CHAPTER VI.. Physical Composition of the Soils.

PHYSICAL COMPOSITION OF THE SOILS.

As has already been stated (chapt. 1, p. 10), the general physical constituents of soils are rock powder or sand and silt, more or less decomposed according to the nature of the original rocks; clay, the product of the decomposition of feldspars and some other silicates; humus, the complex product of the decomposition of vegetable and animal matters on and in the soil mass; as well as vegetable matter not yet humified. Each of these several constituents must now be considered more in detail. Since clay is the substance whose functions and quantitative proportions influence most strikingly the agricultural qualities of land, it should be first discussed.

Clay as a Soil Ingredient.

The plasticity and adhesiveness of clay, together with the extreme fineness of its ultimate particles (said to reach the 1-25000 of an inch), explains its great importance as a physical soil ingredient. It serves to hold together and impart stability to the flocculent aggregates of soil particles that compose a well-tilled soil; for without clay the sand would collapse into close-packed single grains so soon as dried, and loose tilth would be impossible. Sand drifts illustrate this condition.

On the other hand, the fineness of the particles serves to render clay very retentive of moisture as well as of gases and of solids dissolved in water, imparting these important properties to soils containing it; while coarse sandy soils are oftentimes so deficient in them as to render them unadapted to any useful culture, despite the presence of an adequate supply of plant-food.

When to these essential physical properties of clay, there is added the fact that usually the clay-substance as it exists in soils contains the most finely pulverized and most highly decomposed portions of the other soil-minerals, and therefore the main part of the available mineral plant-food, it is easy to understand why soils containing a good supply of clay should be called and considered “strong” land by the farmers of all countries. “Poor” clay soils are exceptional; but sometimes the clay content reaches such a figure that the difficulties of tillage render them too uncertain of production for profitable occupation.

Amount of Colloidal Clay in Soils.—Any and all of the kinds of clay mentioned (p. 57) as occurring naturally may, of course, enter into and form part of soils. But as the amount of true, plastic clay substance contained in them is very indefinite, it becomes necessary, in order to classify soils in respect to their tillableness, to ascertain more definitely the amount of pure, or nearly pure, colloidal clay substance contained in the several classes of soils ordinarily recognized and mentioned in farming practice. That this determination can at best be only approximate, is obvious from the fact mentioned above (chapt. 4, p. 59), that pure kaolinite itself is not plastic, and only becomes so by the indefinite comminution and hydration it experiences in the processes of soil-formation. As the progress of this process is also indefinite, the same soil containing particles ranging from the finest to the chalky scales of pure kaolinite, the drawing of a line must be more or less arbitrary and empirical.

From numerous experiments and comparisons made, the writer has been led to place the limits of “plastic clay” at and below such grain sizes as will remain suspended (afloat) in a water column eight inches high, during 24 hours. To go beyond this point in the examination of soils for practical purposes, would render such examinations so laborious and hence so rare, that this kind of work would be practically excluded from ordinary practice. According to this view the following percentages of such “clay” correspond approximately to the designations placed opposite:

Very sandy soils .5 to 3% clay Ordinary sandy lands 3.0 to 10% “ Sandy loams 10.0 to 15% “ Clay loams 15.0 to 25% “ Clay soils 25.0 to 35% “ Heavy clay soils 35.0 to 45% and over.

It must be distinctly understood, however, that these figures make no claim to accuracy or invariability. For, the tilling qualities of a soil containing one and the same amount of such “clay” may be very materially modified according to the kind and amount of each of the several grain-sizes of rock powder or sand they contain.

Influence of fine powders on plasticity and adhesiveness.—An admixture of a large amount of fine powders diminishes materially the adhesiveness of a clay soil, even though it may render it even more “heavy” in tillage; while the admixture of coarse sand, even in very considerable proportions, does not greatly influence the adhesiveness of the clay. The latter alone cannot therefore serve as a proper guide or basis for the classification of soils in respect to tillage; we must also take into consideration the nature and amount of the several granular sediments mixed with it.

Moreover, the nature and especially the adhesiveness of the clay substance as obtained by analysis may vary considerably in the presence of a very large amount of the finest grain-sizes; among which ferric hydrate or iron rust is especially apt to accumulate predominantly in the clay, considerably increasing its apparent weight and greatly diminishing its adhesiveness. In strongly ferruginous soils, therefore, it becomes necessary to take into special consideration the amount of the ferric hydrate or rust which accumulates in the clay substance. The presence of large amounts of humus or vegetable mold also influences materially the adhesiveness and physical properties of the clay obtained by the method described, although most of it remains with the finer powdery sediments or grain-sizes. There are, besides, other colloidal or at least amorphous substances present in all soils, such as silicic, aluminic and zeolitic hydrates, which are all non-plastic, and yet sufficiently fine to form part of the “clay” obtained as above specified.

This fact emphasizes the impossibility of explaining the plasticity and adhesiveness of clay simply as a function of fineness of grain.

Despite these imperfections, (which however can in a measure be taken into consideration in judging of a soil’s tilling qualities by its clay content), the figures given in the above table approximate much more nearly to a tangible basis for such estimate, than the utterly indefinite mixtures which under the older methods of analysis have been, and still are to some extent, used as a basis for soil classification by writers on agriculture.

Rock Powder; Sand, Silt and Dust.

The powdery (sandy and silty) constituents of soils usually constitute the greater part of their mass; and the proportions present of the several grades of fineness exert a most decisive influence upon their cultural qualities, and very commonly upon their agricultural value also. It is needless to add that the kind of mineral of which they consist or from which they were formed, is also of great importance in determining the quality of soils from the standpoint of the chemist, with respect to their content of mineral plant-food.

