Soils, their formation, properties, composition, and relations to climate and plant growth in the humid and arid regions

Chapter XIX

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THE ANALYSIS OF VIRGIN SOILS BY EXTRACTION WITH STRONG ACIDS.

As stated already, the analysis of soils by extraction with strong acids is intended to enlighten us, not in regard to their _immediate_ productiveness (the “Düngerzustand” of German agricultural chemists), but as to their _permanent value or productive capacity_. As has been seen in the preceding chapter, the efforts to unite investigators upon a generally applicable and acceptable method for the testing of immediate productiveness have not been very successful, and the number of methods employed in different countries and by different chemists within the same country are widely at variance, with no immediate prospect of agreement. Moreover, in most cases the effort is to combine both problems—_temporary_ and _permanent_ productive capacity—in _one_ method or operation; which still farther confuses the issue.

Convinced that the only way to unification lies in the direction of falling back upon a method that is based upon a natural limitation about which there can be no difference of opinion, the writer has, in following the lead of Owen and Robert Peter, endeavored to settle definitely _the natural limit of the action of a suitable acid upon soils, and the time and strength of acid producing the maximum effect_.

_Loughridge’s Investigation._—Systematic work on these points was undertaken, at his suggestion, by Dr. R. H. Loughridge in 1871 and 1872. The results of this work were published in the succeeding year in the Amer. Journal of Science, and in the proceedings of the A. A. A. S. for 1873. They seem to be of sufficient general interest to be reproduced here.

The soil selected for this purpose was a very generalized one, representing large areas in the states of Kentucky, Tennessee, Mississippi and Louisiana, bordering on the east the immediate valley of the Mississippi river, and known locally as the “Table lands;” a noted cotton-producing upland region. The brown or yellow, moderately clayey loam is of great uniformity throughout its region of occurrence, and is evidently derived from such widely-spread sources that it represents no special rock or complex of rocks. Its natural growth is a mixture of oaks and hickories, strong and well-developed trees, such as any land-seeker would at once approve for settlement. Its cotton product when fresh was a 400-pound bale of cotton lint per acre. It may therefore well be considered a typical generalized soil of the humid upland of the Mississippi valley. Its physical analysis is given in chapter 6, it being No. 219 of the table on p. 98.

_Strength of Acid used._—Three different strengths of acid were simultaneously employed, viz., chlorhydric of 1.10, 1.115 and 1.160 density. With these the soil was digested at steam heat in porcelain beakers covered with watch glasses for five days each, then evaporated and analyzed as usual. The results were as follows:

ANALYSIS WITH ACID OF DIFFERENT STRENGTHS.
=========================+=========================
Ingredients. | Sp. G. of Acid.
-------------------------+--------+--------+-------
| 1.10 | 1.115 | 1.160
+--------+--------+-------
Insoluble residue | 71.88 | 70.53 | 74.15
Soluble silica | 11.38 | 12.30 | 9.42
Potash | .60 | .63 | .48
Soda | .13 | .09 | .35
Lime | .27 | .27 | .23
Magnesia | .45 | .45 | .45
Br. ox. Manganese | .06 | .06 | .06
Ferric Oxid | 5.15 | 5.11 | 5.04
Alumina | 6.84 | 8.09 | 6.22
Sulfuric acid | .02 | .02 | .02
Volatile matter | 3.14 | 3.14 | 3.14
| ------ | ------ | -----
| 100.02 | 100.69 | 99.29
| | |
Amount of soluble matter | 24.00 | 27.02 | 22.27
Amount of soluble bases | 13.50 | 14.70 | 12.83
-------------------------+--------+--------+-------

It will be noted that the strongest acid produced the smallest amount of decomposition of the soil silicates, _e. g._ the silica soluble in carbonate of soda solution being 3% less than in the case of the acid of medium strength; a result possibly due to some difficultly-soluble compound formed on the surface of the soil grains. The weakest acid had a stronger solvent power; but the maximum effect was produced by the acid of 1.115 density. This being also the most readily obtainable, by simple steam distillation of acid of any other strength, the writer adopted it as best suited to the purposes of soil analysis.

To ascertain the time required for the desired action, viz., the solution of the plant-food ingredients to the extent likely to be of any avail to growing plants, digestions of the same soil were made in the same manner for periods of 1, 3, 4, 5 and 10 days, with the acid of 1.115 density. The results were as follows:

ANALYSIS AFTER DIFFERENT TIMES OF DIGESTION.
=========================+==================================
| No. of Days’ Digestion.
Ingredients. +------+------+------+------+------
| 1 | 3 | 4 | 5 | 10
-------------------------+------+------+------+------+------
Insoluble Residue | 76.97| 72.66| 71.86| 70.53| 71.79
Soluble Silica | 8.60| 11.18| 11.64| 12.30| 10.96
Potash | .35| .44| .57| .63| .62
Soda | .06| .06| .03| .09| .28
Lime | .26| .29| .28| .27| .27
Magnesia | .42| .44| .47| .45| .44
Br. Ox. Manganese | .04| .06| .06| .06| .06
Ferric Oxid | 4.77| 5.01| 5.43| 5.11| 4.85
Alumina | 5.15| 7.38| 7.07| 7.88| 7.16
Phosphoric acid | .21| .21| .21| .21| .21
Sulfuric acid | .02| .02| .02| .02| .02
Volatile matter | 3.14| 3.14| 3.14| 3.14| 3.14
+------+------+------+------+------
Total | 99.63|100.68|100.55|100.69| 99.80
|
Amount of soluble matter | 19.67| 24.88| 25.57| 27.02| 24.87
Amount of soluble bases | 11.05| 13.68| 13.91| 14.49| 13.68
-------------------------+------+------+------+------+------

While these results pointed clearly to the five-day period as being sufficiently effective so far as the plant-food ingredients are concerned, it was not easy to understand why a ten-day digestion should be less incisive than a five-day one. Instead of repeating the ten-day experiment, it was thought preferable to re-treat the residue from the five-day digestion for five days more. The result was that only more silica and alumina went into solution—in other words, additional clay was alone being decomposed. This being of no interest in the matter of plant nutrition, the five-day period was definitely adopted by the writer for his work; and it, together with the acid of 1.115 density, is the basis of all the results given in this volume, except where otherwise stated. There appeared to him to be no good reason for the acceptance of the arbitrary method of soil-extraction suggested by Kedzie and since adopted by the Association of Official Agricultural Chemists; the more as to do so would throw out of comparison all the previous work done by Owen, Peter, and himself and his pupils, which had already been definitely correlated with the natural conditions and with cultural experience.[117]

[117] While regretting to thus “secede” from the fellowship of his colleagues, the writer cannot but regret equally their voluntary decision to do over again, or lightly reject, all that had been done before in correlating soil-composition and plant-growth. He still thinks that it is idle to expect any unification, national or international, of methods of soil analysis based upon purely arbitrary prescriptions, unless previously shown to be definitely correlated with natural and cultural conditions; as is measurably the case with Dyer’s method.

_Virgin Soils with High Plant-food Percentages are Always Productive._—In strong contrast to the contradictory evidence deduced from the analysis, by any method, of cultivated soils when compared with cultural experience, it seems to be generally true that _virgin soils showing high percentages of plant-food as ascertained by extraction with strong acids_ (such as hydrochloric, nitric, etc.), _invariably prove highly productive_: provided only that extreme physical characters do not interfere with normal plant growth, as is sometimes the case with heavy clays, or very coarse sandy lands.—_To this rule no exception has thus far been found._ The composition of some representative soils falling within this category is given in the annexed table, which at the same time conveys some idea of the proportion of acid-soluble ingredients usually found in the best class of natural soils.

TABLE EXEMPLIFYING HIGH PLANT-FOOD PERCENTAGE IN SOILS.

