CHAPTER VIII.. Soil and Subsoil.

SOIL AND SUBSOIL.

CAUSES AND PROCESS OF DIFFERENTIATION. HUMUS.

Soil and Subsoil Ill-defined.—While the general mass of rock debris formed by the action of the agencies heretofore discussed as soil-material, may under proper conditions become soil capable of supporting useful plant growth, universal experience has long ago recognized and established the distinction between soil and subsoil: by which are ordinarily meant, respectively, the portion of the soil-material usually subjected to tillage, and what lies beneath. There can be no question about the practical importance of this distinction; but the definition of the two terms, as commonly given in some works of agriculture, is both incomplete and, in its application to many cases, partly misleading.

The differentiation of soil and subsoil is due partly to the action of organic matter and micro-organisms, partly to physico-chemical causes, now to be discussed in detail.

THE ORGANIC AND ORGANIZED CONSTITUENTS OF SOILS.

Humus in the Surface soil.—The most obvious mark of distinction between soil and subsoil is, usually, the darker tint of the former, due to the presence of humus or vegetable mold, which becomes most apparent by darkening of the tint when the soil is moistened. Thus soils having a gray tint when dry, may become almost black when wetted. When no such deepening of color occurs in wetting, the absence or great deficiency of humus may safely be inferred. The only other substance whose presence may invalidate the conclusions based upon the darkening of the soil tint, is ferric hydrate (iron rust), which itself possesses the property of darkening on wetting, and may effectually cover either the presence or the absence of humus.

Since the formation of the humus depends upon the decomposition of organic matter (mostly of the cellulose group) derived partly from the roots, partly from the leaves and stems of plants growing and dying on the soil, its accumulation near the surface is natural. But since the depth to which roots penetrate varies greatly not only with different plants, but very essentially in conformity with the greater or less penetrability of the soil and subsoil, the depth to which the dark humus tint may reach vertically varies correspondingly, from two or three inches to several feet. In the case of soils that have been formed by the gradual filling-up of swamps or marshes, the humus-tint may reach to several yards depth.

Surface Soil, and Subsoil.—It is thus apparent that the term “surface soil,” while commonly confined by the farmer to the portion turned by the plow or usually reached in cultivation by any implements, may or may not belong, functionally, to layers of greatly varying thickness. Similarly the term subsoil may or may not refer, in individual cases, to parts of the soil mass materially different from the surface soil. Yet this distinction is of no mean practical importance, because the efficacy of one of the most common measures of soil improvement, viz., subsoil plowing or “subsoiling,” depends materially upon the differences between soil and subsoil in each particular case. Most of the diversity of opinion regarding the merits of this operation is simply the result of a corresponding diversity in the natural facts and cultural practice of each case.

Causes of the Differentiation of Soil and Subsoil.—One of the prominent points of difference between surface soils and subsoils has already been mentioned in the usual predominance of root-mass in the upper layers; to which is added a part at least of the substance of fallen leaves and stems of its vegetation. How much of this vegetable mass ultimately becomes converted into humus, as well as the nature of the product formed, depends upon a great variety of circumstances; some of which have already been mentioned in connection with the general discussion of humification (chapt. 2, p. 20). Briefly stated, the main controlling conditions are: the amount of water or moisture present, the access of air (oxygen), a proper temperature, and the presence of the several organisms which in the course of time take part in the process of soil-formation.

Ulmin Substances; Sour Humus (Germ. Rohhumus).—In the presence of so much moisture or liquid water as will materially impede the access of air, and with the concurrence of reasonably low temperatures, the organisms that at first take the chief role in the transformation of the vegetable tissues into humus-like substances are bacteria. But the antiseptic nature of the compounds thus formed soon puts an end to their activity, and thereafter the process seems to be a purely chemical one, and very slow. In peat bogs, the transition from the fresh, dead stems and roots to brown peat is easily followed downward, white cellulose fibers remaining apparently unchanged to some depth; so that such fiber has been used for tissues and paper. The solid decomposition-products are brown substances, partly soluble in water and imparting to it a brown or coffee color (frequently seen in the drains of marshes) and an acid reaction; the latter due to ulmic (as well as apocrenic) acid, readily soluble in caustic and carbonated alkalies, and forming insoluble salts with the earths and metals; while another portion, ulmin, is insoluble in the same, but gradually becomes soluble by oxidation.

The antiseptic properties of sour humus are well exemplified in the perfect state of preservation in which the remains of animals, wood implements, etc., are found in bogs into which they have sunk in prehistoric times.

The gaseous products formed under these conditions are carbonic dioxid and “marsh gas” (methan, CH₄), the former predominating in the early stages; while later, the carburetted hydrogen predominates, rendering the gas readily inflammable.

Sour Soils.—The “sour” soils thus produced in nature in presence of excess of water bear only “sour” growth, such as sedges and rushes, of little agricultural value; they usually require reclamation processes before becoming adapted to ordinary crops. In old forests of northern climates a peaty and more or less acid layer is sometimes formed on the surface, above the black woods-earth, and retards somewhat the full production of such land when taken into cultivation.

See Müller, Natürliche Humusformen.

Marshes and swamps, both fresh and salt, as above stated usually show coffee-colored waters, which are also characteristic of the streams that drain them, until by intermixture with waters containing lime salts, the ulmic substances are neutralized and precipitated. Such neutralization, preferably by means of lime, is the first step towards the reclamation of lands bearing “sour” vegetation. The acid reaction characterizing the ulmic substances is also characteristic of many woodlands, notably in the United States of the soils of the “Long-leaf-pine” region of the Cotton States, both upland and lowland, as well as of many deciduous forests in northern climates. Hence liming, whether artificial or natural, effects a most notable improvement, together with a marked change of vegetation, in these lands.