WEATHERING IN HUMID AND ARID REGIONS.

Sands of the Humid Regions.—As has already been stated, “sand” is usually understood to be, in the main, quartz more or less finely pulverized, generally intermingled with a few grains of other minerals. With this understanding, since quartz is practically inert with respect to plant nutrition, it follows that soils consisting mainly of this substance contain but little plant-food; hence the common expression “poor, sandy land,” the outcome of the experience had in Europe and in the Eastern United States, and which until recently has been held to be of general application. The “sands of the desert” have, both in ordinary life and in poetry, always stood as the symbol of sterility.

Thus the sandy lands (“sand hammocks”) of Florida, the (long-leaf) pine lands of the Gulf States, the “pine barrens” of New Jersey and of Michigan, are noted both for their sandy soils and their sterility after brief cultivation; necessitating fertilization within a few years from the time of occupation. In Europe, the “Heide” (heather) soils of northeastern Germany are of the same cultural character.

Sands of the Arid Regions.—The experience of arid countries however, has long ago shown that some very sandy lands—e. g., such as form the oases of the north African deserts—may be extremely productive when irrigated, and also of considerable durability. Actual experience and close investigation given this subject in the arid regions of the United States has fully demonstrated that lands appearing to the casual observer to be hopelessly sterile sandy deserts, very commonly prove to be even more productive than the more clayey lands of the same regions. Examination of the sand shows, in these cases, that instead of mere grains of quartz, the minerals of the parent rock, partially decomposed, themselves constitute a large proportion of the sandy mass. But in the regions of deficient rainfall, as has already been stated, (p. 47) the formation of clay (kaolinization) is exceedingly slow; hence the decomposition of the rock powder results in the production of predominantly pulverulent instead of clayey soils. But the mineral plant-food is not on that account less available, provided other physical conditions necessary for the success of plant growth are fulfilled. Among these moisture stands foremost; hence the relative proportions of the several grain-sizes are of vital importance, since upon this depends to a great extent the proper supply and distribution of moisture, without which no amount of plant-food will avail. Moreover, the finest and most highly decomposed powder is the portion from which the roots draw their chief food-supplies.

The point last mentioned is well shown in the results obtained by Dr. R. H. Loughridge, from the analysis of each of the several grain-sizes into which he had resolved a very generalized soil of the State of Mississippi, representing a very large land area in that State as well as in Tennessee and Louisiana. The details of this investigation are given farther on; but summarily it may be stated that he found practically the whole of the acid-soluble mineral plant-food accumulated within the portion of the soil the fineness of whose grains was below .025 millimeters (one-thousandth of an inch); ingredients so fine as to be wholly impalpable between the fingers. Moreover, two-thirds of the total amount was found in the portion described above as “clay.” It is thus readily understood why clay soils are in the regions of summer rains commonly designated as “strong” lands.

The corresponding later investigations of Rudzinski (Ann. Agr. Inst. Moscow, Vol. 9, No. 2, pp. 172-234; Exp. Sta. Record, Dec. 1904, p. 245) and of Mazurenko (Jour. Exp. Landw. 1904, pp. 73-75; Exp’t Stn. Record, Dec. 1904, p. 344) fully corroborate Loughridge’s conclusions, for typical soils of European Russia.

In the arid or irrigation regions, however, the case is different, for the reason that much of the decomposed rock-substance remains adherent to the surface of the larger grains, and plastic clay is formed to a much less extent. Much available plant-food may therefore, in arid lands, be present even in rather coarsely sandy soils almost devoid of clay; such as in humid climates would be likely to be found wholly barren. (See chapt. 19).

PHYSICAL ANALYSIS OF SOILS.

Use of Sieves.—Down to a certain point the separation of the soil into its several grain-sizes may be accomplished by means of sieves. We may thus separate coarse gravel from fine gravel and from sand; and the latter may itself be separated into several sizes by the same means. This presupposes, of course, that the soil has been previously prepared for the purpose by crushing the lumps consisting of aggregates of finer particles, that in the operation of tillage would again be resolved into their fine constituents, or be penetrated by roots. But this preparation of the soil for sifting must not be carried beyond the point mentioned, for a grain consisting of particles somewhat firmly cemented together will under ordinary conditions play in the soil precisely the same part as a solid sand-grain, and must not therefore be broken up, if the soil is to be examined in its natural condition. The pressure of the fingers or of a rubber pestle is as far as trituration should go. The disintegration of these compound particles by means of acids, as prescribed and practiced by the French soil chemists, may wholly change the physical nature of the soil by the breaking-up of mechanical aggregations which in the usual course of tillage would remain intact. This is especially true of strongly calcareous soils, and particularly those containing calcareous sand.

The sieves used for this purpose should not be ordinary wire sieves, but should have bottoms of sheet brass perforated by round holes of the various diameters desired, of fractions of inches, or preferably of millimeters. For the finer grain sizes, silk bolting cloth is used by the U. S. Bureau of Soils.

In the sifting process it will be found that so soon as the finer grain-sizes of the sand are approached, the sieve fails to act satisfactorily; the more so, the more clay was originally contained in the material. The fine particles flock together, forming little pellets, which refuse to be separated by the sieve. This difficulty can, of course, be partly overcome by previously separating the clay from the sand by means of water, as detailed above; but even then it will be found that so soon as the grain-sizes fall much below ¹/₅₀ of an inch (½ millimeter) the same difficulty is experienced, so long as the sand is dry. By playing a small stream of water upon the sieve, however, all the particles beyond the ¹/₅₀₀ of an inch may be successfully separated from the coarser portion; and for many practical purposes the separation need be carried no farther.

