CHAPTER X.. Soil and Subsoil (_continued_)

SOIL AND SUBSOIL (Continued)

THEIR RELATIONS TO VEGETATION.

Physical Effects of the Percolation of Surface Waters.—The muddy water formed by the beating of rains on the soil surface will, in penetrating the soil, carry with it the diffused colloidal clay to a certain depth into the subsoil. We should therefore expect that as a rule every subsoil will be more clayey than its surface soil; and this is found to be almost universally the case in the humid region. Subsoils are therefore almost always less precious and more retentive of moisture, as well as of plant-food substances in solution, than their surface soils, unless these are very rich in humus; and as the finest particles are usually those richest in available plant-food, it follows that subsoils will as a rule be found to contain larger supplies of the latter than the surface soil. Common experience as well as comparative analysis confirm both of these inferences so thoroughly, that it becomes unnecessary to adduce examples in this place.

On the other hand, the reverse, upward movement of moisture caused by surface evaporation tends constantly to bring any soluble salts contained in the soil mass nearer to the surface, thus increasing the stock of easily available plant-food in the surface soil. In extreme cases, especially in the arid region, this accumulation of salts may become excessive, and seriously injurious to plant growth. (See “Alkali Soils”, chapters 21, 22.)

Chemical effects of Water-Percolation.—The accumulation of plant-food in the subsoil is not, however, due only to the mechanically-carried particles, but also to the ingredients carried in solution from the surface soil and redeposited in the more retentive subsoils. Especially is this true of lime carbonate, which is dissolved by the carbonic acid formed chiefly within the humic surface soil, and is often carried off in amounts sufficient to obstruct drain tiles by its deposition in contact with air (see chapt. 3). In the case of moderate rains, however, it is carried no farther than the subsoil, and is there redeposited, in consequence of the penetration of air, following the water, and causing the carbonic gas to diffuse upward; thus leaving the lime carbonate behind. In the majority of cases this results simply in a gradual enriching of the subsoil in this substance; while the surface soil may become so depleted as to require its artificial replacement by liming or marling. The same general process occurs to a less extent, in the case of magnesia.

Calcareous Subsoils.—The fact that subsoils are more calcareous than the corresponding surface soils is often of great practical importance, in enabling the farmer to enrich his depleted surface soil in lime by subsoil plowing. The accumulation of lime carbonate in the subsoil also tends in a measure to offset the extreme heaviness sometimes resulting from the accumulation of clay.

Calcareous Subsoils and Hardpans.—When soils are very rich in lime, and rains occur in limited showers rather than continuously, the lime carbonate dissolved from the surface soil may accumulate in the subsoil so as to either form calcareous “hardpan” by the cementing of the subsoil mass; or it may accumulate and partly crystallize around certain centers and thus form white concretions, known to farmers as “white gravel.” The latter is the form usually assumed in the regions of summer rains; while in the arid regions the deficient rainfall causes this substance to accumulate, and calcareous hardpan to form, at definite depths depending upon the maximum penetration of the annual rainfall; sometimes in crystalline masses of veritable limestone (“kankar” of India), or sometimes merely as crystalline incrustations loosely cementing the subsoil.

“Rawness” of Subsoils in Humid Climates.—From the greater compactness of the subsoil which is almost universal in the humid regions, the absence of humus and of the resulting formation of carbonic and humic acids, it follows that its minerals are less subject to the weathering process than are those of the surface soil. In the farmer’s parlance, the subsoil is “raw” as compared with the surface soil; it is not so suitable for plant-nutrition, and therefore must not be brought to the surface to form the seed-bed, or be incorporated with the surface soil to any considerable extent at any one time, if crop-nutrition is to be normal. It is only in the course of time, by exposure to atmospheric action as well as to that of the humus, and of plant roots, that it becomes properly adapted to perform the functions of the surface soil.

Soils and Subsoils in the Arid Region.—But however pronounced and important are these distinctions and differences in the humid region, they are found to be profoundly modified in the arid; where, as before stated, the formation of colloidal clay is very much diminished, so that most soils formed under arid conditions are of a sandy or pulverulent type. There is then little or no clay to be washed down into the subsoil, hence there is no compacting of the latter; the air consequently circulates freely down to the depth of many feet.