(A) = Buckshot soil. Yazoo Bottom.
(B) = Black Prairie. Rankin County.
(C) = Loamy Sediment. Houma, Terrebonne parish.
(D) = Rio Grande Bottom. Sandy Sediment.
============================+===================+==========+=========
| Mississippi |Louisiana.| Texas.
+---------+---------+----------+---------
| | | |
| (A) | (B) | (C) | (D)
| (Heavy Clay). | (Loam). |
----------------------------+---------+---------+----------+---------
Number of Sample | 390 | 188 | 240 | 37
----------------------------+---------+---------+----------+---------
Chemical Analysis of | | | |
Fine Earth. | | | |
Insoluble matter | 51.06 | 69.95 | 35.48 | 36.04
| 71.77| 74.40| 56.24 | 53.30
Soluble silica | 20.70 | 4.46 | 20.76 | 17.26
----------------------------+---------+---------+----------+---------
Potash (K₂O) | 1.10 | .90 | 1.03 | 1.31
Soda (Na₂O) | .33 | .24 | .13 | .22
Lime (CaO) | 1.35 | 1.04 | .72 | 14.43
Magnesia (MgO) | 1.67 | .91 | .88 | 1.53
Br. ox. of Manganese (Mn₃O₄)| .12 | .12 | .014 | .07
Peroxid of Iron (Fe₂O₃) | 5.82 | 4.77 | 7.10 | 4.09
Alumina (Al₂O₃) | 10.54 | 7.25 | 15.45 | 9.11
Phosphoric acid (P₂O₅) | .30 | .47 | .15 | .20
Sulfuric acid (SO₃) | .02 | .16 | .25 | .04
Carbonic acid (CO₂) | | | | 9.91
Water and organic matter | 7.37 | 10.74 | 18.52 | ?
----------------------------+---------+---------+----------+---------
Total |100.38 |101.01 |100.48 |100.22
----------------------------+---------+---------+----------+---------

(E) = San Diego Co. Colorado Bottom. Silt Sediment.[118]
(F) = Riverside Co. Palm Valley. Micaceous Sandy Soil.
(G) = Tulare Co. Experiment Station. Plains Loam.
(H) = Solano Co. Putah Valley. Dark Loam.
(I) = San Luis Obispo Co. Arroyo Grande Dark Loam.
============================+=========================================
| California.
+---------+------+------+--------+--------
| | | | |
| (E) | (F) | (G) | (H) | (I)
| | | | |
----------------------------+---------+------+------+--------+--------
Number of Sample | 506 | 1092 | 1159 | 110 | 2061
----------------------------+---------+------+------+--------+--------
Chemical Analysis of | | | | |
Fine Earth. | | | | |
Insoluble matter | 58.57 | 71.45| 72.98|67.33 |53.43
| 63.90| | | 71.00| 72.43
Soluble silica | 5.33 | 5.50| 6.60| 3.67 |19.00
----------------------------+---------+------+------+--------+--------
Potash (K₂O) | 1.18 | 1.42| 1.20| .93 | .67
Soda (Na₂O) | .16 | .18| .52| .12 | .18
Lime (CaO) | 8.67 | 2.20| 1.86| .77 | 2.11
Magnesia (MgO) | 2.97 | 2.09| 1.81| 2.29 | 2.26
Br. ox. of Manganese (Mn₃O₄)| .03 | .05| .08| .11 | .06
Peroxid of Iron (Fe₂O₃) | 4.14 | 6.68| 6.86| 8.01 | 5.23
Alumina (Al₂O₃) | 8.40 | 5.78| 5.66| 9.16 | 7.40
Phosphoric acid (P₂O₅) | .13 | .35| .10| .11 | .71
Sulfuric acid (SO₃) | .15 | .01| .03| .12 | .22
Carbonic acid (CO₂) | 7.82 | .18| | | 1.82
Water and organic matter | 3.34 | 4.29| 2.54| 7.12 | 6.63
----------------------------+---------+------+------+--------+--------
Total |100.89 |100.18|100.24|99.74 | 99.72
| | | | |
Humus | | | | | 3.06
Nitrogen in humus | | | | | 22.00
Nitrogen in soil | | | | | .67
Hygroscopic moisture. | | | | |
absorbed at 15°C | | | | | 10.70
----------------------------+---------+------+------+--------+--------
Available phos. acid .14
Available potash .14

[118] The Rio Grande and Colorado bottom soils contain amounts of lime carbonate largely in excess of requirements, 2 to 3% of that compound being all that is needed to insure all the advantageous effects of lime in any soil (see this chapter, page 367).

_Discussion of Table._—It will be noted in this table that while the total of the matters soluble in acids (inclusive of silica) ranges from a little below 50 to over 77 per cent, the total of directly important mineral plant-food ingredients (potash, lime, magnesia and phosphoric acid), constitute in moderately calcareous soils only from about 2.5 to somewhat over four per cent of the whole. Yet if all these were in available form, the supply would be abundant for many hundreds and even thousands of crop years. For, one-tenth of one per cent in the case of the clayey soils of the preceding table would amount to about 3500 pounds per acre-foot, and to 4000 in the case of the sandy ones. Hence the amount of phosphoric acid in _e. g._, the Mississippi delta soil from Houma would suffice for the production of about 440 crops of wheat grain (at 20 bushels per acre) if only one foot depth were drawn upon; but as the roots of grain easily penetrate to twice and half and three times that depth even in the humid region, the number might be tripled. As a matter of fact, however, that soil has produced full crops for from forty to fifty years only; yet this is considered an exceptionally long duration of profitable production without fertilization.

The first and last soils in the above list represent
probably the highest types of productiveness known. The
Yazoo bottom soil has produced up to one thousand pounds of
cotton lint per acre when fresh, and is still producing from
four to five hundred pounds after thirty years’ culture.
The Arroyo Grande soil of California with its extraordinary
percentages of phosphoric acid and nitrogen, as well as
exceptionally high proportion of available phosphoric acid
and potash, has made such a record of productiveness, and
high quality of the seeds produced, that it has for a
number of years been excluded from competition for prizes
offered by seed-producers elsewhere, in order to give other
sections a chance. Both these soils are rather heavy clays,
but readily tillable in consequence of their abundant
lime-content. The remarkably high content of acid-soluble
silica, indicating the presence of much easily available
zeolitic matter, is doubtless connected with the exceptional
productiveness.

Experience, then, proves that lands showing such high plant-food percentages will yield profitable harvests for a long time without fertilization, or with only such partial returns as are afforded by the offal of crops. Also that when fertilization comes to be required, instead of supplying _all_ the ingredients usually constituting fertilizers, only one or two of these will as a rule be actually needed, and even these in smaller amounts than in “poor” lands; thus materially reducing the expense of fertilization. The high production and durability of such lands therefore amply justify their higher pecuniary valuation; for which there would be no rational permanent ground if they required fertilization to the same extent as poor lands. In other words, if the entire amount of soil-ingredients removed by crops had had to be currently replaced equally in _all_ cases (as is implied in the hypothesis, advanced by some, that the chemical composition of soils is of no practical consequence), the high prices which from time immemorial have been paid for black prairie and rich alluvial lands as against meagre uplands and barrens, would have been so much money wasted.

The explanation of these advantages evidently lies largely in the larger amounts of soil ingredients annually rendered available in rich soils by the fallowing effect of the atmospheric agencies, because of the generous totals present. The actual _amounts_ of soil ingredients thus rendered accessible to plants, other things being equal, are evidently more or less directly _proportional to the totals of acid-soluble plant-food ingredients present_. And if this is true in cultivated lands, the inevitable conclusion is that the same must be true of virgin lands; _whose productive capacity and duration can therefore be forecast by such analyses_. It will be observed that the above data, which could be indefinitely increased by corroborative analyses, seem to establish the fact that about one per cent of acid-soluble potash, one of lime, the same, or less, of magnesia, and .15% of phosphoric acid, are thus shown to be “high” percentages of these ingredients in virgin soils.

It is not easy to see how the above conclusions can be successfully controverted; they are, moreover, thoroughly in accordance with cultural experience. Difficulties of interpretation arise mainly in the case of medium soils, which show neither very high nor very low percentages of plant-food; and which raise the question of what amount or percentage constitutes “adequacy” of each of the several substances.