It has been long known that after long-continued cultivation, soils originally of neutral or slightly basic reaction become acid: and the liming of such lands is an ancient practice in Europe. The matter, however, received but scant attention until Wheeler and Hartwell, of the Rhode Island Experiment Station, demonstrated the almost universal acid condition of the older lands of that State, and the excellent effects produced by neutralization with lime, or even with the alkali carbonates. The current neutralization of the humus-acids is unquestionably one of the cardinal advantages of calcareous lands; for such as contain only small amounts of lime carbonate will of course become acid more quickly under cultivation.

Reports of the Rhode Island Exp’t Station, 1895, and ff.

Humin Substances.—In the presence of only a moderate amount of moisture, therefore under the influence of a more or less rapid circulation of air, and in the presence of earthy carbonates (especially that of lime) to prevent the formation of acids, or to neutralize them as formed, the normal process of humification occurs; mainly under the influence of fungous instead of bacterial growths. The various molds take a prominent part in the conversion of the vegetable substance into black, neutral, insoluble humus compounds. Such fungous vegetation is always accompanied by the evolution of carbonic gas, and the resulting fungous tissues are markedly richer in nitrogen and carbon than the substance of the higher plants from which they were derived (see chapt. 9). Comparative analyses show that in the normal process of humification of vegetable substances, oxygen and hydrogen are eliminated in the form of water and carbonic dioxid, while at the same time there is an increase in the percentage of carbon, and generally also of nitrogen; the latter more particularly in the case of vegetable matter not very rich in that element. When once humification is complete, oxidation, especially under arid conditions, bears mainly upon the carbon and hydrogen, so that the nitrogen content may rise to very high figures; while another portion is ultimately wholly oxidized, with the formation of nitrates, under the influence of the nitrifying bacteria, this being the process chiefly efficient in the nutrition of vegetation with nitrogen.

As a matter of course, the several organic compounds contained in plants may continue to exist in soils for some time, varying according to conditions of temperature and moisture. Thus dextrin, glucose, and even lecithin and nuclein have been reported to be found. The activity of the numerous fungous and bacterial ferments under favoring conditions will, of course, limit the continued existence of such compounds somewhat narrowly, so that they can hardly be considered as active soil ingredients save in so far as they favor the development of the bacterial flora.

Porosity of Humus.—One of the essential features of natural humus is its great porosity, whereby it not only becomes highly absorbent of water and gases, but is also gradually oxidized, probably under the influence of bacteria. For this oxidation, as measured by the evolution of carbonic gas, progresses most rapidly under the same conditions as to moisture, temperature and access of air, that are known to be most favorable to fungous and bacterial growth. Hence the formation of carbonic dioxid in the soil is assumed to be the measure of the intensity of such activity.

Physical and Chemical Nature of the Humus Substances.—The humus substances are gelatinous when moist, but are neither markedly adhesive or plastic. Like the other colloidal substances of the soil, they serve to retain both gases and vapors, including moisture, liquid water, and its dissolved solids. In the natural, porous condition they are powerfully absorbent of gases, including especially aqueous vapor. Dry humus swells up visibly when wetted, the volume-weight increasing to the extent of two to eight times; so that humus stands foremost in this respect among the soil constituents. The density of natural humus is about 1.4, being the lightest of the soil constituents. Hence soils rich in humus are “light” not only in the farmer’s sense of being easily tilled when not too wet, but also of light weight for equal volumes when compared with clayey and sandy soils. Some data bearing upon these points are given in the table below, for the substances moderately and uniformly packed:

VOLUME-WEIGHTS OF

Humus. Clay. Quartz Sand.

.3349 1.0108 1.4485

When saturated with water, the same substances gave the following figures:

Air-dry. Saturated Increase. with water. %

Humus .3565 1.1024 209.2 Clay 1.0395 1.6268 55.9 Quartz sand 1.4508 1.8270 25.9

Wollny, Zersetzung der Organischen Stoffe, pp. 242, 243.

Peat pulverized and extracted with alcohol and ether to remove resinous substances.

Peat pulverized and extracted with alcohol and ether to remove resinous substances.

These data show strikingly the effects produced by the several physical soil constituents upon some of its physical properties.

Chemical Nature.—While humus artificially produced by the action of caustic alkalies upon sugar or cellulose is free from nitrogen, all naturally occurring humus contains the latter.

It is not, however, present in the form of ammonia, as it cannot be set free by treatment in the cold with lime or alkalies. When, however, natural humus is boiled with these substances, ammonia is slowly given off, but the process continues indefinitely and it seems to be impossible to expel all the nitrogen in this manner. This behavior being characteristic of amido-compounds, it is presumable, in view of the slightly acid nature of the humus substances, that natural humus is largely of an amidic constitution. Artificial humic acid, formed by the action of caustic alkalies upon sugar, gums or cellulose, combines with ammonia as with other bases, and at first the ammonia can be readily expelled from this as from other ammonia salts. But after the lapse of some time it seems that the amidic condition is assumed, so that caustic lye acts but very slowly and cannot expel the whole of the nitrogen present. This is very important in connection with the practice of fertilization, as any ammonia taken up by or generated in the soil is thus in the course of time rendered comparatively inert, and unavailable to vegetation until nitrified.