Thus one of the most important distinctions between soil and subsoil is to a great extent practically non-existent in the arid region, at least within the depths to which tillage can be made to reach; so that the limitations attached to subsoil-plowing in the countries of summer rains do not apply to the characteristic soils of the arid regions.

Even the distinction in regard to humus is here largely obliterated by the circumstance, already alluded to, that most of that substance must, in the arid regions, be derived from the decay of roots, which moreover reach to much greater depth in these soils. Hence even in the uplands of the arid region it is common to find no change of tint from the surface down to three feet, and even more. This, like the free circulation of the air in consequence of porosity, tends to render the distinction of soil and subsoil practically useless; since it disposes of the objection to “subsoiling” based upon the inert condition of the subsoil, which in humid climates so effectually interferes with the welfare of crops unless subsoiling is restricted to a fraction of an inch at a time.

These fundamental differences in the soils of the two regions are illustrated schematically in the subjoined diagram, which shows on the left the contrast between clay or clay loam soils, in which the depth of the surface soil-sample to be taken is prescribed as nine inches by the rules of the Association of Am. Official Chemists (in the writer’s experience it is more nearly six inches as a rule). Alongside of the Eastern soil thus characterized is placed a typical “adobe” soil from the grounds of the California Experiment station, of which a sample showing uniform blackness to three feet depth was exhibited at the World’s Fair at Chicago in 1893. At the right is a profile of the noted hop soil on the bench lands of the Russian river, Cal., in which the humus-content was determined down to twelve feet, the humus-percentage being .44% at that depth against 1.21% in the surface foot (see chapt. 8, p. 139). In this and similar soils the roots of hops reach down to as much as fourteen feet without much lateral expansion; as shown in plate No. 31 of this chapter. Similar conditions prevail in the sandy uplands, as, e. g., in the wheat lands of Stanislaus county, Cal., mentioned above.

Taking the clay soils as a fair type for comparison, it would seem that the farmer in the arid region owns from three to four farms, one above another, as compared with the same acreage in the Eastern states.

Subsoils and Deep-plowing in the Arid Region.—Up to the present time this advantage is but little appreciated and acted upon by the farmers of the arid region. They still instinctively cling to the practice taught them by their fathers, and which is still promulgated as the only correct practice, in most books on agriculture. There are of course in the arid as well as in the humid region, cases in which deep plowing is inadvisable; viz, that of marsh or swamp lands, as well as sometimes in very sandy, porous soils, the cultural value of which often depends essentially upon the presence of a somewhat consolidated, and more retentive subsoil, which should not be broken up. But in most soils not of extreme physical character, it is in the arid region not only permissible, but eminently advisable to plow, for preparation, as deeply as circumstances permit, in order to facilitate the penetration of the roots beyond the reach of harm from the summer’s drought; while for the same reason, subsequent cultivation should be to a moderate depth only, for the better conservation of moisture, and the formation of a protective surface mulch (see chapter 13).

|TYPE OF EASTERN SOILS| TYPES OF CALIFORNIA SOILS. ---+---------------------+--------------------+----------- Ft.| Upland. | Up land clay-loam. | Beach land. ---+---------------------+--------------------+-----------

FIG. 27.—Soil Profiles illustrating differences in Soils of Humid and Arid Region.]

It must not be forgotten that there are in the lowlands of the arid region (river swamps or tules, sea-coast marshes, etc.,) soils in which surface soil and subsoil are differentiated as fully as in the humid countries; at least so long as they have not been fully drained for a considerable length of time. In swamp areas that have been elevated above the reach of overflow or shallow bottom-water by geological agencies, even the heavy swamp clays are fully aerated down to great depths, and roots penetrate accordingly.

Examples of Plant-growth on Arid Subsoils.—The fact that in the arid region the surface-soil conditions reach to so much greater depths than in the East and in Europe, is so important for farming practice in that region that experimental evidence of the same should not be withheld. Of such, some cases well established as typical of California experience are therefore cited.

