CHAPTER VII.. The Density and Volume-Weight of Soils.

THE DENSITY AND VOLUME-WEIGHT OF SOILS.

Aside from the humus-substances the specific gravity of the common soil constituents, taken individually, do not vary widely; kaolinite being the lightest (2.60), feldspar next (2.62); then quartz (2.65), calcite (2.72). Mica and hornblende range (according to their iron contents) from 2.72 to over 3.0. The average specific gravity of soils of ordinary humus content only will thus range between 2.55 and 2.75; sandy soils approaching very closely to that of quartz alone.

Volume-Weight.—The specific gravity of the soil is, however, of little practical consequence compared with the “volume-weight,” i. e., the weight of the natural soil as compared with an equal bulk of water. A cubic foot of water weighs 62½ pounds; a similar volume of soil usually weighs more, but in the case of peaty lands may actually (when dry) weigh less. The extreme range is from 110 pounds for calcareous, and somewhat less for siliceous sand, to as little as 30 to 50 pounds in the case of peaty and swamp soils. It may be conveniently remembered that while average arable loams range from 80 to about 95 pounds per cubic foot, “heavy” clay soils range from 75 pounds down to 69, observed by the writer in the case of certain alluvial soils, poor in humus, of the Sacramento river, California. Manured garden soils, and the mold surface soil of deciduous forests, generally contain so much humus as to depress their weight considerably, varying according to their state of tilth from 66 to 70 pounds per cubic foot.

This remarkable soil seems to have been derived from the finest “slickens” of the hydraulic gold mines.

Weight per acre-foot.—As for practical purposes and calculations it is often desirable to know approximately the weight in pounds of an acre (43,560 square feet) one foot deep, it is convenient to remember that in the case of sandy land, this weight (per “acre-foot”) may be assumed at four millions of pounds; for loams, at 3½ millions; for clay lands, 3¼ millions; for humus or garden land and woods earth, about 3 millions of pounds; for reedy swamp and peaty lands, 2 to 2½ millions.

The loose tilth and humus-content of the surface soil will in general cause it to weigh less, bulk for bulk, than the underlying subsoil, even when the latter is more clayey; moreover, the continuous pressure from above will tend to consolidate the subsoil and substrata. Warington (Phys. Properties of Soils, pp. 46, 47) gives interesting data on this point from the Rothamstead fields, as follows:

Old pasture, first nine inches 71.3 pounds per cub. ft. Same, fourth “ “ 102.3 “ “ “ “ Arable land, first “ “ 89.4 “ “ “ “ Same, fourth “ “ 101.4 “ “ “ “

The influence of humus and unhumified organic matter, as well as of tillage, in diminishing the volume-weight of soils is here strikingly shown.

Air-space in Natural Soils.—The difference between the specific gravity as usually determined, and the volume-weight of soils, is of course caused by the large amount of air contained in them when dry, but which in wetting them is partially or wholly replaced by water.

Theoretically, assuming all soil grains to be globular, and packed as closely as possible (in oblique order), the space not filled by them would be the same for all sizes, whether that of marbles, or so minute as to be hardly felt between the fingers; and would be 25.95 per cent of the soil volume. If the same globular particles were packed as loosely as possible, i. e., in square instead of oblique order (see figures 10 and 11), the vacant space would be 47.64 per cent If however we imagine each sphere to be itself composed of a number of smaller ones, the empty space will obviously be greatly increased, to an extent proportionate to the diminution of solid mass thus brought about. The pore-space might in that case, with the oblique arrangement of the globules as shown in Fig. 10, be as high as 74.05 per cent But since the soil particles may be of all shapes and sizes within the same soil, and usually fit much more closely than would globular grains, the empty space rarely approaches (only in certain alluvial soils and in loose mulches) to the figure last named. In sandy soils it may fall as low as 20%, and in coarse gravelly soils even as low as 10%. Most cultivated soils range between 35 and 50% of empty space.