_Low Percentages._—On the other hand, whenever in virgin soils acid-analysis shows the presence of but a _very_ small proportion of one or several of the essential ingredients, we have a valuable indication as to the one of these that will first be required to be added when production slackens.

_What are “Adequate” Percentages of Potash, Lime, Phosphoric Acid and Nitrogen?_—It is evident that a very critical discussion of cultural experience can alone answer this question; and at first sight such experience often appears very contradictory when compared with the results of analysis.

One of the chief causes of such apparent discrepancies is
readily intelligible when we consider the differences in
root-development of the same plant in different soils. In
“light” or sandy lands the roots may penetrate to several
times the depth attained by them in heavy clay soils. Having
thus within their reach a soil-mass several times larger,
and aerated to a much greater depth, it is but reasonable
to expect that in deep, sandy lands plants would do equally
well with correspondingly smaller percentages of plant-food
than would suffice in clay soils, in which the root-range
is very much more restricted. The well-known fact that the
production of heavy clay lands may be increased by their
intermixture with mere sand, adding nothing to their store
of plant-food, emphasizes this expectation and elevates it
into a maxim. On this ground alone, therefore, it is evident
that the mere consideration of plant-food _percentages_
found, can be a true measure of productiveness only in the
case of virgin soils with _high_ percentages.

_Soil Dilution Experiments._—The extent to which dilution with mere “lightening” materials can be carried without impairing production, can of course be determined for concrete cases only; but the following experiment made at the California Station is a case in point:

One kilogram of the heavy but highly productive black clay soil of the experimental grounds of the University of California was used in each of five experimental cultures, each made in duplicate, in cylindrical vessels of zinc-covered (“galvanized”) sheet iron, all proportioned alike in height and diameter, but containing respectively one, two, four, five and six volumes of total soil. In the smallest was placed one kilogram of the undiluted, original soil, in the others successively the same amount of the soil thoroughly mixed with one, three, four, and five volumes of a dune sand fully extracted with chlorhydric acid, and washed with distilled water. The water capacity of each of the mixtures was determined and the earth in the pots kept at the point of half-saturation generally admitted to be the optimum (best condition) for plant growth. Each pot was sown with ten seeds of white mustard, subsequently reduced to five plants selected for their vigor.

DEVELOPMENT OF ROOTS OF WHITE MUSTARD IN CLAY SOIL, DILUTED WITH VARIOUS PROPORTIONS OF PURE SAND.]

The (“galvanized”) vegetation pots were made as nearly as possible of similar proportions in depth and width for each dilution, so as to give opportunity for the proportional development of the root systems. The photographs show the latter as nearly as practicable in their natural form, restored after washing off the adherent soil. It was of course extremely difficult to preserve intact the extreme circumferential rootlets and hairs; yet the general development is correctly shown.

The following table shows the percentage composition of the original as well as the diluted soils, while the photographs show the development of the plants in their successive stages, so far as these could be observed; the continued attacks of mildew and plant lice preventing full maturity being attained.

COMPOSITION OF BLACK ADOBE AND SAND DILUTIONS.
============================+========+==============================
|Original| Dilutions.
Chemical analysis of | soil. |
fine earth. | 1:0 | 1:1 1:3 1:4 1:5
----------------------------+--------+-------+-------+-------+------
Insoluble matter | 54.50 | 77.25 | 88.62 | 90.00 | 92.42
Soluble silica | 19.60 | 9.50 | 4.75 | 3.80 | 3.17
Potash (K₂O) | .73 | .36 | .18 | .15 | .12
Soda (Na₂O) | .20 | .10 | .05 | .04 | .03
Lime (CaO) | 1.15 | .57 | .29 | .23 | .19
Magnesia (MgO) | 1.08 | .54 | .27 | .22 | .18
Br. ox. of Manganese (Mn₃O₄)| .04 | .02 | .01 | .01 | .01
Peroxid of Iron (Fe₂O₃) | 8.43 | 4.22 | 2.11 | 1.68 | 1.40
Alumina (Al₂O₃) | 7.92 | 3.96 | 1.98 | 1.58 | 1.32
Phosphoric acid (P₂O₅) | .19 | .10 | .05 | .04 | .03
Sulfuric acid (SO₃) | .04 | .02 | .01 | .01 | .01
Carbonic acid (CO₂) | | | | |
Water and organic matter | 6.54 | 3.27 | 1.64 | 1.31 | 1.09
Loss in analysis | 1.18 | .09 | .04 | .03 | .03
+--------+-------+-------+-------+------
Total | 100.00 |100.00 |100.00 |100.00 |100.00
| | | | |
Humus | 1.21 | .60 | .30 | .24 | .20
“ Ash | .94 | .47 | .23 | .19 | .16
“ Nitrogen, p. cent | | | | |
in Humus | 18.58 | 18.58 | 18.50 | 18.58 | 18.58
“ “ p. cent. | | | | |
in soil | .203| .10 | .05 | .04 | .034
----------------------------+--------+-------+-------+-------+------

The restricted volume of soil occupied by the roots in
the undiluted adobe soil, together with the very abundant
development of root-hairs, is very striking. A marked
change in these respects is manifest in the first dilution,
and increasingly so as dilution increases; the paucity of
root-hairs is very marked in the last (greatest) dilution,
in which, as the photograph of the plants shows, the
development was decidedly behind that in the pot containing
dilution 1:4. The latter in fact showed the best development
not only in this case, but in two other series of tests
conducted at the same and subsequent times; and strangely
enough, also in the pulverulent, “sandy loam” soil of the
southern California substation tract. In the latter series,
which for lack of space cannot be figured here, the main
difference was that in the undiluted soil the roots filled
the entire soil mass, instead of remaining near the surface,
as in the pure adobe. It is possible that the latter was
too wet when given the full half of its water-capacity,
although, as the figures show, the water was slowly
introduced from below by means of glass tubes, ending within
a shield to prevent puddling.

_Limitation of Root Action._—These results, representing five soils of different percentage-composition and physical character, but identical chemical composition and ratios between the several ingredients, and similarly acted upon by the atmospheric agencies in the past, illustrate strikingly the impossibility of judging correctly of a soil’s productiveness from _percentages_ of chemical ingredients alone. It is clear that the physical characters of the land as well as its depth, must be essentially taken into account. But there is obviously a certain limit beyond which greater perviousness and root-penetration cannot make up for deficiency in the absolute amounts of plant-food within possible reach of the plant; for in the case of excessive dilution these are rendered partially inaccessible within the time-limits of a season’s growth.

It is hardly necessary to say that these experiments require repetition with the aid of the experience acquired in these first trials, not only in the laboratory but also in the field. It will be especially interesting to compare with the results obtained in these strongly calcareous soils, the effects of dilution in such soils as those of Florida, mentioned below; the probability being that where lime is naturally deficient, the effects of dilution will be much more pronounced in diminishing production, because of the absence of the previous favorable action of lime upon the availability of the soil-ingredients.

