Chapter XXI: Introduction

_Definition of Soils._—In the most general meaning of the term, a soil is the more or less loose and friable material in which, by means of their roots, plants may or do find a foothold and nourishment, as well as other conditions of growth. Soils form the uppermost layer of the earth’s crust; but the term does not indicate any such definite average texture as is sometimes implied by its popular use to designate certain loose, loamy materials found in older geological formations. We do find in these, not unfrequently, layers that in the past have served to support vegetation, as evidenced by remains of plants found therein. But as a rule, such ancient soils are much compacted and otherwise changed, and would not now be capable of performing the office of plant nutrition without previous, long-continued exposure to the same agencies by which all soils were originally formed from pre-existing rocks. Within the latter category must be included, in scientific parlance, not only the hard rocks known as such in daily life, but also such soft materials as clay, sand, marls, etc., which often compose, partially or wholly, the bodies of wide-spread geological formations.

_Elements Constituting the Earth’s Crust._—More than seventy elementary substances have been found within the portion of the earth accessible to man; most of these are present only in very minute proportions; of those occurring in relatively considerable quantities, a list showing their approximate proportions is given below.

_Average quantitative composition of the Earth’s Crust._—The total thickness of the outer shell of the earth, thus far known to us, does not exceed about 95,000 feet, as observed in the accessible rock deposits. Estimates of the proportions in which the more abundant elements contribute to the composition of these constituent rocks, have repeatedly been made. The latest and most widely accepted of these, by F. W. Clarke, of the U. S. Geological Survey, is given herewith. It includes the constituents of the sea and atmosphere as well; these two constitute about 7 per cent of the whole, 93 per cent being solid rocks.

RELATIVE ABUNDANCE OF THE ELEMENTS TO A DEPTH OF TEN KILOMETERS.

SOLID CRUST OCEAN MEAN,
(93 PER CENT). (7 PER CENT). INCLUDING AIR.

Oxygen 47.29 85.79 49.98
Silicon 27.21 25.30
Aluminum 7.81 7.26
Iron 5.46 5.08
Calcium 3.77 0.05 3.51
Magnesium 2.68 0.14 2.50
Sodium 2.36 1.14 2.28
Potassium 2.40 0.04 2.23
Hydrogen 0.21 10.67 0.94
Titanium 0.33 0.30
Carbon 0.22 0.002 0.21
Chlorin 0.01 2.07 0.15
Phosphorus 0.10 0.09
Manganese 0.08 0.07
Sulphur 0.03 0.09 0.04
Barium 0.03 0.03
Nitrogen 0.02
Fluorin 0.02 0.02
Chromium 0.01 0.01

It will be noted that one-half of the total consists of oxygen, and that nearly 86% (or 47.29% of the 49.98%) of this amount is contained in the solid rocks; nearly 2.50% of the remainder in sea and other water; and .41% in the atmosphere, in the free condition, in which it serves for the respiration of animals and plants, and for the various processes of slow and rapid combustion, or “oxidation.” This relatively small proportion of the whole, is, nevertheless, the most directly important for the maintenance of organic life.

_Oxids Constitute Earth’s Crust._—The vast predominance of oxygen in the above list suggests at once that most of the other elements must exist in combination with it, _i. e._, as “oxids.” H. S. Washington[2] has lately revised the estimates heretofore made, on the basis of a very large number of analyses made by him and others, of rocks within the United States, and gives the following table; alongside of which is placed a revised estimate by Clarke, which also includes rocks from abroad; both being given in terms of oxids of the several elements.

[2] U. S. Geol. Survey, Professional Paper No. 14, p. 108.

WASHINGTON. CLARKE.

Silica SiO₂ 57.78 59.89
Alumina Al₂O₃ 15.67 15.45
Peroxid of Iron Fe₂O₃ 3.31 2.64
Protoxid of Iron FeO 3.84 3.53
Magnesia MgO 3.81 4.37
Lime CaO 5.18 4.91
Soda Na₂O 3.88 3.56
Potash K₂O 3.13 2.81
Water, basic H₂O⁺ 1.42 1.52
Water, acid H₂O⁻ .36 .40
Ferric Sulphid FeS₂ 1.03 .60
Phosphoric acid P₂O₅ .37 .22
Manganese Protoxid MnO .22 .10

The salient point which at once attracts attention in these tables is the great predominance of the oxid of silicon—silica, silicic acid, quartz, etc.,—over all other substances. While quartz occurs alone in enormous masses, as will be shown later, probably the greater proportion is found in combination with other oxids, notably those of aluminum, calcium, iron, magnesium, and the alkali metals potassium and sodium. Chlorin and fluorin, however, do not occur as oxids.[3]

[3] A trifling amount of chlorin is found oxidized in the form of sodium perchlorate, in the nitre deposits of Chile.

_The Chemical Elements Important to Agriculture._—Of the numerous elements known to chemists, only eighteen require mention in connection with either soil formation or plant growth; and of these only thirteen or fourteen participate in normal plant growth. They are the following:

METALLIC ELEMENTS. NON-METALLIC ELEMENTS.

Potassium Carbon
Sodium Hydrogen
Calcium Oxygen
Magnesium Nitrogen
Iron Phosphorus
Manganese Sulphur
Aluminum Chlorin
Titanium Fluorin
Iodin
Silicon.

Of this list, titanium, though a very constant ingredient of soils in the form of titanic dioxid, is not known as performing any important function in soils, and is not, so far as known at present, ever taken up by plants. Aluminum, in the form of its compounds with oxygen and silicon, is a very prominent and physically very important soil ingredient, but does not, apparently, perform any direct function in plant nutrition, and is absent from their ash, except in the case of some of the lower plants (horsetails and ferns).

Iodin appears to be normally present in all seaweeds, and occurs in traces in some land plants. Fluorin is a normal ingredient of animal bones, and its presence in plant ashes is often easily shown. The remaining fourteen, however, are always present in plants; carbon, hydrogen, oxygen and nitrogen forming the volatile or combustible part, while the rest occur in the ashes.

It is true that other elements, or rather their compounds, are sometimes found in plants, being taken up by them from solutions existing in the soil. Thus the alkalies caesium and rubidium, also barium, strontium, zinc, copper, boron and some others, may be absorbed when present in soluble form. But they are neither necessary nor beneficial to plant economy, and when in considerable amounts are harmful. Thus fifteen elements, ommiting iodin and titanium, alone require discussion.

_The Volatile Part of Plants_, as already stated, consists of carbon, hydrogen, oxygen and nitrogen. Of these, carbon is obtained by the plant exclusively from the carbonic (dioxid) gas of the air; hydrogen and oxygen, from the soil in the form of water; nitrogen, directly from the soil but indirectly also from the air, through the agency of certain bacteria. The _ash ingredients_ of course are all derived from the soil through the roots, and must all be present in the latter in an available form, to a sufficient extent to supply the demands of vegetation.

_The Agencies of Soil Formation._—With respect to their mode of formation, soils may be defined as the residual product of the physical disintegration and chemical decomposition of rocks; with, ordinarily, a small proportion of the remnants of organic life. The agencies producing these changes are those classed under the general term “atmospheric” or “meteorological;” they include therefore the action of _temperature_—heat and cold—that of _water_, and that of _air and its ingredients_. In popular parlance, it includes the processes of _weathering_; nearly the same processes are involved in the “fallowing” of soils.