It is well known that in the Sierra Nevada of California the placer mines of the Foothills, worked in the early times, have long disappeared from sight, having been quickly covered by a growth of the bull pine (P. ponderosa). Much of this timber growth has for a number of years past been of sufficient size to be used for timbering in mines, and a second young forest is springing up on what was originally the red earth of the placer mines, which appears to the eye as hopelessly barren as the sands of the desert. In this same red sandy earth not unfrequently cellars and house foundations are dug, and the material removed, even to the depth of eight feet, is fearlessly put on the garden and there serves as a new soil, on which vegetables and small fruits grow, the first year, as well as ever. In preparing such land for irrigation by leveling or terracing no heed is taken of the surface soil as against the subsoil, even where the latter must be removed to the depth of several feet, so long as a sufficient depth of soil material remains above the bedrock.

The same is generally true of the benchlands; the irrigator levels, slopes or terraces his land for irrigation with no thought of discrimination between soil and subsoil, and the cultural result as a rule justifies his apparent carelessness. It is only where from special causes a consolidated or hardpan subsoil is brought to the surface, that the land when leveled shows “spotted” crops. Such is the case in some of the “hog-wallow” areas of the San Joaquin valley of California, and in some cases where by long cultivation and plowing to the same depth, a compact soil-layer or plowsole has been formed, and the land is then leveled for the introduction of irrigation. In these cases a section of the soil mass will usually show a marked difference in color and texture. But, as a rule, in taking soil samples, no noticeable difference can be perceived between the first and the second, and oftentimes as far down as the third and fourth foot. The extraordinary root-penetration of trees, shrubs and taprooted herbs, whose fibrous feeding-roots are found deep in the subsoil and are sometimes wholly absent from the surface soil, fully corroborate the conclusion reached by the eye. The roots of grape vines have been found by the writer at the depth of twenty-two feet below the surface, in a gravelly clay loam varying but little the entire distance. In a similarly uniform and pervious material, the loess of Nebraska, Aughey reports the roots of the native Shepherdia to have been found at the depth of fifty feet.

See Merrill, Rocks and Rock weathering.

Resistance to Drought.—These peculiarities of the soils of the arid region explain without any resort to violent hypotheses, the fact that many culture plants which in the regions of summer rains are found to be dependent upon frequent and abundant rainfall, will in California, and in the country west of the Rocky Mountains generally, thrive and complete their growth and fruiting during periods of four to six months of practically absolute cessation of rainfall; when east of the Mississippi a similar cessation for as many weeks will ruin the crops, if not kill the plants. In continental Europe, in 1892, a six weeks’ drought caused almost all the fruit crops to drop from the trees, and many trees failed to revive the next season; while at the very same time, the same deciduous fruits gave a bountiful crop in California, during the prevalence of the usual five or six months’ drought. This was without irrigation, or any aid beyond careful and thorough surface tillage following the cessation of rains in April or May, so as to leave the soil to the depth of five or six inches in a condition of looseness perfectly adapted to the prevention of evaporation from the moist subsoil, and of the conduction of the excessive heat of the summer sun. This surface mulch will contain practically no feeding-roots, the paralysis or death of which by heat and drought would influence sensibly the welfare of the growing plant.

Root-system in the Humid Region.—It is quite otherwise where a dense subsoil not only obstructs mechanically the deep penetration of any but the strongest roots, but at the same time is itself too inert to provide sufficiently abundant nourishment apart from the surface soil, which is there the portion containing, alongside of humus, the bulk of the available plant-food, and in which alone the processes of absorption and nutrition find the proper conditions; such as access of air and the ready and minute penetration of even the most delicate rootlets and root-hairs. The largest and most active portion of the root-system being thus accumulated in the surface soil, it follows that unless the latter is constantly kept in a fair condition of moistness, the plant must suffer material injury very quickly; hence the often fatal effects of even a few weeks’ drought. The same occurs in the arid region when often-repeated shallow plowing has resulted in the formation of a “plowsole” which prevents the deep penetration of roots; when a hot “norther” will often in a short time not only dry the plowed soil, but will heat it to such extent as to actually bake the roots it harbors. Under the same weather-conditions an adjoining field, properly plowed, may almost wholly escape injury.