King, Physics of Agriculture, p. 116, ff.

Effects of Tillage.—That these figures can be only approximations is obvious from the consideration that one and the same soil will vary materially in its volume-weight according to its temporary condition of greater or less compactness. After land has been beaten by winter rains, its volume-weight will be found to have materially increased from the well-tilled condition brought about by thorough cultivation. This difference is strikingly seen when, in plowing, the height of the ground on the land side is compared with that of the turned furrow-slice in well conditioned loamy land. This loose condition is called tilth, and it results from the formation of relatively large, complex crumbs or floccules, between which there are large air spaces that were wholly absent in the untilled land; the floccules themselves being also more loosely aggregated than was the case before tillage.

The word crumbs, which is generally understood as meaning a relatively large, loose aggregate, seems preferable to the word kernels, suggested for the same by King (Physics of the Soil, p. 110). Kernels are understood to be bodies rather more solid than the surrounding mass, and do not convey the idea of loose aggregates. The word “Krümelstructur” (crumb-structure), adopted by Wollny for this phenomenon, has both fitness and priority in its favor.

Crumb or Flocculated structure.—Figure 11 illustrates the difference between the unplowed land, consolidated especially on the surface by winter rains, and in its upper portion consisting largely of single grains; while the plowed land, toward which the furrow-slices have been turned, is greatly increased in height and volume and consists almost wholly of variously-shaped and-sized aggregates or floccules, loosely piled upon one another and separated by large interspaces. The increase in volume from consolidated clay to crumb-structure is given by Wollny (Forsch., vol. 20, p. 13, 1897) at 41.9%, to powder as 33%. On moistening dry clay increased 36.9%, quartz powder 8.01%. When land is plowed in the proper moisture-condition the crumbs of floccules are held together by the surface tension of the capillary films (menisci) of water at the points of contact. In the case of sands, the crumbs will collapse into single grains whenever the water-films evaporate, unless some cementing substance was dissolved or suspended in the water. (See figure 12). Lime carbonate is one of the substances most commonly found permanently cementing the floccules; hence the ready tillage of most calcareous soils, and especially the loose texture of the “loess” of the western United States, and of Europe and Asia. In these deposits we find sandy and silt aggregates or concretions ranging from ten or more inches in length (loess puppets) to microscopic size, held together by lime carbonate, but collapsing into silt and sand when the material is treated with acid so as to dissolve the cement. The rough surfaces of these aggregates, gripping into each other, explain the stability of the steep loess cliffs in the United States, as well as in northeastern China, as observed by Von Richthofen and Pumpelly.

Clay is most frequently the substance which imparts at least temporary stability to the crumbs and crumb-structure; this is one of its most important functions in soils, as it serves to maintain tilth once imparted by cultivation, even after the land dries out. Beating rains, and cultivation while too wet, will in this case of course destroy the crumbs and the loose tilth.

Other substances which greatly aid the maintenance of tilth are the several humates (of lime, magnesia, iron), which when fresh are colloidal (jelly-like) like clay itself, but unlike the latter, when once dried do not resume their plastic form by wetting (Schloesing). The crumbs thus formed are therefore quite permanent and contribute to the looseness of soils rich in humus. One part of lime humate is said by Schloesing to be equal in cementing power to eleven parts of clay.

Silica, silicates and ferric hydrate are sometimes found cementing soil crumbs, wholly or in part.

The importance of the ready penetration of air, water and roots thus rendered possible is obvious; and the question arises how it happens that wild plants are able to do without tillage.

How Nature Tills.—When we examine the undisturbed soil of woods or prairie in the humid region, we will as a rule find the natural surface soil in a very good condition of tilth; the obvious cause being the presence in it of an abundant network of surface roots and rootlets of grasses and herbs, which in connection with the fallen foliage prevent the beating and compacting of the soil surface; which can be seen to happen before the observer’s eyes whenever a heavy rain falls on a bare land surface, however well tilled.