Progressive Changes.—The natural neutral humin and ulmin, as found, e. g., in the lower portions of peat beds, are in the course of time by oxidation converted into ulmic and humic acids, capable of combining with bases; by still farther oxidation they form apocrenic and crenic acids, readily soluble in water and in part forming soluble salts with lime, magnesia and other bases. These acids act strongly upon the more readily decomposable silicates of the soil, and in the course of time may dissolve out, and aid in the removal by leaching, of most of the plant-food ingredients as well as the ferric hydrate of a soil. Thus red or rust-colored soils may be rendered almost white by continued “swamping” with stagnant water, and be greatly impoverished; and it is doubtless largely through this agency that the underclays of coal beds and the lower portions of peat beds, as well as peat and coal ashes, are almost wholly destitute of mineral plant food.

The Phases of Humification.—The progressive changes involved in the process of humification of vegetable matter are illustrated in the table below, together with the farther changes by which such matter may ultimately be transformed into the several varieties of coal, and finally into anthracite, which already represents nearly pure carbon, but in nature has sometimes been still farther transformed into graphite (black-lead) and diamond.

Data recalculated, omitting ash.

PROGRESS OF HUMIFICATION, AND FORMATION OF COAL. (MOISTURE AND ASH OMITTED FROM CALCULATIONS.) ========+==========+======================+============+ | | Oak Wood. | | | +------+--------+------+ Humin | |Cellulose.|Fresh.|Decayed.| | and | | | +--------+------+ Humic | | | | Light | Dark | Acid. | | | | Brown. |Brown.| | --------+----------+------+--------+------+------------| Carbon | 44.44 | 50.60| 53.60 | 56.20|49.4 to 59.7| Hydrogen| 6.17 | 6.00| 5.20 | 4.90| 2.5 “ 4.5| Oxygen | 49.38 | | | |35.8 “ 47.3| | | 43.40| 41.20 | 38.90| | Nitrogen| | | | | .3 “ 18.7| --------+----------+------+--------+------+------------+

========+======================+================================ | Peat. | Coals. |--------+-------------+--------+-----------+----------- | Brown | Black. | Lignite| Scotch | Penn’a |Surface.+------+------+ Brown | Splint |Anthracite. |(Ulmin.)|40 in.|80 in.| Coal. |Bituminous.| | | | |(Bovey).| | --------+--------+------+------+--------+-----------+----------- Carbon | 57.80 |62.00 |64.10 | 69.50 | 84.20 | 94.80 Hydrogen| 5.40 | 5.20 | 5.00 | 5.90 | 5.80 | 2.60 Oxygen | 36.00 |30.70 |26.80 | 24.00 | 8.80 | } | | | | | | } 2.60 Nitrogen| .80 | 2.10 | 4.10 | .60 | 1.20 | } +-------+--------+------+------+--------+-----------+-----------

Detmer, Landw. Versuchst., Vol. 14, 1871.

The steady increase of carbon and nitrogen, together with a corresponding decrease of oxygen, are well illustrated in the analyses, especially in the strictly comparable series of peat samples from various depths. In this case there is also a steady decrease of hydrogen, and an increase of ash from 2.72% in the surface layer, to 9.16 at 80 inches depth. This increase is due in the main, of course, to the progressive volatilization of the organic matter in the forms of carbonic dioxid and marsh gas (methan, CH₄).

In considering this table it should not be forgotten that while normal humus stands very close to peat, and the latter when compressed in certain stages would be undistinguishable from lignite or brown coal; yet both peat and lignite are known to be formed under conditions permitting much less access of air or oxygen than occurs in the formation of normal black soil-humus. Hence even black peat cannot at once stand in place of soil-humus when removed from its watery bed, but requires considerable time and aeration (oxidation), and in most cases neutralization with lime or marl, before it can serve the purposes of humus in the soil.

Lignite and the progressively more carbonaceous coals are and have been formed under the conjoined action of submergence and pressure, sometimes also aided by heat; and thus they cannot perform the function of soil-humus, any more than the fire-clays or shales underlying them can resume their original soil-functions without prolonged weathering.

Amounts of Humus and Coal Formed from Vegetable Matter.—Only very general and indefinite estimates can be given of the amount of humus or coal formed from a given quantity of vegetable matter, since these must vary according to the conditions under which the transformation occurs. The greater or less access of air and of moisture, the temperature and pressure under which the process occurs, will modify very materially the quantitative as well as the qualitative result. In the hot arid regions the fallen leaves may wholly disappear by oxidation on the surface of the ground, while under humid conditions they are mostly incorporated with the surface soil. If we assume that in the humification of plant debris (estimating their average nitrogen content at 1%), no nitrogen is lost, it would seem that in the humid region one part of normal soil-humus might be formed from 5 to 6 parts of (dry) plant debris; while in the extreme regime of the arid regions, from 18 to 20 parts of the same would be required. But as most probably some nitrogen also is lost in the process of humification, a considerably larger proportion of original substance may be actually required.

As to coal, it is usually assumed that it requires about 8 parts of vegetable matter for one of bituminous coal. Much higher estimates are made by some, and an observation made by the writer at the Port Hudson bluff, Mississippi, in 1869, would seem to justify such estimates. The above figure, from a sketch made at the time, shows the proportions to which a pine log about eight inches in diameter had shrunk in drying into a small sheet of lignitized wood; the original trunk, projecting from a bed of sand some forty feet below the surface, being so porous and spongy that when wet it flattened somewhat by its own weight; it was connected with the little sheet of lignite by a spirally twisted, tapering stipe.

Here evidently the proportion of lignite formed was a very minute one, doubtless because of the long leaching to which the trunk had been subjected. It thus seems impossible, as in the case of humus, to assign any definite proportion as between woody matter and coal formed from it.