Use of Water for Separating Finest Grain-Sizes.—The scientific investigator, however, must of necessity proceed to separate the finer grain-sizes from each other, since, as will presently be shown, they influence the tilling qualities of the soil to a much greater degree than do the coarser particles. Such farther separation can be accomplished only by the aid of water.

Subsidence Method.—When a small amount of soil is stirred up in water, and is afterward allowed to stand for some time, the different grain-sizes will settle consecutively in accordance with their sizes (or weights); the smallest ones settling latest, and the clay only remaining suspended, as stated above. So long, however, as any considerable amount remains suspended in the water, the latter is not only denser but especially more viscid than if the clay were absent. In order therefore to obtain correct results by any method involving the use of water, it is necessary to remove the clay before proceeding to the separation of the granular sediments. This, as has been already stated, is approximately accomplished by allowing the soil, when diffused in water after proper disintegration, to settle for 24 hours from a column of water 200 mm. high, whereby all grain-sizes, of and above .01 mm. diameter are removed from the turbid liquid. This sedimentation is then repeated until after 24 hours the water becomes clear. The clay is then determined in the “clay water” by evaporation or precipitation; the granular sediments may then be successfully separated by sedimentation.

The U. S. Bureau of Soils uses for the separation of clay, instead of subsidence for 24 hours, the more expeditious process of centrifuging the turbid soil water in appropriate glass cylinders, by the aid of an electric motor; and thus in a relatively short time obtains “clay” in which the upper limit of size is one-half of that mentioned above, viz., .005 mm. But for the costliness of the appliances required, including the entire time of an operator, this method of separating the clay would undoubtedly be preferable to the elimination by subsidence; the more as a more minute grain-size for the clay group is thus secured.

The separation of the clay having been accomplished, the various sizes of silt and sand may be separated by again suspending them in water; and interrupting the settling process at stated times, the grain-sizes corresponding to definite velocities in settling may be segregated and weighed. When this process of settling and decanting is carefully and repeatedly carried out, very good results are obtained.

Hydraulic Elutriation.—The sedimentation (or “beaker”) method, long practiced in the arts is, however, quite tedious, requiring the constant close attention of a skilled observer. The desired results may, in the writer’s judgment, be more conveniently obtained by the hydraulic method, whenever no very large volume of work of this kind is required to be done at once.

When instead of allowing the soil to settle in quiet water, the latter is used as an ascending current of regularly graded velocities, it is clear that the soil particles will be carried off by this current in exact conformity with their several sizes (or strictly speaking, volume-weights); and when maintained in such a current for a sufficient length of time, the entire quantity of the sediment corresponding to the prevailing velocity will be carried away. It is of course easy to ascertain to what grain-sizes certain velocities of the upward current (regulated by a stopcock with arm moving on a graduated scale) correspond, and to regulate accordingly the intervals between the different velocities to greater or less detail, as may be desired. A number of instruments have been devised for this purpose.

Schöne’s Elutriator is the one commonly used in Europe; in it the upward current ascends in a conical glass tube, (see figure 6) entering through a narrow, curved inlet tube, in which the soil sample is kept agitated by the current itself. The objection to this plan is twofold: first, the narrow, curved inlet-tube is readily clogged by the soil mass at the lower velocities, which are thereby changed, so that, unless a very small amount of soil only is employed, the whole mass is not kept properly stirred; second, the circulating currents brought about by the conical shape of the tube cause the sediment-particles to coalesce into complex, larger ones (floccules), which will then settle down and fail to pass over at the current-velocity corresponding to their individual component parts.

Churn Elutriator with Cylindrical Tube.—The errors just alluded to are obviated by an arrangement devised by the writer, in which a rapidly revolving stirrer, placed at the base of a cylindrical tube in which the washing process is conducted and which eliminates counter-currents, continually disintegrates these compound particles, and thus enables the entire quantity of the sediment corresponding to the prevailing current-velocity to pass off with a comparatively slight expenditure of time on the part of the operator (see figure 7). A wire screen interposed between the churn and cylindrical glass tube prevents communication of the whirling motion to the column. As the apparatus works automatically, the analyst has only to observe from time to time whether or not the turbidity near the top of the tube has disappeared; and as the sediment accumulates at the bottom of the tall receiver bottle, no harm is done if the attendant should neglect to change the velocity in time, except that water will run to waste.

The figure given of this elutriator in Bulletin No. 24, on physical soil analysis, published by the U. S. Bureau of Soils, shows as the receiver a bottle entirely too low to insure the complete retention of the sediments by settling. The receiving bottle should not be less than twelve inches high and five inches wide.

The conical relay glass below the churn serves to retain the coarser grades of sediments which are not concerned in the velocities employed in the elutriator tube, and thus prevents injurious attrition. But these sediments can at any time be stirred up by the incoming current and brought into the washing tube if desired. In the same manner the passing-off of the finer sediments can be materially accelerated by running off rapidly about two-thirds of the turbid column of water every twenty minutes.

It should be fully understood that prior to attempting such separation, the “colloidal clay” must first be removed by the subsidence or centrifugal method, since otherwise much larger grain-sizes may be carried off at a given velocity.