_Lowest Limit of Plant-food Percentages and Productiveness found in Virgin Soils._—The subjoined table shows some of the very low plant-food percentages found in natural soils, all being of a sandy character:

============================+====================================
| MISSISSIPPI SOILS.
+----------+--------+--------+-------
|Homochitto| Shell | Pine | Pine
| Bottom. |Hammock.| Woods. |Flats.
----------------------------+----------+--------+--------+-------
Number of Sample. | 68 | 83 | 206 | 214
----------------------------+----------+--------+--------+-------
CHEMICAL ANALYSIS OF | | | |
FINE EARTH. | | | |
----------------------------+----------+--------+--------+------
Insoluble matter | | | |
| 92.16 | 96.08 | 93.23 | 95.59
Soluble silica | | | |
----------------------------+----------+--------+--------+------
Potash (K₂O) | .15 | .05 | .26 | .06
Soda (Na₂O) | .04 | .06 | .07 | .05
Lime (CaO) | .12 | .10 | .12 | .02
Magnesia (MgO) | .21 | .12 | .18 | .07
Br. ox. of Manganese (Mn₃O₄)| .28 | .05 | .15 | .05
Peroxid of Iron (Fe₂O₃) | 1.18 | .52 | 1.25 | .46
Alumina (Al₂O₃) | 3.22 | .46 | 2.36 | .85
Phosphoric acid (P₂O₅) | .08 | .10 | .03 | .02
Sulfuric acid (SO₃) | .05 | Trace | .02 | Trace
Carbonic acid (CO₂) | | | |
Water and organic matter | 2.70 | 3.02 | 2.33 | 2.28
+----------+--------+--------+--------
Total | 100.19 | 100.56 | 100.00 | 99.45
----------------------------+----------+--------+--------+--------

============================+====================
| FLORIDA SOILS.
+--------------------
| Pine Lands.
|-------+------------
| First | Second
|Class. | Class.
----------------------------+-------+------------
Number of Sample. | 6 | 7
----------------------------+-------+------------
CHEMICAL ANALYSIS OF | |
FINE EARTH. | |
----------------------------+-------+------------
Insoluble matter | 94.46 | 95.63
| | 96.51
Soluble silica | 1.67 | .88
----------------------------+-------+------------
Potash (K₂O) | .19 | .12
Soda (Na₂O) | .04 | .06
Lime (CaO) | .07 | .06
Magnesia (MgO) | .04 | .04
Br. ox. of Manganese (Mn₃O₄)| .06 | .05
Peroxid of Iron (Fe₂O₃) | .32 | .22
Alumina (Al₂O₃) | .92 | .47
Phosphoric acid (P₂O₅) | .11 | .09
Sulfuric acid (SO₃) | .09 | .06
Carbonic acid (CO₂) | |
Water and organic matter | 1.88 | 1.81
+-------+------------
Total | 99.85 | 99.49
----------------------------+-------+------------

The average of plant-food percentages in all these soils is quite low, and at first sight there seems to be little choice between them. Yet two of them—Nos. 68 and 88, from Mississippi—are not only quite productive at the outset, but also fairly durable. This becomes measurably intelligible when it is known that both are of great depth, and so well drained that roots can descend for many feet; while the composition of the soil-material is almost identical for three or four feet. On the other hand, both Nos. 206 and 214 are quite shallow, being underlaid by sand almost devoid of plant-food at about two feet. In addition, both have extremely low percentages of phosphoric acid; while the rest show near .10% of that ingredient, an amount which, as will be seen hereafter, is considerably above the recognized limit of deficiency. The two Florida soils however bear only pine; they are underlaid by almost clean sand at two or three feet, and are therefore quickly exhausted. It will also be noted that their lime-percentage is only about half of that of the two first-named Mississippi soils, both of which bear a strong growth of deciduous timber trees, grape vines, and other vegetation indicating the presence of lime carbonate.

It is noteworthy, also, that the popular classification of the two Florida soils corresponds exactly with the differences in the percentages of plant-food; those in the “second-class” soil being uniformly lower than those in the one designated as first-class. This indicates, again, that _as between soils of similar character and origin, the production and durability are sensibly proportional to the plant-food percentages_ when the latter fall below a certain limit; a point more fully illustrated farther on.

In the light of the above experiment and tables, it becomes pertinent to consider what _are_ the lowest percentage limits of each of the more important plant-food ingredients compatible with profitable production.

LIMITS OF ADEQUACY OF THE SEVERAL PLANT-FOODS IN VIRGIN SOILS.

It is obvious that the lower limits of adequacy of the critical plant-food ingredients are best ascertained in the case of virgin soils containing very small amounts of some _one_ ingredient, while fairly or fully supplied with the rest. In such cases, which are not at all infrequent, the use of the deficient ingredient as a fertilizer should produce a very marked effect so soon as the first flush of production (always noted in fresh soil) is over. This first productiveness may, even in poor lands, range from one to three years, when there is a sudden decline.

_Lime a Dominant Factor._—When we investigate the cases of such lands, it soon becomes apparent that besides the low percentage of any one ingredient, the _proportions_ of others present require consideration. Among these, _lime_ in the form of carbonate stands foremost. Its presence exerts a dominant and beneficial influence in many respects, as is readily apparent from the prompt change in vegetation whenever it is introduced into soils deficient in it. In discussing the results of soil analysis, its consideration is of first importance in forecasting correctly the adequacy or inadequacy of other soil ingredients (see chapter 20, page 379). For in general, we find that _lower percentages of potash, phosphoric acid and nitrogen are adequate, when a large proportion of lime carbonate is present_.—This has already been referred to in connection with the table of soils of low percentages, given above. In the interpretation of results obtained by analysis this point must always be kept in view; and in the numerical statements made below, it must be understood that they refer to virgin soils sufficiently supplied with lime to assure a constant excess of lime carbonate, maintaining the conditions of nitrification and insuring the absence of acidity. (See chapter 9, page 146).

_Potash._—In respect to potash, the writer was led by his early investigations in the State of Mississippi to conclude that less than one-fourth of one per cent (.25) of potash constituted a deficiency likely to call for early fertilization with potash salts; while as much as .45% of the same seemed to cause the land to respond but feebly to such fertilization. He has not found it necessary to revise materially that early conclusion, whether from his own work or from that of others. Within the last decade, Prof. Liebscher of Göttingen[119] has arrived at this identical figure from analyses made of soils upon which he had conducted a seven-year series of fertilizer tests; he having found that potash fertilization produced no sensible, or at least no paying results on land giving that figure, and otherwise well provided with plant-food. The different (lower) figures given by Schloesing, Risler and other French chemists in discussing the soils of France are doubtless due to the weak acid and short period of digestion employed in the analysis; an unfortunate discrepancy of methods which precludes any direct comparison of results.

[119] Untersuchungen über die Bestimmung des Düngerbedürfnisses der Ackerböden und Kulturpflanzen, von G. Liebscher; Journal für Landwirtschaft 43 (1895), Nos. 1 & 2, pp. 48-216.

These figures apply both to the arid and the humid regions
in the temperate zones. In the tropics we find very much
lower percentages quoted as adequate; thus in the laterite
soils of India and Samoa, according to Wohltmann, in the
soils of Jamaica according to Fawcett, and in those of
Madagascar according to Müntz and Rousseaux.[120] There,
potash-percentages over .10% seem to be high, and in
Madagascar some lands in fair production range as low as
.01%. The soil-extractions have however in these cases been
made with a weaker acid than above specified, so that some
increase of the figures (perhaps 33 to 50%) have to be
allowed for. But even then there can be no question that a
far less amount of potash, as determined by acid-extraction,
is found sufficient for crop production in the tropics;
doubtless because of the very intense decomposing
(“fallowing”) effect of the continuous heat and moisture,
tending also to a rapid decomposition of organic matter and
a proportionally rapid formation of carbonic and nitric
acids. Such soils are of course constantly kept in a leached
condition, as a result of the heavy and continuous rainfall.

[120] _La Valeur Agricole des Terres de Madagascar._ Ann. de la Science Agronomique, 2’me série, tome 1, 1901.

_Phosphoric Acid._—As regards the lower limit of adequacy of phosphoric acid, there is a remarkable agreement in the investigations made everywhere. It was placed at .05% by the writer as long ago as 1860, as the result of investigations made in the State of Mississippi; and the same figure has since been arrived at independently by agricultural chemists in France, Russia, Germany and England. The cause of this remarkable agreement is undoubtedly the readiness with which the phosphates that come under consideration at all for the nutrition of plants, are dissolved by almost any acid treatment likely to be used in soil analysis. Almost the same agreement exists in regard to the “adequacy” of .1% of P₂O₅; while all soils showing percentages between .1 and .05% are considered weak on this side, and liable to need phosphate fertilization soon. One-fourth of one per cent is an unusually high percentage in most countries; .30% and over is exceptional in non-ferruginous soils. But as stated on a previous page, a high percentage of lime carbonate may offset a smaller percentage of phosphoric acid, apparently by bringing about greater availability; and a similar effect seems to result from the presence of a large supply of humus.