Normal humification takes place only under the influence of moderate temperature. When the temperature is too low, bacterial and fungous growth are repressed or arrested; when too high, the fungous vegetation assumes a different phase, the result of which is the almost total oxidation of the organic matter, sometimes so accelerated as to initiate rapid combustion “fire-fanging” of dung; leaving in any case but a trifling organic residue of very high ash contents.

A striking illustration of this is afforded by Naegeli’s experiment of enclosing several loaves of bread in a loosely closed tin-box. After eighteen months there remained only seventeen per cent of air-dry mouldy matter, totally destitute of starch.

Eremacausis.—In the absence of a sufficient degree of moisture to co-operate with the other agencies of humification, the final result in the soil is practically the same as in the “fire-fanging” of dung. The organic matter is almost wholly destroyed by direct oxidation (eremacausis) with or without the aid of minute organisms; leaving essentially only the ash behind to be reincorporated with the soil. This is to a very great extent the predominant process in the arid regions of the Globe; most of the soils formed in these climates being, therefore, very poor in humus-substances, and deriving it almost entirely from the decay of roots only.

The extent to which the humus of a soil may be derived from the vegetable debris falling or growing upon the surface, varies greatly with the climatic conditions as well with the nature of the soil. In the forests of humid climates with loamy soils, not only does the autumnal leaf-fall, as well as decaying twigs and trunks, become obviously incorporated with the surface soil as decay progresses on the lower surface, but active animal agencies (see below) carry the organic remnants bodily down. But where heavy clay soils prevail, these animal agencies are much restricted by the compactness of the material; only a light surface-layer of mold would be formed, and the humus of the lower soil layers must of necessity be derived from the decay of the roots only. This origin is claimed by Kosticheff for the high content of black humus in the tchernozem or black earth of Russia. Following Hellriegel in determining the weight of roots contained in successive equal layers of soil from the surface downwards, Kosticheff gives for each six inches the following data as found in the tchernozem, taking as 100 the root-content of the surface layer:

===========+======+======+======+======+======+====== Number. | 1 | 1 | 2 | 2 | 3 | 3 Depth. |Roots.|Humus.|Roots.|Humus.|Roots.|Humus. -----------+------+------+------+------+------+------ 6 inches. |100. | 5.42 |100. | 8.11 |100. | 9.64 12 “ | 89.1 | 4.83 | 63.9 | 5.19 | 80.3 | 7.77 18 “ | 66.9 | 3.62 | 48.3 | 3.92 | 70.0 | 6.71 24 “ | 47.3 | 2.56 | 35.0 | 2.84 | 58.4 | 5.61 30 “ | 47.3 | 2.59 | 26.0 | 2.11 | 38.2 | 3.57 36 “ | 34.6 | 1.88 | 18.1 | 1.47 | 33.0 | 3.18 42 “ | 23.9 | 1.29 | 6.3 | .51 | 16.2 | 1.56 48 “ | 14.4 | .78 | | .70 | | 54 “ | 6.7 | .36 | | | | -----------+------+------+------+------+------+------

Abstract in Ann. de la Science Agronomique, Tome 2, 1887.

It will be seen that there is a very close correspondence of the humus content with the root development in the several layers, and it seems as if though but little of the humus could be derived from the surface growth, which is that of the grasses of the steppe.

The climate of the black-earth country of Russia is, though not properly arid, yet one of rather deficient and uncertain rainfall. But as a consequence of extremely arid conditions, and in sandy lands, it may even happen that the immediate surface soil contains less humus than what, in the farmers’ habitual parlance, would be called the subsoil; because of the penetration of slow combustion for some distance into the porous soils. It will then be lower down that, in the presence of a favorable degree of moisture and lower temperature, the conditions of normal humification are fulfilled.

It is not always, then, that the commonly recognized distinction between surface soil and subsoil based upon humus content can be maintained. But the observation of everything bearing upon this point is of the utmost importance in determining both the agricultural value and the mode of treatment of the land.

Losses of Humus from Cultivation and Fallowing.—The fact that humus accumulates in woodlands and meadows, where no cultivation is given, would naturally lead to the converse conclusion, viz., that cultivation causes loss of humus and of its constituents. That this is actually the case is recognized and widely acted upon in practice, and there is no question that the general acceptance of stable manure as the most widely useful fertilizer, despite its usually low content of plant-food ingredients, is based upon the fact that it supplies vegetable matter, in a condition highly favorable to its conversion into humus. The most direct and cogent proof of the depletion of the soil of both humus and nitrogen by continuous cultivation of cereal grains has been given by Snyder, who determined the loss both of humus and of nitrogen suffered by a Minnesota soil during eight years’ continuous cultivation of wheat. The total loss of nitrogen was 1700 pounds per acre, while only 350 pounds were utilized by the crop; about 1400 pounds being dissipated as gas or leached out as nitrates. A conservative estimate of the loss of humus suffered during the same period was about a ton per acre annually, and this loss seriously decreased not only the nitrogen-content, but rendered the soil more compact and less retentive of moisture. But by rotation of the wheat with clover in alternate years, very nearly an equilibrium of both humus and nitrogen-content was obtained. In addition, the amount of available mineral plant-food was decreased by continuous grain culture. Ladd has made similar observations in North Dakota, with similar results.

Bull. No. 70 Minn. Exp’t Station, 1905.

That excessive aeration results in serious losses of humus as well as of nitrogen, is very obvious in the arid region, where it is the habit to maintain on the surface of orchards and vineyards during the dry, hot summers, a thick mulch of well-tilled soil, thus preventing loss of water by evaporation. In the course of years this surface soil becomes so badly depleted of humus that good tilth becomes impossible, the soil becoming light-colored and compacted; while the loss of nitrogen is indicated by the small size of the orchard fruits. Similar losses are of course sustained in the practice of bare summer-fallow, which at one time was almost universal in portions of the arid region. The complete extirpation of weed growth thus brought about, at first considered an unmixed benefit, has ultimately had to be made up for by the practice of green-manuring; since in the arid region the use of stable manure encounters many difficulties.