Crusting of Soils.—In some soils, especially of the Gulf States, the beating of rain followed by warm sunshine so effectually compacts the surface that in the case of taprooted plants like cotton, it becomes necessary to cultivate after each rain, so as to break the crust that would otherwise not only prevent the proper circulation of air, but would also serve to waste the moisture of the land. The same land in the wild condition suffered no such change, being protected by the native vegetation, and by fallen leaves. (See chapt. 8).

Soils of the arid region.—In the regions of deficient rainfall the conditions are modified in several respects. Grass sward rarely exists, nearly all grasses assuming the habit of growing in tufts or bunches some distance (a foot or two) apart; hence the name of “bunch grass” commonly used, which however means not any one definite kind of grass, but serves to distinguish the grasses of the uplands from those of the moist lowlands, where true sward may be found. Between these bunches of grass the soil is fully exposed, and being free from roots and leaf-covering is compacted, unless its nature is such that the usually gentle rains do not produce a serious crusting of the surface.

That such is actually the predominant nature of the soils formed under arid influences has already been stated; and thus the hard-baked soil-surface so often seen in the Eastern United States in unplowed bare land, or during the prevalence of a drought, is rarely seen in the arid region. The clay lands that do exist are usually sufficiently calcareous to possess the property of “slaking” into crumbs whenever wetted after drying. But where this is not the case, the stony hardness brought about by the long dry and warm season is long in being removed by the winter rains.

Changes of soil-volume on wetting and drying.—The behavior of colloidal clay in the above respects has already been described above (see chapt. 4, page 59). It is obvious that whenever soils contain a large proportion of such clay, their behavior on wetting and drying will approximate to those of the pure clay. This is exemplified in the heavy clay, or so-called “prairie soils” of the United States, which when thoroughly wetted in spring will, during a dry summer, form wide, gaping cracks. These in the long summers of the arid region may extend to the depth of several feet, with a width of as much as three and more inches at the surface of the ground. This, of course, contributes greatly to the drying-out of the soil to the same depth, and results as well in the mechanical tearing of the root-system of growing plants; sometimes causing the total destruction of vegetation. In some clay soils it happens that after a rain or irrigation, the shrinkage occurring upon the advent of warm sunshine will cause the surface crust to so contract around the stem, e. g., of grain, as to constrict and injure the bark, causing serious injury to the crop. In soils of this character very thorough tillage in preparing for a crop, and the maintenance of a loose surface during its growth, are of course extremely essential.

In the arid region it will frequently happen that such soils when not tilled to a sufficient depth, will during the later part of the summer so shrink and crack beneath the shallow-tilled surface layer that the latter will bodily fall into the cracks, exposing the roots to all the deleterious influences of mechanical lesion and drying-out. It is thus obvious that the cultivation of such soils should not be undertaken at all by those not naturally able and willing to bestow upon them, to the fullest extent, the deep and thorough tillage which is absolutely essential in the utilization of their usually high productive power.

Extent of Shrinkage.—The extent of this shrinkage in drying, and subsequent expansion in wetting, have been measured by the writer by the use of the sieve cylinder described below (chapt. 11, p. 209), as serving for the determination of the water capacity of soils. When a soil of the kind above referred to is placed in the sieve cylinder in the tilled (flocculated) condition, then allowed to absorb its maximum of water and then dried at 100 degrees C., the contraction in drying can be very strikingly seen, and its amount measured by filling up the empty space with mercury; then measuring the latter after expelling the surplus by means of a ground glass plate laid on top. The contraction of several heavy clay soils, thus measured, has been found by the writer to range from 28 to as much as 40 per cent of the original bulk. The soil thus contracted, when again wetted, does not return altogether to its original bulk, but remains in a more or less compacted condition, like that of a soil which has been rained upon.

Wollny (Forsch. Vol. 20, p. 13 ff., 1897) records similarly high shrinkages in his experiments.