On the other hand, very large percentages of finely divided ferric hydrate may, especially in the absence of lime carbonate, render even large supplies of phosphoric acid inert and useless, by the formation of the totally insoluble ferric phosphate. Aluminic hydrate probably acts in a similar manner. The following table gives examples in point, as regards ferric hydrate.

HAWAIIAN SOILS SHOWING HIGH CONTENTS OF FERRIC OXID.
(Rept. Cal. Exp. Sta. 1894-5, page 27.)
=============================+===============+=======================
| Oahu. | Hawaii.
-----------------------------+-------+-------+-------+-------+-------
NUMBER OF SAMPLE. |No. 21.|No. 22.|No. 24.|No. 26.|No. 27.
-----------------------------+-------+-------+-------+-------+-------
Coarse Materials. 0.55ᵐᵐ | 2.00 | 2.50 | 4.00 | 3.00 | 5.00
Fine Earth | 98.00 | 97.50 | 96.00 | 97.00 | 95.00
| | | | |
CHEMICAL ANALYSIS OF | | | | |
FINE EARTH. | | | | |
Insoluble matter | 15.84 | 14.49 | 26.99 | 28.66 | 21.07
Soluble Silica | 14.07 | 30.37 | 10.26 | 7.35 | 2.68
Potash (K₂O) | .45 | .26 | .40 | .61 | .44
Soda (Na₂O) | .14 | .08 | .26 | .17 | .25
Lime (CaO) | .26 | 1.04 | .52 | .68 | .28
Magnesia (MgO) | .65 | .80 | .96 | 1.04 | .60
Br. ox. of Manganese (Mn₃O₄) | .05 | .03 | .21 | .20 | .07
Peroxid of Iron (Fe₂O₃) | 39.05 | 19.68 | 19.10 | 18.23 | 30.10
Alumina (Al₂O₃) | 14.61 | 18.29 | 21.41 | 20.18 | 14.38
Phosphoric acid (P₂O₅) | .19 | .32 | .64 | .70 | .97
Sulfuric acid (SO₃) | .03 | .09 | .32 | .21 | .29
Carbonic acid (CO₂) | | | | |
Water and organic matter | 14.18 | 14.59 | 18.60 | 21.65 | 28.60
+-------+-------+-------+-------+-------
Total | 99.52 |100.04 | 99.67 | 99.61 | 99.73
| | | | |
Humus | 3.35 | 3.24 | 4.84 | 5.43 | 9.95
“ Ash | 3.12 | 2.22 | 2.76 | 3.56 | 6.70
“ Nitrogen, p.c. in Humus | 3.30 | 9.800| 2.800| 3.100| 1.71
“ “ , p.c. in soil | .112| .314| .134| .168| .17
Phosph. acid in humus ash | .110| .166| .580| .500|
Soluble in 2% Citric acid | .004| .020| .035| .037| .025
in Nitric acid, 1.20 sp. g.| .190| .320| .640| .700| .970
in Chlorhydric acid | | | | |
(1.115 sp.g.) | .430| .350| 1.600| 1.280|
Hygroscopic moisture 15°C. | 18.50 | 21.25 | 23.07 | 23.14| 23.81
-----------------------------+-------+-------+-------+-------+-------

_Unavailability of Ferric Phosphate._—It will be noted
that in the soils from Oahu with an overwhelming amount
of ferric oxid (mostly in the form of hydrate or rust)
the citric acid has taken up only an insignificant amount
of phosphoric acid; nitric acid took up 40 to 50 times as
much, and chlorhydric doubled even this. In the much less
ferruginous Hawaiian soils, though containing more alumina,
the citric acid extracted nearly ten times as much; proving
that it is chiefly ferric oxid, and not the alumina as has
been supposed, that causes the insolubility of phosphoric
acid in soils and doubtless also in fertilizers. The
very unusually high content of phosphoric acid in the
Hawaiian soils, exceeding all others on record, so far
as known to the writer, emphasize the effects of ferric
hydrate upon soluble phosphates; while the fact that these
very soils are greatly benefited by the use of phosphate
fertilizers, proves that the Dyer (citric acid) method for
the determination of available phosphoric acid which in
soils Nos. 21 to 26 yielded results largely in excess of the
established limit in European soils, cannot be successfully
applied to these highly ferruginous soils. It should also
be noted that the amounts of phosphoric acid found in the
humus extracted by the Grandeau method is in the first
two Hawaiian soils over ten times the amount extracted by
citric acid, but that while they rise and fall together, no
definite quantitative ratio exists between the two.

It is obvious that in such soils, fertilization with water-soluble phosphates would be likely to result in the quick partial withdrawal of the same from useful action, and that any excess not promptly taken up by the crop, is likely to become inert and useless. It will evidently be desirable to use the phosphates in the form of bone meal or basic slag (Thomas Phosphate), which because of their difficult solubility will be acted upon but very slowly, if at all, by the ferric and aluminic hydrates.

_Nitrogen._—In determining the nitrogen-content of the soil, a great variety of methods has been followed. Some include all that can be obtained by the combustion of the organic matters of soil and from the nitrates present in the same; while others, the writer among the number, believe that the mainly important source of nitrogen to the plant being the nitrification of the humus-nitrogen, the determination of the humus by the method of Grandeau, and of the nitrogen contained in it, should be the standard; the unhumified vegetable matter being of no definitely ascertainable value, and the nitrates varying from day to day and being liable to be lost by leaching at any time; therefore forming no permanent feature of the soil. Considering the variety of methods, the unanimity with which about one-tenth of one per cent (.10) has been assumed as the ordinarily adequate percentage is remarkable. In view of the extremely variable amount of nitrogen in the humus (ranging from 1.7 to nearly 22%), the amount of the latter cannot, of course, afford even an approximation to the nitrogen-content; except that as in the humid region, the nitrogen-percentage is not known to exceed about 5 or 5.5%, an approximate estimate can be made on that basis. In the arid region, according to location, the nitrogen-percentage may be from three to six times greater for a similar amount of humus. (See chap. 8. p. 135). In the writer’s experience, a nitrogen-percentage of .1% in the arid region is a very satisfactory figure, indicating that the need of nitrogen-fertilization is not likely to arise for a number of years.

_Nitrification of the Organic Matter of the Soil._—In order to test the question whether or not the nitrogen of the unhumified debris existing in surface soils is directly nitrifiable, the writer selected a soil which in its natural condition sustains intense nitrification, so that at some points it contains as much as 1200 pounds of sodic nitrate per acre. The composition of this soil, representing the land of the “ten-acre tract” of the southern California substation, is as follows:

SOIL FROM “TEN-ACRE TRACT,”
SOUTHERN CALIFORNIA SUB-STATION, NO. 1284.

Coarse Materials > 0.55ᵐᵐ 1.00
Fine Earth 99.00
------
100.00
CHEMICAL ANALYSIS OF FINE EARTH.

Insoluble matter 62.62} 70.92
Soluble silica 8.30}
Potash (K₂O) .95
Soda (Na₂O) .50
Lime (CaO) 5.07
Magnesia (MgO) .84
Br. ox. of Manganese (Mn₂O₄) .06
Peroxid of Iron (Fe₂O₃) 6.43
Alumina (Al₂O₃) 3.88
Phosphoric acid (P₂O₅) .21
Sulfuric acid (SO₃) .06
Carbonic acid (CO₂) 3.66
Water and organic matter 6.02
-----
Total 99.70

Water-soluble matter, per cent. .137
Sodic nitrate, per cent. .020

Humus 1.99
“ Ash 1.13
“ Nitrogen, per cent. in Humus 10.30
“ “ , per cent. in soil .203
Total Nitrogen in soil .330
“ “ in unhumified matter .127
Available Potash {citric} .03
Available Phosphoric acid {method}
Hygroscopic Moisture
absorbed at 15° C. 5.81

It will be noticed that this is a rather strongly calcareous soil, (nearly 9% of calcic carbonate), slightly impregnated with alkali, of which about one-ninth is saltpeter. One portion of this soil was thoroughly leached with distilled water until not a trace of nitrates could be detected in the leachings. Another portion was treated for the removal of humus according to the Grandeau method (see chapter 8, page 132); the extracted soil showed under the microscope an abundance of vegetable debris, some slightly browned as from incipient humification.