Comparison of root development in the arid and humid regions.—Figures 28, 29 given here show the differences as actually seen in the case of fruit trees as grown in Wisconsin and California, respectively, both in the absence of artificial water-supply.

Adaptation of humid species to arid conditions.—Figures, in No. 30, show the root systems respectively of the riverside grape (Vitis riparia) as grown in the Mississippi Valley states, and the natural development as found in the Rock grape of Missouri and also in the wild grape vine of California. It will be noted at once that the latter directs its cord-like roots almost vertically from the first, until it reaches a depth varying from 12 to 18 inches, where it begins to branch more freely, but still with a strong downward tendency in all. The roots of the riverside grape, on the contrary, tend to spread almost horizontally, branching freely at the depth of a few inches and manifestly deriving its supply both of plant-food and moisture mainly from the surface soil. It is curious to observe the behavior of this vine when cuttings are planted in California vineyards as a resistant grafting-stock. Its first roots are sent out horizontally, very much as is its habit in the East, so long as the soil moisture is maintained near the surface. But as the season advances, the more superficial rootlets are first thrown out of action by the advancing dryness and heat of the surface soil, and many finally die the first year.

Not unfrequently the entire root system developed by the uppermost bud perishes; but usually its main roots soon begin to recede from the threatening drought and heat of the surface, curving, or branching downward in the direction of the moisture supply, and without detriment to their nutrition because of the practical identity of the surface soil and subsoil. As the portions of the roots near the surface thicken and mature, their corky rind soon prevents their being injured by the arid conditions to which they are subjected; while the root-ends, finding congenial conditions of nutriment and aeration in the moist depths, develop without difficulty as they would in their humid home. Practically the same process of adaptation takes place in every one of the trees, shrubs, or perennials belonging to the humid climates, until their root system has assumed nearly the habit of the corresponding native vegetation.

The photograph of the roots of a hop plant, grown on bench lands of the Sacramento river, shows the roots extending to 8 feet depth, but where broken off the main root is still nearly two millimeters in thickness, proving that it penetrated at least two feet beyond the depth shown in figure 31.

In the case of native annuals, either the duration of their vegetation is extremely short, ending with or shortly after the cessation of rains; or else their tap roots descend so low, and the nutritive rootlets are developed at such depth, as to be beyond reach of the summer’s heat and drought. For while it is true that rootlets immersed in air-dry soil may absorb plant-food, this absorption is very slow and can only be auxiliary to the main root system which, instead of terminating in the surface soil as in the humid region, will be found to begin to branch off at depths of 15 and 18 inches, and may then in sandy lands descend to from 4 to 7 feet even in the case of annual fibrous-rooted plants like wheat and barley. In the case of maize the roots of a late-planted crop may sometimes be found descending along the walls of the sun-cracks in heavy clay land poorly cultivated; and it frequently matures a crop without the aid of a single shower after planting. See figures 33, 34.

Shaler (Origin and Nature of Soils; 12th Rept. U. S. Geol. Survey, p. 311) says: “Annual plants cannot in their brief period of growth push their roots more than six to twelve inches below their root-crowns”—a generalization measurably true for the humid region only. According to F. J. Alway, the roots of cereals penetrate to 5-7 feet in Saskatchewan, also.

The annexed plate (No. 32) shows the main roots of two native perennial weeds of California, the goosefoot (Chenopodium californicum) and the figwort (Scrophularia californica), common on the lower slopes of the coast ranges. The soil was a heavy clay loam or “black adobe” resulting from the weathering of the clay shale bedrock, fragments of which are so abundantly intermixed with the substrata that excavation of the roots became very difficult. Yet the main root of the goosefoot went down below the depth of eleven feet.

The main root of the figwort, also, was followed below the depth of ten feet without reaching the extreme end. This proves clearly that the great penetration of the goosefoot was not, as might be supposed, due to its bulbous root. Yet such thickening of the root just below the crown is a rather common feature in arid-region plants, and can here be noted even in the figwort, within whose botanical relationship bulbous roots are almost unknown.

