IMPORTANT as is the rainfall in making dry-farmingsuccessful, it is not more so than the soils of the dry-farms. On a shallow soil,or on one penetrated with gravel streaks, crop failures are probable even under alarge rainfall; but a deep soil of uniform texture, unbroken by gravel or hardpan,in which much water may be stored, and which furnishes also an abundance of feedingspace for the roots, will yield large crops even under a very small rainfall. Likewise,an infertile soil, though it be deep, and under a large precipitation, cannot bedepended on for good crops; but a fertile soil, though not quite so deep, nor underso large a rainfall, will almost invariably bring large crops to maturity.
A correct understanding of the soil, from thesurface to a depth of ten feet, is almost indispensable before a safe Judgment canbe pronounced upon the full dry-farm possibilities of a district. Especially is itnecessary to know (a) the depth, (b) the uniformity of structure, and (c) the relativefertility of the soil, in order to plan an intelligent system of farming that willbe rationally adapted to the rainfall and other climatic factors.
It is a matter of regret that so much of ourinformation concerning the soils of the dry-farm territory of the United States andother countries has been obtained according to the methods and for the needs of humidcountries, and that, therefore, the special knowledge of our arid and semiarid soilsneeded for the development of dry-farming is small and fragmentary. What is knownto-day concerning the nature of arid soils and their relation to cultural processesunder a scanty rainfall is due very largely to the extensive researches and voluminouswritings of Dr. E. W. Hilgard, who for a generation was in charge of the agriculturalwork of the state of California. Future students of arid soils must of necessityrest their investigations upon the pioneer work done by Dr. Hilgard. The contentsof this chapter are in a large part gathered from Hilgard's writings.
The formation of soils
"Soil is the more or less loose and friablematerial in which, by means of their roots, plants may or do find a foothold andnourishment, as well as other conditions of growth." Soil is formed by a complexprocess, broadly known as weathering, from the rocks which constitute theearth's crust. Soil is in fact only pulverized and altered rock. The forces thatproduce soil from rocks are of two distinct classes, physical and chemical. Thephysical agencies of soil production merely cause a pulverization of the rock; thechemical agencies, on the other hand, so thoroughly change the essential nature ofthe soil particles that they are no longer like the rock from which they were formed.
Of the physical agencies, temperature changesare first in order of time, and perhaps of first importance. As the heat of theday increases, the rock expands, and as the cold night approaches, contracts. Thisalternate expansion and contraction, in time, cracks the surfaces of the rocks. Intothe tiny crevices thus formed water enters from the falling snow or rain. When wintercomes, the water in these cracks freezes to ice, and in so doing expands and widenseach of the cracks. As these processes are repeated from day to day, from year toyear, and from generation to generation, the surfaces of the rocks crumble. The smallerrocks so formed are acted upon by the same agencies, in the same manner, and thusthe process of pulverization goes on.
It is clear, then, that the second great agencyof soil formation, which always acts in conjunction with temperature changes, isfreezing water. The rock particles formed in this manner are often washeddown into the mountain valleys, there caught by great rivers, ground into finer dust,and at length deposited in the lower valleys. Moving water thus becomes anotherphysical agency of soil production. Most of the soils covering the great dry-farmterritory of the United States and other countries have been formed in this way.
In places, glaciers moving slowly down the canonscrush and grind into powder the rock over which they pass and deposit it lower downas soils. In other places, where strong winds blow with frequent regularity, sharpsoil grains are picked up by the air and hurled against the rocks, which, under thisaction, are carved into fantastic forms. In still other places, the strong windscarry soil over long distances to be mixed with other soils. Finally, on the seashorethe great waves dashing against the rocks of the coast line, and rolling the massof pebbles back and forth, break and pulverize the rock until soil is formed.Glaciers, winds, and waves are also, therefore, physical agencies of soilformation.
It may be noted that the result of the actionof all these agencies is to form a rock powder, each particle of which preservesthe composition that it had while it was a constituent part of the rock. It may furtherbe noted that the chief of these soil-forming agencies act more vigorously in aridthan in humid sections. Under the cloudless sky and dry atmosphere of regions oflimited rainfall, the daily and seasonal temperature changes are much greater thanin sections of greater rainfall. Consequently the pulverization of rocks goes onmost rapidly in dry-farm districts. Constant heavy winds, which as soil formers aresecond only to temperature changes and freezing water, are also usually more commonin arid than in humid countries. This is strikingly shown, for instance, on the Coloradodesert and the Great Plains.
The rock powder formed by the processes abovedescribed is continually being acted upon by agencies, the effect of which is tochange its chemical composition. Chief of these agencies is water, which exertsa solvent action on all known substances. Pure water exerts a strong solvent action,but when it has been rendered impure by a variety of substances, naturally occurring,its solvent action is greatly increased.
The most effective water impurity, consideringsoil formation, is the gas, carbon dioxid. This gas is formed whenever plantor animal substances decay, and is therefore found, normally, in the atmosphere andin soils. Rains or flowing water gather the carbon dioxid from the atmosphere andthe soil; few natural waters are free from it. The hardest rock particles are disintegratedby carbonated water, while limestones, or rocks containing lime, are readily dissolved.
The result of the action of carbonated waterupon soil particles is to render soluble, and therefore more available to plants,many of the important plant-foods. In this way the action of water, holding in solutioncarbon dioxid and other substances, tends to make the soil more fertile.
The second great chemical agency of soil formationis the oxygen of the air. Oxidation is a process of more or less rapid burning, whichtends to accelerate the disintegration of rocks.
Finally, the plants growing in soils arepowerful agents of soil formation. First, the roots forcing their way into the soilexert a strong pressure which helps to pulverize the soil grains; secondly, the acidsof the plant roots actually dissolve the soil, and third, in the mass of decayingplants, substances are formed, among them carbon dioxid, that have the power of makingsoils more soluble.
It may be noted that moisture, carbon dioxid,and vegetation, the three chief agents inducing chemical changes in soils, are mostactive in humid districts. While, therefore, the physical agencies of soil formationare most active in arid climates, the same cannot be said of the chemical agencies.However, whether in arid or humid climates, the processes of soil formation, aboveoutlined, are essentially those of the "fallow" or resting-period givento dry-farm lands. The fallow lasts for a few months or a year, while the processof soil formation is always going on and has gone on for ages; the result, in qualitythough not in quantity, is the same--the rock particles are pulverized and the plant-foodsare liberated. It must be remembered in this connection that climatic differencesmay and usually do influence materially the character of soils formed from one andthe same kind of rock.
Characteristics of arid soils
The net result of the processes above describedIs a rock powder containing a great variety of sizes of soil grains intermingledwith clay. The larger soil grains are called sand; the smaller, silt, and those thatare so small that they do not settle from quiet water after 24 hours are known asclay.
Clay differs materially from sand and silt, notonly in size of particles, but also in properties and formation. It is said thatclay particles reach a degree of fineness equal to 1/2500 of an inch. Clay itself,when wet and kneaded, becomes plastic and adhesive and is thus easily distinguishedfrom sand. Because of these properties, clay is of great value in holding togetherthe larger soil grains in relatively large aggregates which give soils the desireddegree of filth. Moreover, clay is very retentive of water, gases, and soluble plant-foods,which are important factors in successful agriculture. