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,[29] 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.
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.
Fig. 10.—Various possible arrangements of soil particles.
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.[30] 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.