The expansion and contraction of a heavy clay soil on wetting and drying are well illustrated in the figure below, in which the soils are shown in the shallow cylinder which serves for the determination of water-holding power (see chapt. 11, p. 209). The middle figure shows in profile the expansion of a dry, pulverized “black adobe,” struck level, when allowed to absorb its maximum of water; it rises above the rim of the sieve-box to nearly the half height of the latter. The outside figure to the right shows the same soil after drying; that to the left, a red clay soil similarly treated. It is easily seen that these variations in volume may bring about very marked results in the fields; the surface of which, apart from the cracks usually formed, may be several inches lower in the dry season than during wet weather.

FIG. 13.—Expansion on Wetting and Contraction on Drying of heavy clay soils.]

Contraction on Wetting.—In the case of alkali soils containing much carbonate of soda, a very notable contraction occurs in wetting the loose, dry soil. The cause is here obviously the collapse of the crumbs, formed in dry tillage or crushing, into single grains, closely packed. The same result is observed in the naturally depressed “alkali spots” (see chapt. 22).

“Hog-wallows.”—In the field the wetting of cracked clay soils produces some very curious effects. The effect of the first light rains usually is to crumble off the edges or angles near the surface, the materials thus loosened falling into the lower portion of the cracks. This is repeated at each successive shower followed by sunshine, the crevices thus becoming partly filled with surface soil. When, subsequently, the heavier and more continuous rains wet the land fully, also causing the consolidated mass in the crevices to expand, the latter cannot close on account of the surplus material having fallen into them; the result being that the intermediate portions of the soil are compelled to bulge upward, sometimes for six or more inches, creating a very uneven, humpy surface, well-known in the southwestern United States as “hog-wallows.”

A totally different kind of “hog-wallows,” occurring in California and the arid region generally, have been described in a previous chapter under the head of Aeolian soils (See chapt. 1, p. 9).

Such a surface is always therefore an indication of an extremely heavy soil, difficult to cultivate; yet embracing some of the most highly and permanently productive lands known in the United States, and in India, where the “regur” lands of the Deccan are of this character; they have been cultivated without fertilization for thousands of years. The subjoined physical analyses of lands of such extreme character as to be almost uncultivatable will serve to exemplify their physical composition.

PHYSICAL ANALYSES OF HEAVIEST CLAY SOILS.

=================================================================== |No 242 Miss.|No. 643 Cal. +------------+------------ |Hog-wallows |Black Adobe. | soil. |Contra Costa | Jasper Co. | Co. |Mississippi.|California. -----------------------------------------+------------+------------ Weight of gravel over 1.2 mm. diameter } | .83 | “ “ between 1.2 and 1 mm } | | “ “ between 1 and 0.6 mm | 1.19 | Fine earth | 97.98 | 100.00 +------------+------------ | 100.00 | 100.00 FINE EARTH. | | | | Hydr. Value. | Diameter. | | -------------------------+---------------+ | Clay <.0023 mm | ? | 48.00 | 45.96 | | | { <0.25 mm | .010 | } 35.18 | 37.64 { 0.25 mm | .016 | } | Silt { 0.5 mm | .025 | 5.50 | 2.74 { 1.0 mm | .036 | 3.74 | 3.31 { 2.0 mm | .047 | 2.54 | 2.95 { 4.0 mm | .072 | .20 | 2.39 | | | { 8.0 mm | .120 | .27 | 1.68 { 16.0 mm | .160 | .90 | .79 Sand { 32.0 mm | .30 | 1.67 | 2.36 { 64.0 mm | .50 | 2.00 | | +------------+------------ | | 100.00 | 100.00 -------------------------+---------------+------------+------------

It will be noted that in both these extremely heavy soils the sum of the clay and finest sediments is a little over 83%.