Yoder’s Centrifugal Elutriator.—A very ingenious instrument which combines the elutriation and sedimentation processes into one, has been devised by P. A. Yoder, of the Utah Expt. Station. The elutriator bottle is placed in a centrifuge driven by an electric motor; it is closed by a glass stopper carrying a delivery tube to a short distance above the bottom of the elutriator bottle, as well as an outflow tube ending at the base of the stopper; the latter also carries a funnel coinciding with the center of rotation. Into this funnel flows gradually the muddy water containing the soil in suspension; and the rate of its flow, together with the velocity of rotation, determines the size of the sediment-granules that will be deposited in the slack-water below the mouth of the delivery tube. The muddy soil-water is kept agitated in a funnel-shaped reservoir by air-bubbles from a constant-pressure chamber.

While the principle of this instrument is good, it is quite complicated and the results obtainable from it in practice have not as yet been made public. The inventor claims that an analysis may by its means be completed in less than three hours.

In all hydraulic elutriators a provision for constant pressure in the reservoir supplying the current of water is needed; although in Schöne’s and some other instruments a gradually decreasing pressure in a plain reservoir is employed. A large glass bottle or carboy fitted with the proper tubes so as to constitute a Mariotte’s bottle (in which the air enters near the bottom of the vessel), is a very convenient arrangement.

Number of Sediments.—The number of grain-sizes or sediments into which the soil mass is to be segregated is of course entirely within the option of the operator. Experience has shown that it is unnecessary to discriminate very closely between the several sizes of the coarser portion of the sand, such as those lying between one-fourth and one-half of a millimeter. But below this point, and especially between one-tenth of a millimeter and the clay, a proper discrimination becomes very important. The series first devised by the writer in 1872 is based upon a consecutive doubling of the velocities of the current from a quarter of a millimeter per second to thirty-two millimeters per second; the sediment of sixty-four millimeter-velocity corresponding to a diameter of one-half of a millimeter, will remain in the elutriator. Above this, as before remarked, the sieve (especially when aided by a jet of water) effects a satisfactory segregation.

The table below shows the elements of these series both as regards current-velocities and maximum quartz-grain diameters carried off by each. In a great many cases, however, it is altogether unnecessary to go into such detail, and a subdivision into six or seven divisions is quite sufficient. Such a subdivision, based upon the doubling of grain-sizes instead of current-velocities, has been adopted by Prof. Milton Whitney, of the U. S. Department of Agriculture, and others.

TABLE OF DIAMETERS AND HYDRAULIC VALUES OF SEDIMENTS.

=============================================================== |Velocity per second,|Maximum diameter Designation of materials.| or hydraulic | of quartz | value. | grains. -------------------------+--------------------+---------------- | Mm. | Mm. | | Grit | (?) | 1-3 { | (?) | .5-1 { | 32-64 | .50 Sand { | 16-32 | .30 { | 8-16 | .16 { | 4- 8 | .12 { | 2- 4 | .072 { | 1.0- 2 | .047 Silt { | .5- 1 | .036 { | .25-0.5 | .025 { | 0.25 | .016 { | < 0.25 | .010 | | Clay | < 0.0023 | -------------------------+--------------------+----------------

Results of such analyses.—A tabular presentation of the results of analyses made in accordance with the above plan will give a good idea of the differences between the various grades of soils recognized in farm practice, to any one accustomed to the study of figures. But a much more satisfactory showing is made by placing the several grain-sizes segregated, into small vials or tubes of identical diameter and placing them in parallel series alongside of each other. The curves formed by the surfaces of the several sediment-columns in each series show to the eye very strikingly the relations of the several grades of soils to each other, and suggest at once that while gentle slopes or gently undulating curves belong to soils of intermediate, loamy character, steep grades and zigzags show soils of extreme types. This is exemplified in the subjoined Figures:

Convenient stands for this purpose, used by the writer since 1872, may be cut from L-shaped moldings of wood, such as can be readily ordered from any planing mill. The vials can be cemented, wired or tied.

Physical composition corresponding to popular designations of Soil quality.—The subjoined table illustrates the physical composition of a number of soils from the State of Mississippi, selected for their representative character, in order to deduce therefrom approximate definitions of physical character corresponding to popular designations. This table, published in 1873 in accordance with results obtained during the two preceding years, does not require any material modification on account of subsequent investigations. It lacks, however, a characteristic representative of the predominant soils of the arid region, viz., the silty soils so prevalent in dry climates, only approximately represented by No. 165 of the table; hence two such, from California, exemplifying respectively the valley deposits of the Sacramento and Colorado rivers, have been added to the list.

It must not, however, be understood that these typical soils necessarily represent correctly the physical constitution of all soils falling under the same popular designation; for we are far from being able as yet to predict accurately in every case the tilling qualities of a soil material from its physical composition. To do this it would be necessary not only to know with some degree of precision the several physical coefficients of each of the several grain-sizes, and perhaps of many more intermediate ones; but we would also have to construct a formula according to which each could be given its proper weight when present in varying proportions, and of varying shapes, surface condition, and material. For this our present knowledge is wholly inadequate, if indeed the problem is not beyond the limits of mathematical computation. We must for the present at least be satisfied with the empirical approximations afforded us by the constantly increasing number of such analyses, correlated with farming experience.