The calcic and magnesic carbonates withdrawn in the humus-extraction were then restored to the soil in the form of finely divided precipitates and thoroughly mixed in, first in the dry and then in the wet condition; the extracted soil being repeatedly wetted with turbid water from the leached soil, in order to replace and reinfect it with the nitrifying bacteria. Both soils were then spread out in flat glass dishes and placed in a wooden box containing also a similar flat dish with distilled water, upon which played the draught from the inlet pipe opening into the outer air, with outlet-holes in the cover at the opposite end; thus keeping the air within fairly moist. In addition, the soils themselves were moistened with distilled water every three days and restored to a loose condition by stirring. The whole was placed so as to maintain, during the greater part of the 24 hours, a temperature of from 30 to 35 degrees C. At intervals the samples of both soils were leached and color-titrated for their nitrate content by the picric-acid test. The results, calculated as sodic nitrate, during two years were as follows:

It will be noted that in the course of four months, nitrification had not sensibly set in the extracted soil; while in the leached natural soil the nitrate-content had reached to three-fifths the amount originally present, and in the course of a year the nitrate-content of the latter was more than double that of the original (unleached) soil; while that in the extracted soil had only reached one-seventh of the same. At the end of two years we find a still farther increase of nitric nitrogen in both, the ratio between the two remaining about the same (1:14). At the same time the ratio of increase attained at first had materially diminished in the water-leached soil, probably on account of the accumulation of the niter itself.

It thus appears that although the nitrogen of the unhumified organic matter constituted about 40% of the total in the original soil, it would during the entire year have contributed only to an insignificant extent to the available nitrate-supply; while the fully humified “matière noire” contributed fourteen times as much. During the ordinary growing-season of four or five months the unhumified organic matter would have yielded practically nothing to the crop.

_Functions of the unhumified Vegetable Matter._—The chief utility of the unhumified matter in the soil consists of course in its gradual conversion into true humus, in the course of which it evolves carbonic gas to act on the soil minerals; while at the same time it helps to render the soil more porous and thus facilitates the action of the aerobic bacteria, for which it serves as food. Hence the addition of vegetable matter to soils not already too “light” is always advantageous, so long as it does not introduce injurious, non-humifiable ingredients, like turpentine in the sawdust of resinous pines. But it is always advisable to first use such matter as litter for stock, in order to better prepare it for the processes of humification, under the influence of ammoniacal fermentation, such as occurs in the decay of green plants or animal matter. A portion of the ash ingredients also is quickly utilized by solution in the soil-water.

_Matière Noire the Only Guide._—According to these results it is clear that in order to gain any tangible indications with respect to crop-bearing, it is the nitrogen in the humus proper, the _matière noire_ only, that should serve as the basis; and that as a current source of nitrogen to the plant, the unhumified matter is hardly entitled to more consideration than the “insoluble silicates.” For, the favorable conditions for nitrification under which the above experiment was conducted, will very rarely be even approached under field conditions.

_What are the Adequate Nitrogen Percentages in the Humus_?—The nitrification of the _matière noire_ being, apparently, the main source of plant-nutrition with that element under ordinary conditions, the question naturally arises as to what may be considered an adequate nitrogen-content of that substance, so as to permit a full supply of nitrates to the crop.

The data extant on this subject are rather scanty, and thus far have all been obtained at the California Experiment Station.[121] But they seem to be very cogent in proving that the growth of crops removed from the soil causes a rapid depletion of the nitrogen in the humus-substance, and that _so soon as the nitrogen-percentage in the same falls below a certain point, the soil becomes “nitrogen-hungry;”_ so that the application of nitrogenous fertilizers is needed and is very effective. The data in the table below, as well as the figure of a culture experiment (No. 52 below), illustrate this point.

[121] _The Supply of Soil Nitrogen_, Rep. Cal. Expt. Station, 1892-93, page 68; ibid., 1894-95, page 28; _The Recognition of Nitrogen Hungriness in Soils_, in Bull. 47, Div. of Chemistry, U.S. Department of Agriculture, 1895; Landw. Presse, No. 53, July 1885. See also for detailed data chapter 8, page 135.

[122] Calculated upon the true humus substance (matière noire), _not_ by determining total (incl. unhumified) nitrogen in the soil.

Nos. 6 and 1679 show the usual humus-and
nitrogen-percentages in the “black adobe” or “prairie”
soils of California. Nos. 1842 and 1841 represent the same
soil as 1679, upon which, however, ramie and ray grass had
respectively been growing, without fertilization, for about
ten years; showing that while the _humus-content of the
soil has increased, the nitrogen-content of the humus has
decreased_ in the case of ramie by 72.78%, in that of
the grass by 76.78%; reducing the land to figures commonly
found in the humid region. In the case of the ramie, the
partial return through the leaves has resulted in a higher
humus-content, together with higher nitrogen-percentage,
than in the case of the grass, which in the several cuttings
annually made, caused a greater depletion in nitrogen and a
smaller accession of humus. The grass was very weak in its
growth and partially dying out.

No. 29, the sugar-cane land from Maui, was still in fair
production, but beginning to weaken as against its first
production. No. 27, the guava land from Hawaii, originally
bore a luxuriant cover of wild guava, but after bearing one
fair crop of seed-cane and one of ratoons, the cane planted
on it “spindled up” and died so soon as the seed-cane
planted was exhausted. Both the island soils, originally
derived from the weathering of the black basaltic lavas
of the region, were well supplied with mineral plant-food
(see above, page 356), and the humus-content in both was
exceptionally high; and neither was in an acid condition.
The difference in their nitrogen-content, both in the totals
and in the humus itself, suggested that notwithstanding
the relatively high total of nitrogen in No. 27, it might
be nitrogen-hungry, in view of the low percentage of the
nitrogen in the humus.

_Confirmatory Experiment._—A pot-culture with wheat, the results of which are shown in the figure below, fully confirm this suspicion. One kilogram of soil was used in each of two pots, one being fertilized with half a gram of Chile saltpeter. The experiment could not be carried to full completion on account of the overwhelming invasion of mildew; but the figures speak for themselves. Moreover, a field trial made on the island with saltpeter, in pursuance of the writer’s recommendation, resulted in a luxuriant growth of the cane.

_Data for Nitrogen-adequacy._—It appears from the facts shown above, that for the growth of grasses a nitrogen-percentage in the humus of 1.7 is wholly inadequate, no matter how much humus may be present. A percentage of 3.15 in the Maui soil, No. 29, containing nearly 11% of humus, gave only a fair crop of sugar-cane; on the Berkeley grass plot, with 3.40% and only 1.65 of total humus, the ray grass was barely maintaining life. The ramie, with 4.17% of nitrogen in the soil-humus, was still doing fairly well.

It is doubtless impossible to give one and the same absolute figure for nitrogen-deficiency for all plants and soils. Where the conditions of nitrification are favorable, as in the presence of much of the earth carbonates, a smaller percentage may suffice for the same plants that elsewhere suffer; and it is highly probable that different minima will be found for plants of different relationship and root-habits. But there is every reason to believe that _in the nitrogen-percentage of soil-humus, considered in connection with other chemical and physical conditions and soil derivations_, we have a means of ascertaining the needs of plants with respect to nitrogen-fertilization, if proper study be given to the subject. Broadly speaking, it appears to be necessary _to keep the nitrogen-percentage of soil-humus near 4% to insure satisfactory production_.