Estimation of Humus in Soils. It has been usual to determine the amount of humus in soils by means of (dry or wet) combustion, calculating the humus from the carbonic dioxid so formed, while measuring the nitrogen gas directly. But in this process the entire organic matter of the soil, humified and unhumified, is indiscriminately included; and it is wholly uncertain to what extent the latter will ultimately become humus, from the nitrification of which plants are presumed to chiefly derive their nitrogen. In order to obtain definite results, the actual, functional humus must be extracted from the soil mass by some solvent which discriminates between the humified and unhumified organic matter. This cannot be done by direct extraction with caustic soda or potash, which inevitably dissolve unhumified matters and tend to expel ammonia from the humus; besides themselves acting as humifiers (see this chapter, p. 125.)

Grandeau Method: Matière Noire.—The only method now known which accomplishes this separation, practically excluding the unhumified while fully dissolving the humified matter—is that of Grandeau: the extraction of the soil, first with dilute acid, in order to set the humic substances free from their combinations with lime and magnesia; and their subsequent extraction with moderately dilute solutions of ammonia (or other alkali hydrates). Upon the evaporation of the ammonia solution the humus is left behind in the form of a black lustrous substance (“matière noire” of Grandeau) much resembling the crust of soot formed in flues from wood fires. As it contains a variable amount of ash, it must be burnt and the ash subtracted from the first weight.

The humus determinations thus made, which include nearly all those made by German chemists, give the humus-content from 40 to 50% too high. The French determinations are mostly made by the method of Grandeau.

Amounts of Humus in Soils.—While in peat, marsh and muck lands the humus-content may rise above twenty per cent, in ordinary cultivated lands it rarely exceeds about five per cent, and very commonly falls below three per cent, even in the humid regions. In properly arid soils we find a very much lower average, rarely exceeding one per cent, and frequently falling to .30 and even less. This scarcity of humus manifests itself plainly in the prevalently light gray tint of the arid soils.

Meadows and woodlands generally show the highest humus-content in their surface soils, gradually increasing while in that condition; while when taken into cultivation the humus-content gradually decreases, owing to the free aeration and consequent “burning-out” caused by tillage. Hence the humus must be from time to time replaced by the use of stable manure, or green-manure crops, to prevent injurious changes in the tilling qualities of the land. Not only humus as such, but according to Schloesing also the insoluble colloid humates, produce in the soil a loosening effect or tilth (Germ. Bodengare), which apparently cannot be brought about by any other substance.

The decrease of humus from wheat culture in the soils of Minnesota and North Dakota has been studied by H. Snyder and E. F. Ladd, respectively. In the prairie lands of the latter State the total organic matter in the first six inches of soil ranges from 15 to as much as 26%, and the humus alone from 4 to 7.8%.

Humates and Ulmates.—That the insoluble humates of lime, magnesia, iron, manganese and alumina are present in most soils is conclusively shown by the composition of the solution obtained by the extraction of soils with weak acid, as above mentioned in connection with the quantitative determination of humus according to Grandeau; since these bases are almost always extracted by the weak acid. When the brown solution of alkali humate obtained in this process is carefully neutralized with sulfuric or hydrochloric acid, or is mixed with solutions of the above bases, flocculent, insoluble precipitates are formed, while the solution is discolored. Similar precipitates may be obtained with other metallic solutions, notably with that of copper, which precipitates the humus-acids most completely. Doubtless these compounds contribute greatly to the conservation of the humus-content of soils, protecting it to a certain extent from oxidation, and also preventing excessive acidity. The brown tint of certain subsoils in the northern humid regions have been shown by Tollens and others to be due not to ferric hydrate, as had been supposed, but to calcic, magnesic and aluminic humates. None of the mineral bases or acids present can be detected in the humic solution by the usual reagents.

Mineral Ingredients in Humus.—That the mineral plant-food ingredients present in the humus extracted by the Grandeau process, and which remain as ash when the matière noire is burned, are capable of nourishing plant growth, was directly shown by Grandeau, Snyder and others. The former was inclined to consider that those substances were mainly thus taken up by plants, under natural conditions. This theory, however, has not been sustained by subsequent investigations; the mineral plant-food thus extracted is not a measure of the immediate productiveness of the soils, as demonstrated by Snyder, and the residual soils are not sterile. It is also still doubtful to what extent the mineral bases and acids are naturally combined with the humus-substances, it being contended by some that they are brought into organic combination by the acid and ammonia extraction. The investigations of Snyder and Ladd, above referred to, prove however to some extent at least that the humus-substances are naturally combined with them, and that probably they are largely made available to plants through the direct and indirect action of the humus compounds. This subject is farther considered in chapter 19.

The nature and amounts of these mineral substances are well exemplified in the subjoined full analysis by Snyder, of the ash of the humus and humates extracted from a compound sample of prairie soils of Minnesota, which had been thrown down from the ammonia solution by simply neutralizing the liquid:

ASH OF HUMUS FROM MINNESOTA PRAIRIE SOILS.

Insoluble matter 61.97 Potash (K₂O) 7.50 Soda (Na₂O) 8.13 Lime (CaO) 0.09 Magnesia (MgO) 0.36 Peroxid of Iron (Fe₂O₃) 3.12 Alumina (Al₂O₃) 3.48 Phosphoric acid (P₂O₅) 12.37 Sulfuric acid (SO₃) .98 Carbonic acid (CO₂) 1.64

Precipitation with an excess of acid does not greatly change the results.

In California soils this is mostly silica soluble in carbonate of soda.

The large amounts of the soluble alkalies potash and soda thrown down with the humic matters are very striking, as is the very large proportion of phosphoric acid. Lime and magnesia had, of course, been mainly eliminated by the preliminary acid treatment.

Functions of the Unhumified Organic Matter.—The unhumified plant debris in the soil are not to be regarded as useless, even aside from their potential conversion into active humus. Not only do these remnants of vegetation lighten the soil, rendering it more pervious to air and water, but in their progressive decay they give off carbonic gas, which is active in soil-decomposition; and they serve as nourishment to the soil bacteria upon which its thriftiness so greatly depends. See below, chapter 9.

The Nitrogen-Content of Humus.—Since soil-humus is doubtless the chief depository of soil-nitrogen, and the main source from which, through the process of nitrification, the nitrogen-supply to plants is usually derived, its content of that element is a matter of great interest. It has been customary to estimate approximately the nitrogen-content of soils by the proportion of humus-substance present; and as the light tints of the soils of the arid region indicate a small humus-content, a scarcity of nitrogen seemed to be also indicated for these lands. As this in a number of cases did not seem to accord with actual experience, an investigation of the subject was made at the California experiment station, with the results shown in the subjoined table. In considering these results it must be kept in mind that while arid conditions can rarely be fulfilled in the humid region, humid conditions are quite frequently locally represented in the arid, in lowlands and on high mountains; while moderately moist benchlands represent the semi-arid regime.

Hilgard and Jaffa. On the Nitrogen-content of Soil-humus in the Humid and Arid regions. Rep. Cal. Exp’t Station for 1892-4; Agric. Science, April, 1894; Wollny’s Forsch. Geb. Agr. Phys., 1894.

HUMUS PERCENTAGE AND NITROGEN CONTENT IN SOILS OF THE ARID AND HUMID REGIONS. =====+==============================================+=====+========+======== | |Humus|Nitrogen|Nitrogen Station| Soils arranged in order of nitrogen | in | in | in Number| percentages in humus. |soil,| Humus, |soil, | | per | per | per | |cent.| cent. |cent. -----+----------------------------------------------+-----+--------+-------- | SOILS OF THE ARID REGION (California). | | | | | | | 2061 |Dark clay loam, Arroyo Grande Valley, | 3.06| 22.00 | .670 | San Luis County | | | 2291 |Red soil, Orland, Glenn Co. | .71| 21.10 | .150 1904 |Sediment Soil, Porterville, Tulare Co. | .90| 19.50 | .180 1901 |Sandy soil near Ceres, Stanislaus Co. | .64| 18.75 | .120 704 |Sandy soil of plains, near Fresno, Fresno Co. | .60| 18.66 | .112 6 |Black adobe soil, Stockton, San Joaquin Co. | 1.05| 18.66 | .196 1679 |Black adobe soil, Berkeley, Alameda Co. | 1.20| 18.58 | .203 2324 |Clay soil of desert, Imperial, San Diego Co. | .38| 18.40 | .070 1167 |Black clay loam soil, near Tulare, Tulare Co. | 1.66| 18.19 | .302 1536 |Brown loam soil, Windsor Tract, Riverside, | .20| 18.00 | .036 | Riverside County | | | 1126 |Sandy loam soil, Paso Robles, | .55| 17.27 | .095 | San Luis Obispo Co. | | | 2301 |Red hill soil, Upper Lake, Lake Co. | .81| 16.90 | .137 1607 |Plateau soil of desert, Lancaster, | .25| 16.80 | .042 | Los Angeles Co. | | | 1159 |Sandy plains soil, Tulare, Tulare Co. | .37| 16.75 | .062 1900 |Sandy soil, near Modesto, Stanislaus Co. | .84| 16.65 | .140 1113 |Clay loam soil (slate), Jackson, Amador Co. | .54| 16.60 | .090 1149 |Adobe clay soil, near Paso Robles, | .47| 16.18 | .074 | San Luis Obispo County | | | 1538 |Mesa soil, Chino, San Bernardino Co. | .65| 16.08 | .105 1147 |Sandy loam soil, Paso Robles, | .66| 16.06 | .106 | San Luis Obispo Co. | | | 2403 |Valley Soil, Wheatland, Yuba Co. | 1.50| 16.00 | .240 1281 |Red Mesa soil, Pomona, San Bernardino Co. | .58| 15.50 | .090 1117 |Sandy granitic soil, near Jackson, Amador Co. | .80| 15.27 | .123 1406 |Red loam soil, Arlington Heights, Riverside, | .30| 15.00 | .045 | Riverside County | | | 1172 |Red clay loam soil, east of Tulare, Tulare Co.| .72| 14.75 | .106 1958 |Sandy Mesa soil, Nipomo, San Luis Obispo Co. | .85| 14.45 | .122 1423 |Chocolate-red soil, Carisa plain, | .39| 14.36 | .056 | San Luis Obispo County | | | 1291 |Sandy hill land, near Jackson, Amador Co. | .76| 14.34 | .109 585 |Wire-grass loam soil, Visalia, Tulare Co. | 1.00| 14.10 | .146 863 |Red ridge loam soil, Grass Valley, Nevada Co. | 2.89| 13.91 | .402 1907 |Dark loam soil, near Chino, San Bernardino Co.| .92| 13.26 | .121 1115 |Sandy granitic soil, near Jackson, Amador Co. | .85| 13.20 | .112 332 |Plateau desert soil, Mojave, Los Angeles Co. | .28| 12.50 | .035 2126 |Gravelly soil, East Highlands, | .62| 11.75 | .070 | San Bernardino Co. | | | 1910 |Ojai Valley soil, Nordhoff, Ventura Co. | 1.64| 11.21 | .183 2187 |Sandy loam soil, Soledad, Monterey Co. | .97| 11.10 | .110 1759 |Sandy soil, Perris Valley, Riverside Co. | .53| 11.04 | .059 774 |Bench slope soil, Ontario, San Bernardino Co. | 1.29| 10.85 | .140 1984 |Red soil, East Highlands “ “ “ | .58| 10.50 | .060 2325 |Silt soil of desert, Imperial, San Diego Co. | .65| 10.70 | .070 1906 |Light sandy soil, Pomona, San Bernardino Co. | .95| 9.80 | .093 2430 |Hillside adobe, Berkeley, Alameda Co. | 1.85| 8.70 | .160 | +-----+--------+-------- | Average of arid uplands | .91| 15.23 | .135 | | | | | SUB-IRRIGATED ARID SOILS (California). | | | | | | | 586 |Sandy plains soil, Tulare, Tulare Co. | 1.14| 10.79 | .123 1466 |Loam soil, Miramonte, Kern Co. | .60| 10.66 | .064 1284 |Moist land loam soil, Chino, | 1.99| 10.20 | .203 | San Bernardino Co. | | | 1148 |Swale soil, near Paso Robles, | 1.16| 9.65 | .112 | San Luis Obispo Co. | | | 1714 |Bench soil, Santa Clara River, Piru, | .78| 9.56 | .074 | Ventura Co. | | | 77 |Alluvial soil, Tulare Lake bed, Tulare Co. | .47| 9.37 | .045 1880 |Creek bench soil, Niles, Alameda Co. | 1.19| 8.90 | .109 1903 |Sediment soil, Porterville, Tulare Co. | 1.12| 8.50 | .140 168 |Alluvial soil, Santa Clara river, Santa Paula,| .84| 7.99 | .067 | Ventura Co. | | | 1760 |Green-sage land, Perris Valley, Riverside Co. | .91| 7.70 | .070 506 |Alluvial soil, Colorado River, Yuma, | .75| 7.47 | .050 | San Diego Co. | | | 1636 |Red soil, Manton, Tehama Co. | 2.00| 6.86 | .137 1758 |Alkali soil, Perris Valley, Riverside Co. | .60| 6.83 | .071 1963 |Sandy loam soil, Willows, Glenn Co. | .36| 6.05 | .022 2080 |Sandy soil, Santa Maria Valley, | 1.64| 5.36 | .090 | Santa Barbara Co. | | | | | | | | Average of sub-irrigated arid soils | 1.06| 8.38 | .099 | | | | | HUMID SOILS FROM ARID AND HUMID REGIONS | | | | (California). | | | | | | | 207 |Eel River Alluvial soil, Ferndale, | 1.25| 6.96 | .085 | Humboldt Co. | | | 2319 |Alluvial soil, Hupa Valley, Humboldt Co. | 7.83| 6.70 | .514 213 |Marsh soil, Novato, Meadows, Marin Co. | 1.54| 6.36 | .089 1704 |Valley soil, Hollister, San Benito Co. | .94| 5.21 | .049 2295 |Tule soil, Upper Lake, Lake Co. | 1.70| 4.50 | .077 110 |Alluvial soil, Putah Creek, Dixon, Solano Co. | 1.71| 4.25 | .072 37 |Redwood Valley soil, Pescadero, San Mateo Co. | 2.28| 3.07 | .070 | | | | | Average for California | 2.45| 5.29 | .135 | | | | | OTHER STATES. | | | | | | | 26 |Bog soil, Michigan |33.02| 6.08 | 2.012 |Back-land clay loam, Houma, Louisiana | 5.07| 4.20 | .218 |Duff soil, Oregon |13.84| 3.49 | .483 |Sandy prairie soil, Harris Co., Texas | 2.13| 3.66 | .184 | | | | | Average for other States | 7.01| 3.78 | .295 | | | | 23 |Red soil, Oahu Island, Hawaii (maximum) | 1.57| 5.07 | .078 27 |Guava soil, Hawaii Island (minimum) | 9.95| 1.71 | .170 |Average of 5 soils, Oahu Island | 3.01| 6.07 | .237 |Average of 2 soils, Maui Island | 9.07| 2.13 | .286 |Average of 4 soils, Hawaii Island | 6.17| 2.54 | .146 | | | | | Average for Hawaiian Islands | 5.26| 3.69 | .169 | | | | | Total for Humid soils, average | 4.58| 4.23 | .166 -------+----------------------------------------------+-----+--------+-------

Introduced only for comparison of the nitrogen percentage in Humus and not included in the average.

It thus appears that on the average the humus of the arid soils contains about three and a half times as much nitrogen as that of the humid; that in the extreme cases, the difference goes as high as over six to one (see Nos. 37 and 704); and that in the latter cases, the nitrogen-percentage in the arid humus considerably exceeds that of the albuminoid group, the flesh-forming substances.

It thus becomes intelligible that in the arid region a humus-percentage which under humid conditions would justly be considered entirely inadequate for the success of normal crops, may nevertheless suffice even for the more exacting ones. This is more clearly seen on inspection of the figures in the third column, which represent the product resulting from the multiplication of the humus-percentage of the soil into the nitrogen-percentage of its humus; as appears in comparing the respective averages, or Nos. 1167 and 110 and others. An additional consideration is the probable greater ease with which the nitrifying bacteria can act upon a material so rich in nitrogen.