Since the finest grain-sizes above those classed as “clay” do not tend to “lighten” soils, but even to render them more intractable (“putty soils”), while coarser ones gradually change the dense clay-texture into the “loamy,” it is clear that in between there must be a neutral point, some grain sizes which by themselves do not influence soil texture either way. Discussion of numerous physical analyses, and some direct experiments, have led the writer to conclude that this theoretically neutral grain-size lies at or near the diameter of .025 mm., or .5 mm. hydraulic value. In correlating the results of analysis with the tilling qualities of the soil as to “heaviness and lightness,” therefore, that grain-size may usually be left out of consideration.

PHYSICAL ANALYSES OF SOILS AND SUBSOILS.

(A) = 238 White Pipe Clay. Tishomingo Co. (Z) = Hygroscopic Moisture (+7 = to +21°C) ===========+==============+================+======= | Diameter | Velocity | |(Millimeters) | (Hydr. V.) | Designation| | | of +--------------+----------------+------- Materials. | | | | | Millimeters | (A) | | per sec. | -----------+--------------+----------------+------- Grit | 1-3 | | | | | { | .5-1 | | { | .50 | 64 |} Sand { | .30 | 32 |} 0.06 { | .16 | 16 |} { | .12 | 8 | 0.08 | | | { | .072 | 4 | 0.02 { | .047 | 2 | 0.04 Silt { | .036 | 1 | 0.08 { | .025 | 0.5 | 0.08 { | .016 | 0.25 | 2.00 { | .010 | <0.25 | 21.15 | | | Clay | ? | <0.0023 | 74.65 +------- | 98.16 | (Z) | 9.09 Ferric Oxid | 0.13 -------------------------------------------+-------

(B) = 248 Tallahoma Subsoil. Jasper Co. (C) = 165 Flatwoods Soil. Chickasaw Co. (D) = 206 Pine Hill Soil. Smith Co. (E) = 209 Pine Hill Subsoil. Smith Co. (F) = 397 Oxford Subsoil. Lafayette Co. (G) = 219 Table Lands Subsoil. Benton Co. (H) = 173 Prairie Subsoil. Monroe Co. (I) = 230 H’y Flatwoods Soil. Pontotoc Co. (J) = 246 Red Hills Subsoil. Attala Co. (K) = 196 Hog-wallow Subsoil. Jasper Co. (Z) = Hygroscopic Moisture (+7 = to +21°C) ==========+=============================================================== | MISSISSIPPI UPLANDS. +------------------+------------------+------------------------- Designation| Sandy. | Loam. | Clay. of +-----+------+-----+-----+-----+------+-----+------+-----+------ Materials.| | | | | | | | | | | (B) | (C) | (D) | (E) | (F) | (G) | (H) | (I) | (J) | (K) | | | | | | | | | | ----------+-----+------+-----+-----+-----+------+-----+------+-----+------ Grit | 6.94| 2.90 | 0.36| 0.36| |} |} | 0.33 |} | 0.83 | | | | | |} 0.23|}2.10| |}1.97| {|17.65| 6.96 | 2.98| 0.83| |} |} | 0.35 |} | 1.19 {|18.81| 2.81 | 6.62| 6.21|} | 1.47|} | | 0.72| 1.96 Sand {|10.16| 4.41 | 7.75| 3.38|}0.79| 2.33|}0.62| | 2.32| 1.64 {| 2.66| 3.13 | 3.01| 3.85|} | 1.17|} | | 2.09| 0.88 {| 1.66| 2.02 | 1.59| 1.49| 0.18| 0.78| 0.20| 0.23 | 0.70| 0.26 | | | | | | | | | | {| 1.02| 2.23 | 1.19| 0.64| 0.78| 0.76| 1.26| 0.18 | 1.29| 0.19 {| 0.88| 5.06 | 3.56| 2.63| 3.56| 9.79| 2.92| 1.61 | 1.80| 2.49 Silt {| 1.96| 9.67 | 6.50| 5.40|13.12| 7.26| 7.36| 2.66 | 3.60| 3.67 {| 7.89|14.18 |13.97| 7.77|16.64| 13.14| 8.81| 9.13 | 2.73| 5.39 {| 8.40|22.03 |14.20|16.65|27.28| 15.07| 7.85|26.64 | 3.30| 10.31 {|15.53|15.62 |29.36|37.75|18.87| 26.50|35.22|32.35 |25.33| 24.18 | | | | | | | | | | Clay | 8.63| 7.86 | 4.58|10.70|17.23| 19.19|33.16|25.48 |40.25| 47.03 +-----+------+-----+-----+-----+------+-----+------+-----+------ |99.28|98.68 |95.67|97.77|98.35| 97.65|99.50|97.87 |96.11|100.00 | | | | | | | | | | (Z) | 1.80| 3.36 | 2.48| 7.69| 8.79| 7.24|11.35| 9.33 |18.60| 14.48 Ferric Oxid| 1.10|(1.45)| 1.25| 4.45| 2.53| 5.11| 5.42|(5.90)|10.50| 4.00 ----------+-----+------+-----+-----+-----+------+-----+------+-----+------