Any one accustomed to the cornfields of the Middle West, where in the after-cultivation of maize it is necessary to restrict very carefully the depth of tillage to avoid bringing up a mat of white, fibrous roots, will be at once impressed with the remarkable adaptability of maize to different climatic conditions, as exhibited in such cases and shown in figures 33, 34. In southern California, in the deep mesa or bench soils, corn stalks so tall that a man standing on horseback can barely reach the tassel, and with two or three large ears, are quite commonly grown under similar rainfall-conditions.

Importance of proper Substrata in the Arid Region.—The paramount need of deep penetration of roots in the arid region renders the substrata below the range of what is usually understood by subsoil in the humid climates, of exceptional importance. A good farmer anywhere will examine the subsoil to the depth of two feet before investing in land; but more than this is necessary in the arid region, where the surface soil is often almost thrown out of action during the greater part of the growing season, while the needful moisture and nourishment must be wholly drawn from the subsoil and substrata; an examination of which should therefore precede every purchase of land, or planting of crops.

Such examinations are most quickly made by means of a probe consisting of a pointed, square steel rod five or six feet long, provided at one end with a loop for the insertion of a cross-handle like that of a carpenter’s auger. The handle being grasped with both hands, the probe is forced into the soil with a slight reciprocating motion, by the weight of the operator; who soon learns how to interpret the varying kinds of resistance, and on withdrawing the probe carefully will generally be able to determine if bottom water has been reached. Should this easy method of examination not convey all the needful information, the post-hole auger may be resorted to; and it is desirable that extra (three-foot) rods or gaspipe joints be provided for the purpose of lengthening the probe or auger, when necessary, to nine or twelve feet. It will rarely be necessary to go to the trouble of digging a pit for such examinations; but even this is to be recommended rather than “buying a cat in a bag” in the guise of an unexplored subsoil.

Faulty Substrata.—A number of examples of “faulty lands,” i. e., such as are underlaid by faulty substrata, are given in the annexed diagram Fig. 35; the examples being taken from California localities because of their having been most thoroughly investigated. Similar cases, as well as others not here illustrated, of course occur more or less all over the world.

No. 1 shows a case which, though at first sight an aggravated one of a rocky substratum, is in reality that of some of the best fruit lands in the State. The limited surface-soil is very rich, and is directly derived (as a “sedentary” soil) from the underlying bedrock slate. But this it will be noted stands on edge, and the roots of trees and vines wedge their way along the cleavage planes of the slate to considerable depth, deriving from them both nourishment and moisture. Under similar conditions the California laurel, usually found on the banks of streams, grows on the summits of rocky ridges in the Coast Ranges.

The case of No. 2 is quite otherwise. Here the shale lies horizontally, and though much softer than the slate of the first column, obstinately resists the penetration of roots; so that the land, though fairly provided with plant-food, is almost wholly useless for cultivation. It is naturally covered with low, stunted shrubs or chaparral; only here and there, where a cleft has been caused by earthquakes or subsidence, a large pine tree indicates that nourishment and moisture exists within the refractory clay stratum, and suggests blasting as a means of rendering the land fit for trees at least.

FIG. 35.—Faulty Lands, California.]

No. 3 is a case similar to that of No. 2, only there is here a dense unstratified mass of red clay, of good native fertility. It is here that the expedient of blasting the tree holes with dynamite was first successfully employed, in central California. For lack of this, extensive tracts of similar land in southern California, planted to orchards, have completely failed of useful results after three years of culture.

No. 4 shows a typical case of calcareous hardpan obstructing the penetration of roots, even though usually interrupted at intervals, because of the formation occurring mostly in swales, along which the sheets lie more or less continuously. Here also, blasting will generally permit the successful growth of trees and vines, whose roots frequently will, in time, wholly disintegrate the hardpan and thus render the land fit for field cultures. The depth at which such hardpan is formed usually depends upon the depth to which the annual rainfall penetrates. (See below, page 183).

Nos. 4, 5, and 6 all illustrate cases of intrinsically fertile, very deep soils, shallowed by obstructions which in the case of No. 4 are hardpan sheets, while in No. 5 the intervention of bottom water limits root penetration, hence restricts the use of the land to relatively shallow-rooted crops, and the use of only a few feet of the profusely fertile soil. Such is the case where bottom water has been allowed to rise too high, through the use of leaky irrigation ditches.

No. 6 illustrates a case not uncommon in sedimentary lands, where bottom water is quite within reach of most plants, but is prevented from being utilized by the intervention of layers of coarse sand or gravel, through which the water will not rise; and the roots, while they would be able to penetrate, are not near enough to feel the presence of water underneath and therefore spread on the surface of the gravel, suffering from drought within easy reach of abundance of water. The “going-back” of large portions of orange orchards in the San Bernardino Valley of California has been thus brought about; and unfortunately this state of things is almost beyond the possibility of remedy.

Injury from Impervious Substrata.—The injurious effects of a difficultly penetrable subsoil have already been discussed and are self-evident. When the substratum is a dense clay, the rise of moisture from below being very slow, it can easily happen that the roots cannot penetrate deep enough in time for the coming of the dry season, and that thus the crop will suffer. The case will be still worse when hardpan cemented by lime or silex limits root-penetration, as well as proper drainage. In such cases the culture of field crops often becomes impracticable, even with irrigation, as its frequent repetition, besides being costly, can rarely be commanded. In the case of trees, the limitation of root-penetration results in the spreading-out of the roots on the surface of the impenetrable layer; as shown in figure 36, which exhibits a root-development that would be quite normal in the regions of summer rains, but is wholly abnormal in the arid region, and results in the unprofitableness or death of the trees. It has often been attempted in such cases to plant trees in large holes dug deep into the subsoil and refilled with surface earth and manure. All such attempts result in failure, if only because the excavation inevitably fills with water, which will soak away but very slowly into the dense substrata, and will thus injure or drown out the roots. Besides, the latter will remain bunched in the loose earth, and will thus be unable to draw either moisture or nourishment from the surrounding land. It is absolutely necessary to remedy this by loosening the substrata, if success is to be attained.

Shattering of Dense Substrata by Dynamite.—The permanent loosening of dense substrata is best accomplished by moderate charges (½ to ¾ pounds) of “No. 2” dynamite at a sufficient depth (3 to 5 feet). The shattering effect of the explosive will be sensible to the depth of eight feet or more, and will fissure the clay or hardpan to a corresponding extent sidewise. If properly proportioned the charge will hardly disturb the surface; but if this be desired, from 1½ to 2½ pounds of black powder placed above the dynamite will throw out sufficient earth to plant the tree without farther digging. Where labor is high-priced this proves the cheapest as well as the best way to prepare such ground for tree planting; and it has often been found that in the course of time, the loosening begun by the powder has extended through the mass of the land so as to permit the roots to utilize it fully, and even to permit, in after years, of the planting of field crops where formerly they would not succeed.

Leachy Substrata.—While we may thus overcome the disadvantages of a dense subsoil or hardpan, there is another difficulty not uncommonly met with in alluvial lands, which cannot be so readily remedied. It is the occurrence, at from two to six feet depth, of coarse sand or gravel, through which capillary moisture will not ascend, but through which irrigation water will waste rapidly, leaving the overlying soil dry. Then unless very frequent irrigation can be given, the crop will suffer from drought, unless indeed the gravel itself is filled with bottom water upon which the root-ends can draw.

This case is a common one in the larger valleys of the arid region, and in time of unusual drought the sloughs originally existing, but since filled up, will be clearly outlined by the dying crops, while outside of the old channels there may be no suffering.

“Going-back” of Orchards. On such land as this, and on such as has a shallow soil underlaid by an impervious subsoil, trees will often grow finely for three to five years; then suddenly languish, or turn yellow and die, as the demand of their larger growth exceeds what moisture or plant-food the shallow soil and subsoil can supply. Enormous losses have arisen from this cause in many portions of the arid region, but more especially in California, owing to the implicit confidence reposed even by old settlers, and still more by newcomers, in the excellence of the lands, as illustrated by farms perhaps a short distance away, but differently situated with respect to the country drainage and the geological formations. All such disappointments could have been avoided by an intelligent observation of the substrata, either by probing or digging. Important as is such preliminary examination in the region of summer rains, it is a vitally needful precaution in the arid region, where the margin between adequate and inadequate depth of soil and moisture-supply is much smaller.

