Delhi, Meerut, Agra,
etc., inhabited by much more hardy and progressive races than the
humid and highly productive tropical portions of the Indian peninsula.
Throughout the extensive and important portion of northern India,
irrigation is necessary to maintain regular production; and in default
of it, periodic famines ravage the country. Thousands of years ago,
millions upon millions of treasure were expended there upon irrigation
works, as has again been done in modern times; yet in the rainy,
forested districts we still find large areas practically tenanted by
wild beasts. In Asia Minor, as well as in Central Asia, the remains of
ancient cities once surrounded by richly productive irrigated fields,
are found where at present only the herds of nomads pasture. The
Khanates of southern Turkestan with their historic cities, illustrate
the same obstinate bias in favor of arid climates. Similarly, in the
New World, it was not in the moist and exuberantly fertile forest lands
of the Orinoco and Amazon, but on the arid western slopes of the Andes,
that the civilization of the Incas was developed. In Mexico, also, it
was the high central, arid plateau, not the bountifully productive
_tierra caliente_, over which the Aztecs chose to establish the main
centers of their empire. Even to northward, the inhabitants of the
high, dry plains of Arizona and New Mexico were, as their descendants
of the Pueblos are to-day, superior in social development to their
forest-dwelling neighbors of the Algonquin race. From time immemorial
they have practiced irrigation in connection with cultivation,
maintaining a comparatively dense population on very limited areas.
It might be thought that the desire to avoid the labor of clearing the
forest ground was the motive which guided the choice of the ancient
nations toward the cheerless-looking, treeless regions.
But if we consider the cost and labor of establishing and maintaining
irrigation ditches, it certainly seems that a stronger motive, based on
the intrinsic nature of the case, must have influenced their selection.
Neither can we with any degree of plausibility ascribe the preference
for the arid open country to the fear of enemies lurking in the forest,
since war was in early times practically the normal condition of
mankind, and was waged with little hesitation wherever booty was in
sight. It has also been asked how the ancients could have known of the
high productive capacity of arid lands; but no one who has ever seen
the springing-up of luxuriant vegetation after the periodic overflows
of the arid-region streams, or the same surrounding the springs in the
deserts, would ask that question.
_Irrigation necessitates Co-operation._—Irrigation enterprises
can be accomplished in a very limited degree only by individuals
or even families. Its permanently successful execution requires
the co-operation of at least several social groups, ultimately of
communities and states, if it is not to give rise to acrimonious
contentions or actual warfare; witness the “shotgun policy” resorted to
in the arid West in times not very remote. Irrigation, in other words,
compels co-operative social organization quite different from and far
in advance of that necessary in humid countries. And such organization
is manifestly conducive to the preservation and development of the arts
of peace, which means civilization. The most ancient systematic code of
laws known to us is that of Hammurabi, the king of arid Assyria.
_The high and permanent productiveness of arid soils induces permanence
of civil organization._—In humid countries, as is well known,
cultivation can only in exceptional cases be continued profitably
for many years without fertilization. But fertilization requires a
somewhat protracted development of agriculture to be rationally and
successfully applied in the humid regions, and the Germanic tribes,
like the North-American Indians, seem to have shifted their culture
grounds frequently in their migrations. No such need was felt by the
inhabitants of the arid regions for centuries, for the native fertility
of their soils, coupled with the fertilizing effects of irrigation
water bringing plant-food from afar, relieved them of the need of
continuous fertilization; while in the humid regions, the fertility
of the land is currently carried into the sea by the drainage waters,
through the streams and rivers, causing a chronic depletion which has
to be made up for by artificial and costly means. What with the greater
intrinsic fertility and the great depth of soil available for plant
growth, much smaller units of land will suffice for the maintenance of
a family in arid countries; a fact which is even now being illustrated
in the irrigated region of the United States, where ten acres of
irrigated land instead of 40 or 160, as in the East, form the unit.
The arid regions were, therefore, specially conducive to the
establishment of the highly complex polities and high culture, of
which the vestiges are now being unearthed in what we are in the habit
of calling “deserts;” the very sands of which usually need only the
life-giving effects of water to transform them into fruitful fields
and gardens. It is also quite natural that the wealthy and prosperous
communities so formed should in the course of time have excited the
cupidity of the “barbarous” forest-inhabiting races, and as history
records, have been over and again overwhelmed by them—a similar fate
often afterwards overtaking the conquerors in their turn, after the
Capuan ease of their existence had weakened their warlike prowess. At
the present time, the arid regions of the old world are still largely
suffering from having been overrun by the nomadic Turanians, whose
original habitat—Mongolia and Turkestan—while also arid, does not
permit of the ready realization of the advantages above outlined, on
account of the rigorous climate brought about by altitude. Mahometanism
first expelled, and has since repelled, occidental civilization from
the arid regions of the Old World, remaining to-day as an obstacle
to its progress. The peaceful aggression of railroads and telegraphs
now seems likely to gradually overcome this repulsion; and when
Constantinople and Bagdad shall be linked together by the steel bands,
the desert will lose its terrors, and Mesopotamia and Babylonia will
again become garden lands, as of old, under the abundant waters of the
Euphrates and Tigris. Until the water-supplies of the arid countries
shall have been more definitely gauged, it is impossible to foretell
to what extent food-production may be increased by their cultivation
under irrigation, after the relief from political misrule shall have
rendered such undertakings safe. But it can even now be foreseen that
with improved modern methods of cultivation, the productive area of
the world can be vastly increased by the utilization of the countries
where, as the Turcomans say, “the salt is the life of the land.”