We must not, then, be misled by the smallness of many humus-percentages in the arid region, into an assumption of a deficiency in the supply of soil-nitrogen.

Decrease of Nitrogen-Content in Humus with Depth.—Since the oxidation of the carbon and hydrogen in the humus-substance, and the consequent increase of its relative nitrogen-content, are manifestly dependent upon the presence of air and heat, it is reasonably to be expected that the nitrogen-percentage of the humus should decrease with the depth of the soil. That this is really the case is plainly shown in the subjoined table, which gives the humus-percentages and the nitrogen-content of the humus from the surface foot down to twelve feet, in a soil on the bench of the Russian River, Cal., which is sub-irrigated, and liable to more or less rainfall during the summer. It will be seen that not only does the absolute humus-percentage decrease quite regularly down to seven feet, at which point there evidently was at one time a strong root development, causing a notable increase of the humus-content; from which again there is a regular decrease down to the twelfth foot. It will be noted that the nitrogen-percentage in the humus, while not decreasing with the same regularity as the humus-content itself, yet exhibits a general recession from 5.30 to 1.15 in the ninth foot, to which direct oxidation doubtless never penetrates.

HUMUS AND NITROGEN-CONTENT OF RUSSIAN RIVER SOIL. ==============+==============+=================+================ Depth in feet.| Per cent | Per cent | Per cent | Humus | Nitrogen | Humus-Nitrogen. | in soil. | in Humus. | in soil. --------------+--------------+-----------------+---------------- 1 | 1.21 | 5.30 | .064 2 | 1.16 | 4.32 | .054 3 | 1.14 | 3.87 | .044 4 | 1.17 | 3.76 | .044 5 | .74 | 2.16 | .016 6 | .60 | 2.66 | .016 7 | .47 | 2.54 | .012 8 | .78 | 1.54 | .012 9 | .54 | 2.24 | .012 10 | .52 | 1.15 | .006 11 | .53 | 1.51 | .008 12 | .44 | 1.81 | .008 --------------+--------------+-----------------+----------------

Influence of the Original Materials on the composition of Humus.—The great variability of the composition of humus formed from different substances is well shown in the subjoined table, representing the results of experiments made by Snyder, who caused various substances to humify by mixing the pulverized material intimately with a soil poor in humus, and allowing the process to continue for a year. At the end of that time the humus formed was extracted by the method of Grandeau, outlined above, and analyzed, with the following results.

========+======+======+=======+======+========+=======+========== |Sugar.| Oat | Green |Wheat |Sawdust.| Meat | Cow | |Straw.|Clover.|Flour.| |Scraps.|Manure. --------+------+------+-------+------+--------+-------+---------- Carbon | 57.84| 54.30| 54.22| 51.02| 49.28 | 48.77| 41.93 Hydrogen| 3.04| 2.48| 3.40| 3.82| 3.33 | 4.30| 6.26 Nitrogen| 0.08| 2.50| 8.24| 5.02| 0.32 | 10.96| 6.16 Oxygen | 39.04| 40.72| 34.14| 40.14| 47.07 | 35.97| 45.63 --------+------+------+-------+------+--------+-------+---------- |100.00|100.00| 100.00|100.00| 100.00 | 100.00| 100.00 --------+------+------+-------+------+--------+-------+----------

Bull. No. 53, Minn. Exp’t Station, p. 12, Chem. of Soils and Fertilizers, p. 94.

The figures for cow manure are so far out of range with any others thus far observed, that it seems reasonable to suppose that they are influenced by unchanged substances present in the excreta.

While it may be questioned whether the process of humification had in these materials really reached the point of sensible completion in all cases (notably in those of sawdust and cow manure), the great variability of the products from different materials is very striking. When the nitrogen-content is deducted the percentage composition of the products agrees more nearly. Considering that the nitrogen is probably present in the amid form, it is natural that hydrogen should in a measure vary with it, as in the case of the clover, flour and meat humus. Nitrogen being the most variable ingredient of humus, it seems probable that the variation of the proportion of the humus-amids present is the most potent factor in the variability of the composition of natural soil-humus.

Arranging these results in the order of their nitrogen-content as in the table below, we see that the latter approximately corresponds to the original protein-content of the humified substances.

Humus from meat scraps 10.96 % Nitrogen. “ “ green clover 8.24 “ “ cow manure 6.16 “ “ wheat flour 5.05 “ “ oat straw 2.50 “ “ sawdust .32

While the above data prove the correlation between the first products of humification and the original substance, it must be remembered that subsequently, under proper conditions, the nitrogen-percentage in humus may, in the course of time, increase very greatly, even to a proportion considerably above that contained in flesh itself. When we consider that ordinarily, the latter, and the albuminoid substances generally, decompose in contact with air with an abundant evolution of ammonia compounds, sometimes leaving only a little fat (adipocere) behind, it is surprising that the decomposition within the soil should have exactly the opposite result, viz., an accumulation of the nitrogen. The causes of this marked difference are not yet well understood, but it is probably due to the differences in the kinds of bacteria that are active in the two cases.

Snyder has also shown that the richer the organic matter humified is in nitrogen, the more energetically it acts in rendering available the mineral matters of the soil for plant nutrition. Correspondingly, Ladd has shown that with the increase of humus in the soil, there is also a corresponding increase in the amounts of mineral plant-food extracted from the soil by a four per cent solution of ammonia, such as is employed in the Grandeau method of humus-determination.

Bull., S. Dakota Station, Nos. 24-32, 35, 47.