(L) = 390 Buckshot Soil. Issaquena Co. (M) = 237 Loess. Claiborne Co. (N) = 365 Tallahatchie All. Soil. Panola Co. (O) = 377 Frontland Subsoil. Sunflower Co. (P) = 395 Dogw. Ridge Soil. Coahoma Co (Q) = Southwest Pass. Plaquemine Par. (R) = Southwest Mud-lump. Plaquemine Par. La. (Z) = Hygroscopic Moisture (+7 = to +21°C) ==========+======================================================= | MISSISSIPPI RIVER BOTTOM. +-------+-------+--------------------------------------- Designation| Swamp.| River.| River Deposit. Delta of +-------+-------+-------+-------+-------+-------+------- Materials.| | | | | | | | (L) | (M) | (N) | (O) | (P) | (Q) | (R) | | | | | | | ----------+-------+-------+-------+-------+-------+-------+------- Grit | 0.09 |} |} | | | | | |} 0.24 |} 0.09 | | | | {| 0.05 |} |} | | | | {| | 0.37 | 0.04 |} 0.32 | 0.15 |} 0.18 |} 0.10 Sand {| 0.36 | 0.61 | 0.05 |} | |} |} {| | 0.93 | 0.21 | 2.79 | | 0.47 | 5.02 {| 0.31 | 1.65 | 1.30 | 2.41 | 3.75 | 7.03 | 3.68 | | | | | | | {| 0.27 | 1.95 | 2.68 | 16.90 | 21.46 | 12.38 | 5.34 {| 1.56 | 14.25 | 9.38 | 19.97 | 21.83 | 13.27 | 10.09 Silt {| 2.23 | 16.20 | 9.88 | 13.90 | 14.01 | 15.87 | 5.58 {| 3.68 | 20.08 | 20.37 | 4.27 | 9.93 | 8.25 | 9.54 {| 8.97 | 5.59 | 19.79 | 1.89 | 9.58 | 7.26 | 8.01 {| 38.19 | 33.38 | 25.30 | 30.08 | 8.65 | 19.67 | 34.46 | | | | | | | Clay | 44.30 | 2.51 | 9.64 | 5.51 | 10.35 | 12.20 | 18.18 +-------+-------+-------+-------+-------+-------+------ |100.01 | 97.74 | 98.73 | 98.04 | 99.72 | 96.58 |100.00 | | | | | | | (Z) | 14.31 | 4.18 | 6.12 | 5.68 | 3.95 | | Ferric Oxid|(5.82) | 3.27 | 2.58 | 2.31 | 2.69 | | ----------+-------+-------+-------+-------+-------+-------+-------

(S) = 10 Sacramento. Sacramento Co. (T) = 506 Gila. San Diego Co. (Z) = Hygroscopic Moisture (+7 = to +21°C) ===========+================== | CALIFORNIA. +------------------ Designation | River Deposit. of +---------+-------- Materials. | | | (S) | (T) | | -----------+---------+-------- Grit | | | | { | | { | | .13 Sand { | | .15 { | | .11 { | .32 | .75 | | { | 3.16 | 2.51 { | 10.27 | 8.32 Silt { | 13.67 | 12.64 { | 13.11 | 11.28 { | } 43.61 | } 31.79 { | } | } | | Clay | 12.06 | 23.97 +---------+-------- | 96.20 | 91.71 | | (Z) | 9.18 | 9.26 Ferric Oxid | | -----------+---------+--------

Number of soil grains per gram.—It is of some interest to consider the number of grains of different sizes that may be contained in, e. g., a gram of soil. If for this purpose we assume all the soil grains to be spherical, we shall obtain the minimum figures, for most other shapes will pack more closely. King (Physics of Agriculture, p. 117) calculates such figures for different grain-sizes, assuming the density to be that of quartz (2.65), with the result that while with a diameter of one millimeter (1-25 inch) the number of grains would be 720, and with one-tenth of a mm. 720,000; if made of the finest particles only, viz., one thousandth of a mm., the number would be 720,000 billions. Probably few of the clayey soils we ordinarily deal with are of this order; it is doubtless approached in certain fine plastic clays.

Surface afforded by various grain-sizes.—The amount of surface afforded by a similar amount of soil must naturally be considered in this connection, since upon it depends not only the amount of moisture which the soil may hold in the form of superficial films, but also the extent of surface upon which the weathering agencies as well as the root hairs of plants may act. Quoting again from King’s work, we find on the same premises given above for the number of grains, that their surface would in the case of grains of one mm. diameter be eleven square feet per pound (about half a pint) of material; while in the case of the finest grade we should have 110,538 square feet, or more than two and a half acres.

From actual experiments made with the flow of air through various soils, King calculates that while in ordinary loam soils the total surface is about an acre per cubic foot, in fine clay soils it rises to as much as four acres. If we imagine this large surface to be covered with even a very thin film of water, it is readily seen how large an amount may be present in a cubic foot of moist soil.

E. A. Mitscherlich (Bodenkunds für Land-und-Forstwirthe; Berlin, 1905) attributes to the surface offered by the soil particles supreme importance in determining the productiveness of soils. According to him the internal soil-surface determines directly the ease with which roots can penetrate the soil; and he proposes the determination of this factor by means of the heat produced in wetting the soil (“Benetzungswärme”), measured in a calorimeter, as a substitute for all methods of physical soil analysis, which are vitiated by the varying shapes and densities of the particles; while his method gives directly the actual surface. To the consumption of energy required by difficult penetration he attributes most of the differences in production, and hence refers to the internal soil-surface as governing nearly all the other physical factors. The introduction of many arbitrary assumptions, and the failure to show that the admitted inaccuracy of the ordinary mechanical soil analyses are of any practical importance, greatly detract from the cogency of the rigorous mathematical discussion carried through his work by Mitscherlich.