It having been suggested that the frequent and disastrous crop failures on the noted tchernozem or black-earth soils of Russia might be due in part at least to nitrogen-depletion of the humus, the writer obtained through the courtesy of Prof. P. Kossovitch of St. Petersburg soil samples from the center of the Black-earth region, both cultivated and uncultivated. These samples are in appearance exactly like some of the dark alluvial soils of Louisiana and California, and approach them very nearly in the essentials of composition, as will be seen from the table below:

ANALYSES OF BLACK SOILS,
=============================+=================+====================
| Tchernozem | Alluvial
| (Russia.) | Black clay lands.
+------+----------+---------+----------
| | |Louisiana|California
| | | No. 240.| No. 1167.
|Virgin|Cultivated+---------+----------
| | |Back-land|Black-land
| | | Houma. | Tulare.
-----------------------------+------+----------+---------+----------
CHEMICAL ANALYSIS OF | | | |
FINE EARTH. | | | |
(No coarse material in soils.)| | | |
| | | |
Insoluble matter | 48.38| 55.09 | 35.48 | 62.43
Soluble silica | 13.21| 12.28 | 20.76 | 16.99
Potash (K₂O) | .72| .52 | 1.03 | 1.09
Soda (Na₂O) | .20| .13 | .13 | .77
Lime (CaO) | 1.51| 1.31 | .72 | 1.46
Magnesia (MgO) | .73| .75 | .88 | 1.44
Br. ox. of Manganese (Mn₃O₄) | .05| .03 | .01 | .06
Peroxid of Iron (Fe₂O₃) | 7.12| 4.80 | 7.10 | 4.98
Alumina (Al₂O₃) | 5.22| 4.73 | 15.45 | 6.87
Phosphoric acid (P₂O₅) | .14| .13 | .15 | .12
Sulfuric acid (SO₂) | .07| .08 | .25 | .02
Carbonic acid (CO₂) | | | |
Water and organic matter | 22.78| 19.94 | 18.52 |
Total |100.13| 99.79 | 100.48 | 100.59
| | | |
Humus | 5.11| 5.54 | 5.07 | 1.33
“ Ash | 1.80| 1.40 | .91 | .36
“ Nitrogen, per cent. | | | |
in Humus | 4.63| 4.22 | |
“ “ per cent. | | | |
in soil | .27| .24 | |
Available Potash | | | |
(citric acid method) | .014| .010 | |
Available Phosph. acid | .011| .008 | .08 | .01
(citric acid method) | | | |
Hygroscopic Moisture | | 12.07 | 18.82 | 5.38
absorbed at | | 17°C | 13°C | 15°C
-----------------------------+------+----------+---------+----------

It will be seen that the Russian soil is of high fertility according to the standards given above, and that the nitrogen-content of the abundant humus is amply within the limits of adequacy suggested by the experience in California and Hawaii. The humus-content of the arid California soils is characteristically low as compared with the Russian tchernozem as well as with the Houma backland of humid Louisiana; but its nitrogen-content is doubtless at least three times that of the latter, as is that of the humus of similar lands in which it has been determined.

INFLUENCE OF LIME UPON SOIL FERTILITY.

Assuming as substantially correct the numerical data given above in respect to the three leading ingredients of plant-food—phosphoric acid, potash and nitrogen,—the dominant role of lime in soil fertility, already mentioned, requires some farther illustration and discussion.

“_A Lime Country is a Rich Country._”—The instant change of vegetation when we pass from a non-calcareous region to one having calcareous soils, has already been alluded to. (See this chapter, p. 354). But it is not necessary to be a botanist to see the change in the prosperity of the farming population as one enters a lime district. The single log-cabin with, probably, a wooden barrel terminating the mud-plastered chimney, is replaced, first by double log-houses, then by frame, and farther on by brick buildings, with the other unmistakable evidences of prosperity. Thus this is seen in passing from the mountain region of Kentucky into the “blue-grass” country, which is throughout underlaid by calcareous formations; and thus, likewise, in crossing the strike of the formations of Alabama, Mississippi and Louisiana, or any other region where underlying calcareous formations have contributed to the formation of the soils, as compared with some adjacent district where this is not the case. The calcareous loess areas bordering on the Mississippi river and some of its chief tributaries, are conspicuous cases in point, as are also the prairies of Illinois and Indiana.

_Effects of High Lime-content in Soils._—The table below illustrates the fact that in the presence of high lime-percentages, relatively low percentages of phosphoric acid and potash may nevertheless prove adequate; while the same, or even higher amounts, in the absence of satisfactory lime-percentages prove insufficient for good production.[123]

[123] This statement appears contradictory of the observations of Schloesing upon the solubility of phosphoric acid in presence of lime carbonate (Am. Sci. Agron., tome 1, 1899), but the natural conditions seem to justify fully the above conclusion.

SOILS SHOWING LOW PHOSPHORIC ACID PERCENTAGE.
============================+======================================
| HIGH LIME.
+-----------+---------+----------------
|Mississippi|Louisiana| California
+-----------+---------+------+---------
| Kemper | Vernon | Yuba | Amador
| County | County |County| County
----------------------------+-----------+---------+------+---------
Number of Sample. | 139 | 171 | 499 | 1113
----------------------------+-----------+---------+------+---------
Chemical Analysis of | | | | Slate
Fine Earth. | | | | Soil
----------------------------+-----------+---------+------+---------
Insoluble matter | | 53.19 | 78.79| 49.96
| 67.08 | 74.29| | 64.92
Soluble silica | | 21.10 | 3.80| 14.96
----------------------------+-----------+---------+------+---------
Potash (K₂O) | .70 | .33 | .25| 1.48
Soda (Na₂O) | .14 | .06 | .04| .43
Lime (CaO) | 1.37 | 1.40 | 1.02| .60
Magnesia (MgO) | 1.00 | .74 | .40| 2.21
Br. ox. of Manganese (Mn₃O₄)| .25 | .15 | .02| .05
Peroxid of Iron (Fe₂O₃) | 6.75 | 4.52 | 5.81| 11.52
Alumina (Al₂O₃) | 13.07 | 11.36 | 6.28| 12.31
Phosphoric acid (P₂O₅) | .03 | .05 | .04| .05
Sulfuric acid (SO₃) | .08 | .12 | .02| .02
Carbonic acid (CO₂) | | | |
Water and organic matter | 9.45 | 7.27 | 3.64| 6.63
+-----------+---------+------+---------
Total | 99.91 |100.29 |100.19|100.22
+-----------+---------+------+---------
Hygroscopic Moisture | 11.45 | 18.11 | 4.80 | 5.74
absorbed at °C | 8.0 | 25.5 | 15.0 | 15.0
----------------------------+-----------+---------+------+---------

============================+=======================================
| LOW LIME.
+-----------------+---------------------
| Mississippi | California
+---------+-------+-----------+---------
|Chickasaw|Carroll| Shasta |Humboldt
| County |County | County | County
----------------------------+---------+-------+-----------+---------
Number of Sample. | 164 | 48 | 559 | 207
----------------------------+---------+-------+-----------+---------
Chemical Analysis of | | | |
Fine Earth. | | | |
----------------------------+---------+-------+-----------+---------
Insoluble matter |93.62 | | 76.27 | 65.35
| 94.98| 89.39 | 80.38| 72.24
Soluble silica | 1.36 | | 4.10 | 6.90
----------------------------+---------+-------+-----------+---------
Potash (K₂O) | .09 | .19 | .50 | 1.13
Soda (Na₂O) | .07 | .08 | .04 | .28
Lime (CaO) | .07 | .08 | .10 | .11
Magnesia (MgO) | .13 | .07 | .40 | 3.33
Br. ox. of Manganese (Mn₃O₄)| .02 | .12 | .01 | .12
Peroxid of Iron (Fe₂O₃) | 1.09 | 1.21 | 6.67 | 6.99
Alumina (Al₂O₃) | 1.47 | 4.37 | 8.48 | 10.24
Phosphoric acid (P₂O₅) | .03 | .05 | .04 | .17
Sulfuric acid (SO₃) | .01 | .05 | .01 | .02
Carbonic acid (CO₂) | | | |
Water and organic matter | 2.00 | 4.09 | 3.97 | 5.63
+---------+-------+-----------+---------
Total |99.94 | 99.70 |100.62 | 100.24
+---------+-------+-----------+---------
Hygroscopic Moisture | 1.80 | 4.66 | 5.05 | 7.87
absorbed at °C |11.0 | 11.0 | 17.0 | 13.0
----------------------------+---------+-------+-----------+---------

Nos. 139 and 171 are heavy black prairie soils of high
productive capacity, whose production had, at the time of
sampling, lasted almost undiminished for over twenty years.
Nearly the same is true of the two California soils, Nos.
499 and 1113; which, however, are ferruginous loams of only
moderate clay-content. In all, the percentage of phosphoric
acid shown by the analysis is at or below the recognized
limit of deficiency, while the lime-content of all is as
high as is required for the welfare of any soil, however
constituted. The potash-percentage also is low in all except
the “red foothill soil,” No. 1113.