When farmers note such distress in the orchard, the first idea usually is that fertilization is needed. This in the almost universally very rich lands of the arid region is rarely the case until after many years of exhaustive cultivation, and is scarcely ever of more than passing benefit in such cases. The first suggestion should always be an examination of the substrata, and especially of the deeper roots; in the diseased or thirsty condition of which the cause of the “die-back” or yellowing will commonly be found. Of course no amount of fertilization can permanently remedy such a state of things, arising from impervious substrata, coarse gravel, or shallow bottom water.

Hardpan.—By “hardpan” is understood a dense and more or less hardened layer in the subsoil, which obstructs the penetration of both roots and water, thus materially limiting the range of the former both for plant-food and moisture, and giving rise to the disadvantages following such limitation, as described in the case of dense subsoils. The hardpans proper differ from the latter, however, in being usually of limited thickness only; the direct consequence of their mode of formation, which is not direct deposition by water or other agencies, but the infiltration of cementing solutions into a pre-existing material originally quite similar to that of the surface soil. Such solutions usually come from above, more rarely from below, and are of very various composition. The solutions of lime carbonate in carbonated water have already been referred to in this connection; as has also the fact that corresponding solutions of silica, associated more or less with other products of rock decomposition (see chapters 2 and 4) are constantly circulating in soils. The surface soil being the portion where rock-weathering and other soil-forming processes are most active, these solutions are chiefly formed there; and according as their descent into the substrata is unchecked, or is liable to be arrested at some particular level, whether by pre-existing close-grained layers or by the cessation of rains, the subsequent penetration of air, and evaporation of the water alone by shallow-rooted plants, may cause the accumulation of the dissolved matter at a certain level, year after year. Finally there is formed a subsoil-mass more or less firmly cemented by the dissolved matters, sometimes to the extent of stony hardness (lime carbonate in the arid regions, kankar of India), more usually soft enough to be penetrated by the pick or grubbing hoe, and sometimes by the stronger roots of certain plants; but resisting both the penetration and the assimilation of plant food by the more delicate feeding roots.

Nature of the Cements.—The nature of the cements that serve to consolidate the hardpan mass is substantially the same as those already mentioned in the discussion of sandstones (chapt. 4, p. 55); with the addition of those formed, usually in connection with siliceous solutions, by the acids of the humus group. The latter class of hardpans is especially conspicuous in the case of swampy ground and damp forests, where “moorbedpan” and reddish “ortstein” (the latter particularly developed in the forests of northern Europe, where it has been studied in detail by Müller and Tuxen, are characteristic.) The latter gives for a characteristic sample of the reddish hardpan underlying a beech forest in Denmark a content of from 2.20 to 4.40% of ulmic compounds, and shows that the color is due to these and not, as had been supposed, to ferric oxid, which is present only in minute quantities.

See “Studien über die natürlichen Humusformen,” by Dr. P. E. Müller.

Bog ore, Moorbedpan, Ortstein.—It is otherwise with moorbedpan, which often consists of a mass of bog iron ore permeated more or less with humous substances, which impart to it the dark brown tint so often seen also in the “black gravel” spots of badly-drained land. On the whole, however, ferric cements are much less frequently found in hardpans than in sandstones formed above ground.

Clay substance washed from the surface into the subsoil by rains (chapter 10, p. 161) always helps materially to render the hardpan impervious when afterwards cemented, a much smaller proportion of the cementing material sufficing in that case to form a solid layer. In such cases however the cement is rarely of a calcareous nature, since lime prevents the diffusion and washing-down of the clay. It is mostly siliceous or zeolitic; if the former, acid will have little or no effect upon the solidity of the hardpan; while if zeolitic, acid will pretty promptly disintegrate it. The presence of humus acids in the cements, if not apparent to the eye, is readily demonstrated by immersing the hardpan fragment in ammonia water or a weak solution of caustic soda; when if humus acids are the main cementing substance the fragment will fall to crumbs, or be softened to an extent corresponding to the amount of the humus present. Calcareous hardpan is, of course, readily recognized by its quick disintegration by dilute acid, with evolution of carbonic gas.