It should be stated that both these soils after being thoroughly wetted become so adhesive that it is almost impossible to travel over the tracts occupied by them, and that they are practically almost untillable, being too adhesive when wet; yet if allowed to dry to a certain extent (varying within very narrow limits) they turn up by the plow in large clods, which after a few hours of sunshine become of stony hardness and will resist all efforts at pulverization or the production of tilth.

In driving a light carriage over the land represented by No. 643 above, after a light rain, the wheels gathered up so much soil within a hundred yards as to render it necessary to stop and chop it off the tires by means of a hatchet. This is a common experience in the black prairie lands of Texas.

Calcareous Clay Soils crumble on drying.—The heavy clay soils of some of the calcareous prairies of the Southwest, instead of contracting into a stony mass on drying, on the contrary resolve into a mass of crumbs, thus producing excellent tilth. This occurs even though the land may have been plowed when wet, and of course is a great advantage. The most striking exemplification of this peculiarity occurs in the heavy but profusely fertile “buckshot” clay lands of the Yazoo bottom, in Mississippi, where it is usual to plant corn and sweet potatoes in the semi-fluid mud left after an overflow, after turning a shallow furrow, then covering by turning another. To the onlooker it seems impossible that such plantings could be successful; but within a short time the muddy surface becomes a bed of crumbs (“buckshot”), forming a seed-bed not readily excelled by any made by artificial means. Hence, largely, the almost invariable success of crops in the Yazoo region.

Port Hudson Bluff.—The same clay produces a most unpleasant result at the foot of the Port Hudson bluff, where it crops out some feet above low water. When after a freshet the water level falls below this stratum, on drying the clay disintegrates into crumbs just as does the Yazoo buckshot soil; with the result that at the next rise, the loose mass subsides into the river as a flood of mud. Thus the foot of the bluff is being constantly undermined, and the falling of the bluff scarp has obliged the town above to recede many hundreds of feet from its original historic site.

The exact proportions of lime carbonate necessary to produce this phenomenon, and its necessary relations to clay substance and other physical soil ingredients, yet remain to be investigated.

Schübler (Grundsätzed. Agrikulturchemie, 1838) ascribes the crumbling of calcareous clay soils to the difference in the contraction of calcareous sand and the clay substance. But it is doubtless more directly connected with the flocculation of the latter by lime.

Loamy and Sandy Soils.—It is largely the absence of these extreme changes of volume that renders the cultivation of loamy or even sandy lands so much more easy, and the success of crops so much more safe, than is the case in clay soils. Whenever the content of colloidal clay diminishes below 15%, the shrinkage in drying from the wet condition becomes so slight as to cause no inconvenience; while in sandy soils properly speaking, no perceptible change in volume occurs.

Peaty soils, however, and all those containing a relatively large amount of humus, are also liable to visible shrinkage when passing from the wet to the dry condition. But on account of their looseness and porosity such shrinkage does not usually result in the formation of cracks or rupture of the roots, as is the case in heavy clay lands. The entire mass of the soil then shrinks downwards, but rarely forms cracks on the surface. Hence the introduction of humus into “heavy” soils is among the best means of improving their tilling qualities.

Formation of Surface Crusts.—Some soils, especially those of a clay-loam character, are very liable to the formation of hard surface crusts from the beating of rains, and from surface irrigation; owing, doubtless, to the ready deflocculation of their clay substance. It is not easy to define the precise physical composition conducive to this crust formation; but the subjoined physical analyses show examples of soils in which this tendency is very prominent and is frequently annoying, in that when they occur in the regions of frequent summer rains, it becomes necessary after each one to till the surface in hoed crops (e. g., in cotton-fields) in order to prevent the injurious effects of such consolidation of the surface. It may, of course, be prevented by mulching, or on the large scale by green-manuring, to such extent as to prevent contraction.

The subjoined physical analyses of two soils from the Brown-Loam region of Northern Mississippi (see chap. 24), shows the composition of lands excellent in every respect other than the tendency to crust after each rain:

PHYSICAL ANALYSES OF CRUST-FORMING SOILS.