Passing to the soils of low lime-content, we find the two
Mississippi soils, poor in both potash, lime and phosphoric
acid, so low in production as to be wholly unprofitable in
cultivation without previous fertilization; No. 559, from
California, produced two fair crops of barley and then no
more. No. 207, is the soil of Eel river bottom, California;
profusely productive at first, by virtue of its high content
of both potash and phosphoric acid; but “giving out” under a
few years’ culture of clover or alfalfa (which draw heavily
upon lime), and quickly restored to productiveness under the
influence of dressings of quicklime. In this case the soil
had become acid, a condition which always militates against
the success of culture plants, and more especially against
those of the leguminous relationship.

_What are Adequate Lime Percentages?_—We have in the presence or absence of the natural vegetation peculiar to calcareous soils (“calciphile”) an excellent index of the presence or absence of such amounts of lime carbonate as fulfil the conditions of its beneficial effects. Lists of such plants for the United States are given farther on; they agree almost throughout with such plants as are everywhere recognized by American farmers as indicating productive soils.

All soils bearing such vegetation show with red litmus paper, when wetted, a neutral reaction at first, which after the lapse of twenty or thirty minutes turns to a blue alkaline one; such as is given under the same conditions by the carbonates of lime and magnesia.

But the reverse is not necessarily true; for we occasionally find soils containing considerable amounts of lime carbonate that yet fail to bear lime vegetation. This is the case of extremely heavy clay soils, as exemplified in the table below in the case of the last three soils; while the first, No. 220, exemplifies a case where although potash is exceptionally high, only scrubby oak growth is produced in presence of an amount of lime that in sandy lands would show profuse lime growth.

TABLE ILLUSTRATING THE NEED OF HIGH LIME-PERCENTAGES
IN HEAVY CLAY SOILS.
============================+===========================+===========
| Mississippi. |California.
+---------+-------+---------+-----------
|Flatwoods| Hog- | Ridge | Yellow
| Pontotoc| wallow| Prairie,| ridge,
| Co. | Jasper|Smith Co.|Alameda Co.
| | Co. | |
----------------------------+---------+-------+---------+-----------
No. Sample. | 230 | 242 | 203 | 4
----------------------------+---------+-------+---------+-----------
CHEMICAL ANALYSIS OF | | | |
FINE EARTH. | | | |
----------------------------+---------+-------+---------+-----------
Insoluble matter | 77.85 | 76.76 | 51.75 | 86.00
Soluble silica | | | |
----------------------------+---------+-------+---------+-----------
Potash (K₂O) | .75 | .53 | .53 | .19
Soda (Na₂O) | .11 | .19 | .22 | .15
Lime (CaO) | .18 | .42 | .48 | .48
Magnesia (MgO) | .83 | .67 | 1.01 | .45
Br. ox. of Manganese (Mn₃O₄)| .17 | .56 | .10 | .04
Peroxid of Iron (Fe₂O₃) | 5.90 | 4.12 | 23.79 | 4.01
Alumina (Al₂O₃) | 10.30 | 10.06 | 10.85 | 5.53
Phosphoric acid (P₂O₅) | .05 | .06 | .15 | .06
Sulfuric acid (SO₃) | .03 | .06 | .02 | .02
Carbonic acid (CO₂) | | | |
Water and organic matter | 3.69 | 5.73 | 11.39 | 4.05
+---------+-------+---------+-----------
Total | 99.86 | 99.17 | 100.29 | 100.99
+---------+-------+---------+-----------
| | | |
Hygroscopic Moisture | 9.3 | 6.8 | 19.7 |
absorbed at °C | 22.0 |air-dry| 17.0 |
----------------------------+---------+--------+---------+-----------

All of the soils in this table are heavy clays, very difficult to till; in all, the lime-percentage falls below .5%; and none bear any lime vegetation, the Mississippi soils having a stunted growth of black jack and post oaks, such as is universally known to indicate soils too poor for profitable cultivation. The California soil bears stunted live-oak (_Q. agrifolia_); but not being as heavy as its brethren from Mississippi, though unthrifty, is more readily improved.

Comparison with the two first sandy soils in the table on
p. 352 shows, that with plant-food percentages equal to, or
even much below those here shown, not only was vigorous lime
growth present, but crop-production was good and even high.

We are thus led to the conclusion that the greater the clay percentage in a soil, the more lime carbonate it must contain in order to possess the advantages of a calcareous soil; and that while in sandy lands lime growth may follow the presence of only .10% of lime, in heavy clay soils not less than about .6% should be present to bring about the same result. This is apparent to the eye in that the dark-tinted humus characteristic of truly calcareous lands, does not appear in clay soils until the lime-percentages rise to nearly 1%; while in sandy lands a much smaller amount (say .2%) will produce this effect.

_European Standards._—It is of interest to consider,
in connection with preceding discussions, the estimates
given by Maercker of Halle, of the practical value of soils
corresponding to chemical composition as ascertained by
analysis with strong acids, substantially in accordance with
the methods adopted by the writer.

It will be observed that according to Maercker’s valuation,
the average California soil is “rich” in potash and lime,
but only “medium” as regards its contents of phosphoric
acid and nitrogen. In this respect, and almost throughout,
Maercker’s ratings are in remarkable agreement with those
made by the writer as far back as 1860.[124] It also appears
that Maercker’s figures for “normal” soils correspond to
those of the American humid regions; the “arid” figures for
potash and lime being “abnormally” high.

[124] See discussions of analyses of Mississippi soils in the Report on the Agriculture and Geology of Mississippi, 1860; same in Rep. On Cotton Production, Tenth Census, 1880, Vol. 5; also Appendix to the Report on the Experiment Stations of the University of California, 1890, p. 163.

Unfortunately neither Maercker’s method of preparing the soil extract, nor his ratings as given in the table, are accepted by all soil chemists even in Germany. As will be seen by reference to Wohltmann’s work on the soils of Samoa and Kamerun (chap. 21, p. 404), his methods and numerical estimates differ widely from those given by Maercker, and also from those adopted by the Prussian soil surveys. Reference to the analyses of the soils of Madagascar by Müntz and Rousseaux, given in the same chapter, page 406, shows still another different method, although as it happens their numerical estimates do not differ very widely from those of Wohltmann. In both cases, a special, more incisive extraction is made for the determination of potash. Why the same more energetic action is not used for the other ingredients also, is not stated, and is obscure. Fortunately, in all cases the action is at least sufficiently strong to secure the dissolution of all the lime existing in the form of carbonate, and of all, or nearly all, the phosphoric acid not securely locked up as ferric phosphate; the latter being inert, is of no special interest (see Analyses of Hawaiian Soils, this chapter, page 256).

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Soils, their formation, properties, composition, and relations to climate and plant growth in the humid and arid regionsChapter XIX

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