In “alkali” soils containing sodic carbonate (“black alkali”) there is commonly found at the depth of two or three feet an exceedingly refractory hardpan resulting from the accumulation of puddled clay (see above chapt. 4, p. 62) in the subsoil, or sometimes even on the surface of depressed spots. This hardpan, easily destroyed by the use of gypsum and water, is described more in detail in chapter 22, on alkali soils; it blues red litmus paper instantly.

The Causes of Hardpan.—The recognition of the cause of hardpan is of considerable importance to the farmer, because of the influence of the nature of the cement and the causes of its formation upon the possibility and methods of its destruction, for the improvement of the land.

It may be said in general that inasmuch as the cause of the formation of hardpan is a stoppage of the water in its downward penetration, the re-establishment of that penetration will tend to prevent additional induration; moreover, experience proves that whenever this is accomplished even locally, as around a fruit tree in an orchard, the hardpan gradually softens and disappears before the frequent changes in moisture-conditions and the attack of roots. The use of dynamite for this purpose in California has already been referred to; it seems to be the only resort when the hardpan lies at a considerable depth. When it is within reach of the plow, it may be turned up on the surface by the aid of a subsoiler and will then gradually disintegrate under the influence of air, rain and sun. But when the hardpan is of the nature of moorbedpan, containing much humic acid and perhaps underlaid by bog iron ore, the use of lime on the land is indicated, and will in the course of time destroy the hardpan layer. This is the more desirable as in such cases the surface soil is usually completely leached of its lime content, and is consequently extremely unthrifty.

Woodlands of northern countries bearing beech and oak are especially apt to be benefited by the action of lime on the “raw,” acid humous soil and underlying hardpan, which is commonly underlaid by a leaden-blue sandy subsoil (“Bleisand” of the Germans, “Podzol” of the Russians) colored brown by earth humates and mostly too moist in its natural condition to permit of adequate aeration. These soils are usually of but moderate fertility, and are best suited to forest growth unless somewhat expensive methods of improvement can be put into practice.

“Plowsole.”—An artificial hardpan is very commonly formed under the practice of plowing to the same depth for many consecutive years. The consolidated layer thus created by the action of the plow (hence known as plowsole) acts precisely like a natural hardpan, and is sometimes the cause of the formation of a cemented subsoil crust simulating the natural product. This is most apt to occur in clayey lands, and greatly increases the difficulty of working them, while detracting materially from the higher productiveness commonly attributed to them as compared with sandy lands. Of course it is perfectly easy to prevent this trouble by plowing to different depths in consecutive years, and running a subsoil plow from time to time. In this case, also, lime will generally be very useful and be found to aid materially in the disintegration of the “plowsole.”

It is hardly necessary to insist farther upon the need of the examination of land to be occupied, for the existence of hardpan or other faulty subsoil, which may totally defeat for the time being the farmer’s efforts, or make him lose his investment in plantations after a few years. Probing by means of the steel rod described above (p. 177) or boring with a post-hole auger; or finally, if necessary, digging a pit to the proper depth (from four to six feet in the arid region), should precede every purchase of new or unexplored agricultural land.

Marly Substrata.—Among the causes of failure occasionally found in the case of the “going-back” of orchards, is the occurrence of strongly calcareous or marly substrata, at depths which in the humid region would not be reached by the roots, but in the course of a few years are inevitably penetrated by the roots of trees in the arid region. Then there appears a stunting of the growth, and sometimes a yellowing of leaves, or chlorosis, due to the influence of excessive calcareousness at the depth of four or five feet. For this of course there is no remedy except the planting of crops which, like the mulberry, Texas grapes, Chickasaw plum and others, are at home on such lands; which in the Eastern states are naturally occupied by the crab apple, honey locust and wild plums.