Influence of the several grain-sizes on soil texture.—Undoubtedly the most potent of all the sediments appearing in the above table in influencing soil texture, is the “clay.” That the materials included under this empirical designation may vary considerably in different soils, has already been sufficiently insisted on; and it is doubtful that in the present imperfect state of our knowledge of the functions of the several physical grain-sizes, we would be much wiser were we to go to the extreme advocated by Williams (Forsch. Agr. Phys., vol. 18, p. 225, ff.), of determining with precision the actual amount of such extremely fine clay particles as cease altogether to obey the law of gravity when once suspended in water. It is at least doubtful that the essential property of adhesive plasticity belongs only to these, for this property doubtless increases gradually as the size diminishes, although unquestionably not a mere function of the latter, since it belongs only to the hydrated silicate of alumina.

Ferric Hydrate.—Probably the body which most commonly modifies materially the adhesive and contractile properties of the clay substance, is ferric hydrate; the more as on account of its high density it tends to exaggerate materially, in many cases, the apparent content of true clay, and the estimate of the soil’s plasticity based upon it. A good example in point is the case of soil No. 246 (Miss.) of the above table. This is a heavy clay soil, yet not excessively adhesive; scarcely as much so as No. 230 (Miss.), the heavy gray “flatwoods” soil, and not nearly as “sticky” when wet as No. 173 (Miss.), the prairie subsoil, although containing apparently 15% more clay than the former, and 7% more than the latter. But No. 246 is a highly ferruginous clay, in which the ferric hydrate is in a very finely divided condition, and materially influences the physical qualities of the clay substance. Were it all accumulated in the “clay,” it would diminish the percentage of true clay by 11.75%, reducing the clay-percentage to 28.5% which accords more nearly with the soil’s only moderate adhesiveness, and not excessively heavy tillage.

But it must be remembered that the iron oxid shown in the analysis is not nearly always in this finely diffused condition. Frequently it incrusts the sand grains; quite commonly it forms small concretions of limonite, which themselves act as sand grains; and again, it may be present in the form of “black sand” or magnetic oxid, as is commonly the case in California and on the Pacific slope generally. To take this point properly into account, therefore, it would be necessary to determine the amount of ferric hydrate actually present in the “clay” as separated by subsidence of the granular constituents.

Other substances.—This circumstance as well as the inevitable presence of other modifying substances, clearly shows the desirability of being enabled to examine the physical properties of this “clay” directly, by collecting its entire amount as obtained in analysis, instead of merely determining it by weighing fractional portions. When this is done the analysis is much more valuable as indicating the true tilling qualities of the land. The increase of bulk suffered by this substance after wetting, is a very fair index of its content of true clay, and is preferable to the chemical analysis proposed by some investigators. For it is quite impossible to distinguish the silica and alumina derived from the kaolinitic substance proper, from that which is due to the decomposition of zeolites.

It is possible, however, to determine the possible maximum of the kaolinite ingredient by taking into consideration the quantitative ratio according to which silica and alumina combine to form it, viz., approximately 46% of the former to 40 of the latter, the rest being water. By using this calculation we can often demonstrate clearly the presence in the “clay” of considerable amounts (up to 33%) of aluminic hydrate; since no zeolitic mass can contain as much alumina as does kaolinite. Whether the aluminic hydrate be in the form of gibbsite, bauxite, diaspore, or in the gelatinous state, the nature of the soils containing it proves that it is totally destitute of plasticity and adhesiveness; and this consideration will often serve to explain the fact that soils showing in their chemical analysis high percentages of alumina, nevertheless show quite low degrees of plasticity, adhesiveness and water absorption. What part it may take in modifying the physical properties of the soil we can thus far only conjecture.

Bauxite is not only the most abundant of the three hydrates of alumina known to occur naturally, but also stands nearly midway between the two others in its water content, viz., a little over 25%; that of diaspore being nearly 15%, gibbsite about 35%.

Influence of the granular sediments upon the tilling qualities of Soils.—Considering the granular sediments by themselves, in the absence of clay, it may be stated in a general way that while in a moist condition they flocculate sufficiently to produce a fair tilth, they will nevertheless on drying collapse into a close arrangement resulting from the single-grain structure. The form of the grains being angular instead of rounded, they are apt to form a very closely packed mass far from suitable to vegetable growth; as will be seen by an example taken from one of the culture stations of the University of California, from a piece of land which on the surface would be called a very sandy loam, but after we descend increases in its content of fine grains until at a depth varying from eighteen inches to three feet we find what appears to be a hardpan, which is equally impervious to roots and water and causes the water to stagnate to such an extent that after heavy rains the land becomes so boggy as to render plowing almost impossible without endangering the team. A close examination of this hardpan shows that, unlike others, it is devoid of any cement, and when taken out can be readily crushed between the fingers, and softens in water, but does not become plastic. Its imperviousness is therefore due solely to the close packing of the sand grains, for it contains practically no plastic clay, and under the microscope the grains are seen to be angular-wedge-shaped and composed of the remnants of granite. The physical analysis shows the following result:

MECHANICAL ANALYSIS OF HARDPAN.

================================== Designation.|Diameter.|Percentage. ------------+---------+----------- { | .50 mm. | 10.93 Sand { | .30 “ | 21.23 { | .12 “ | 7.27 | | { | .072 “ | 9.63 { | .047 “ | 12.00 Silt { | .036 “ | 7.19 { | .025 “ | 1.25 { | .016 “ | 14.20 | | “Clay” | ? | 8.64 ------------+---------+-----------

It is doubtful whether this condition of things can be remedied by the usual measure of breaking up the hardpan either by hand or by means of giant-powder blasting. Experience seems to show that the effect is only temporary, and that in the course of time, by the action of the percolating waters, the particles settle back into their original impervious condition. It is just possible, however, that if once penetrated by roots, the intervention of these would permanently destroy the close structure, so as to make this a fair subsoil for the growth of trees and other plants. The writer is not aware that this kind of purely physical hardpan without cement has ever been observed elsewhere.