=========================================================== |Diameter.|Hydr. Value.|No. 219.|No. 197. ------------------+---------+------------+--------+-------- Coarse materials | 1-3 mm. | | } | | | | } .23 | { |.5-1 “ | | } | { | .50 | 64 mm. | 1.47 | Sand { | .30 | 32 “ | 2.33 | .79 { | .16 | 16 “ | 1.17 | { | .12 | 8 “ | .78 | .18 | | | | { | .072 | 4 “ | .76 | .78 { | .047 | 2 “ | 9.79 | 3.56 Silt { | .036 | 1 “ | 7.20 | 13.12 { | .025 | .50 | 13.11 | 16.64 { | .016 | .25 | 15.07 | 27.28 { | .010 | <.25 | 26.36 | 18.87 | | | | Clay | ? | <.0023 | 19.10 | 17.23 ------------------+---------+------------+--------+--------

These soils agree in having a sufficient amount of clay (17 to 19%) to characterize them as clayey loams, associated with a very large proportion of the grain-sizes of less than .025 mm., or .5 mm. hydraulic value. A higher proportion of clay, even though associated with a similarly high or even larger proportion of these fine sediments, seems to prevent crusting, probably because the swelling of the clayey ingredient on wetting and its extravagant contraction in drying breaks up the continuity of the surface. The heaviest clay soils, such as those shown on a preceding page, neither crust nor crumble on drying after wetting, but contract into lumps of stony hardness, as a whole.

The burning-out of the humus from well-tilled surface soils during the extended heat and dryness of rainless summers, brings about such a contraction or packing of the surface soil of orchards in California as to greatly reduce their productiveness, and to render necessary diligent green-manuring as the only practical remedy. In many cases, liming of the surface also serves well to prevent this injurious effect, which to some extent of course follows surface irrigation as well as rains.

In most soils, repeated alternate wetting and drying in place produces a loose, flocculated texture, so long as no deflocculation is brought about by mechanical causes, such as beating rains or running water.

Effects of Frost on the Soil.—The expansion suffered by water in freezing necessarily tends to separate the soil particles previously held together by the surface tension of the capillary water, or otherwise flocculated or cemented. Freezing of the soil is therefore of material assistance in disintegrating cloddy, ill-conditioned soils, leaving them in loose, crumbly condition after the ice has melted and the surplus water drained off; so as to materially facilitate tillage and root penetration. When, however, soils thus circumstanced are tilled or trodden while too wet, they quickly become puddled, being practically reduced to single-grain structure. (See this chapt. p. 110). Hence the injury caused by allowing cattle to range in winter on cultivated land subject to freezing and thawing, which it sometimes takes years to correct.

A disagreeable effect often produced by the freezing and thawing of wet lands is the “heaving-cut” of grain, resulting from the upward expansion of the surface soil in freezing, that may readily rupture the roots; while on thawing, the soil surrounding the upheaved stool is apt to settle down, especially in case of a rain, leaving the stool and roots exposed either to drying or freezing, as the case may be. Hence the desire of grain farmers in northern climates, for a sufficient covering of snow to protect the fall-sown grain, rather than an “open winter,” during which the grain is exposed to alternate freezes and thaws, or extreme cold.

In certain soils, notably in those liable to crusting (p. 117), instead of heaving the soil, the water in freezing emerges bodily from small cracks, in foliated or wire-like forms (“ice-flowers”) resembling those of native silver, and formed substantially in the same way, by a kind of “wire-drawing” process, aided by crystallization.

Small ice-crystals formed on the surface of small crevices filled with water cause others to be formed at their lower ends, and the expansion occurring in freezing, forces the ice upward; the process repeating itself under favorable conditions, until the stalks or sheets of ribbed ice grow to a height of several inches. This phenomenon is especially frequent in the middle cotton States—Arkansas, Tennessee, northern Mississippi, etc., where frequent changes from rainstorms or thaws to cold northwest winds occur in winter.