This physical condition is doubtless responsible for two other phenomena, viz., the “putty soils,” and also certain difficulties experienced in irrigation.

“Putty Soils” is the name popularly given in the Cotton States, and probably elsewhere, to soils usually occurring in low ground and also known as “cray-fishy.” They consist of very uniform, powdery sediment, with little or no coarse sand and still less of clay to render them coherent. When wet these soils behave precisely as would glazier’s putty, adhering to the surface of even the best-polished plowshare, so that no furrow-slice can be turned and the plow is soon dragged out of the ground. At a very closely limited condition of moisture such lands may plow fairly well; but when this limit is passed in the least (as sometimes happens in the course of a single day), it turns up only hard clods, which in a few hours of sunshine become so hard that no instrument of tillage short of a sledgehammer will make any impression upon them. The physical analysis of these usually gray soils shows that they contain only a trifling amount of clay; perhaps 1 or 2%, playing the part of linseed oil in making putty out of whiting. Even the addition of lime does not help such soils much, because there is little or no clay to flocculate. They are, as a matter of fact, among the most refractory lands the farmer has to deal with. A soil showing similar behavior, though not quite as extreme as in the case of the Gulf or Cotton States’ soils in question, occurs at the culture substation at Paso Robles, California, and is probably closely correlated to the physical hardpan referred to above. The physical analysis of this soil yielded the following result:

MECHANICAL ANALYSIS OF SOIL.

================================== Designation.|Diameter.|Percentage. ------------+---------+----------- { | .50 mm. | 14.24 { | .30 “ | 15.17 Sand { | .16 “ | 8.88 { | .12 “ | 5.60 | | { | .072 “ | 6.75 { | .047 “ | 8.35 Silt { | .036 “ | 8.55 { | .025 “ | 6.03 { | .016 “ | 17.77 | | “Clay” | ? | 7.50 ------------+---------+-----------

It would seem the best and almost only remedy to be applied to such soils as these is the introduction of vegetable matter or green-manuring, by which their texture is loosened: for the hauling of mere clay upon the land would hardly accomplish the purpose intended, within the limits of farm economy.

Dust Soils, which during the dry season are even in their natural condition so loose as to rise in clouds and render travel very uncomfortable, are not uncommon in arid countries, e. g., in Washington and adjacent parts of Oregon, on the uplands bordering the Columbia, Yakima and Snake rivers. The physical analyses of three of such soils, given in the table below, will convey some idea of their peculiarities in this respect.

PHYSICAL ANALYSIS OF DUST SOILS.

========================================================= |Hydr. Value.|Diameter. |No 17.|No. 37.|No. 79. ----------+------------+----------+------+-------+------- Clay |<.0023. mm. |<.10--? | .93| 3.59 | 1.27 { |<.25 mm. | .010 | 30.93| 13.06 | 32.29 Silt { | .25 to .5| .016 | 3.20| 5.82 | 12.75 { | .5 to 2.0| .025-.047| 7.18| 27.37 | 37.51 { |2.0 to 8.0| .047-.120| 21.88| 43.78 | 10.92 Sand |8.0 to 64.0| .12- .50 | 32.39| 49.57 | 3.97 | | +------+-------+------- Total | | | 96.57| 98.18 | 98.72 ----------+------------+----------+------+-------+-------

Slow penetration of Water.—Soils of this class are wetted with extreme slowness by irrigation water; so that when first taken under cultivation it sometimes takes twenty-four hours to soak the land for twelve inches in each direction. Irrigation furrows must be placed very close together and in large numbers, in order to ensure the wetting of the soil so that the crop shall not suffer from lack of moisture at a distance of two or not more than three feet. Where the irrigation furrows are drawn farther apart a fine stand of grain may be seen within eighteen inches of the same, while farther away the crops may be dying from lack of moisture. This difficulty is by no means infrequent in the arid region, and is difficult to overcome except by frequent and thorough tillage, which gradually increases the rapidity of water-penetration; as has been shown in the soils of the alluvial prairies of the Yakima country in the State of Washington. It is necessary, however, to take care that they shall always contain an adequate amount of humus or vegetable matter, in order to prevent re-consolidation by the burning-out of the humus during the warm, rainless season.

There is an unmistakable resemblance between these dust soils of the Northwest and the “putty” soils mentioned above; both showing a very low percentage of clay with a relatively large amount of the finest sediments, with a sudden downward break of the curve before the coarser grain-sizes are reached. It would seem as though the absence of these intermediate grains favors the close packing of the fine sediments in the interstices of the coarse ones, thus bringing about the imperviousness, which is the chief obstacle to their cultivation.

Effects of coarse Sand.—Coarse sand intermingled with heavy clay soils has but little effect in improving the tilling qualities, unless carried to such excess as renders it financiallyimpracticable. In actual practice it is frequently possible to improve such soils by properly distributing upon them the washings of the adjacent hills, which will always carry sands of many grades; and when it is intended to improve garden land by hauling sand it is important to choose the latter so as to complement the deficient grain-sizes of the soil. The sand of wind drifts or dunes is generally well adapted to such improvement, being, as Udden has shown, of a fairly definite composition of sufficiently wide range of grain-sizes for the purpose.

The effects of humus in modifying soil texture are discussed farther on.

The Mechanical Composition of Wind Deposits, Bull. No. 1, Augustana Library Publications; 1898.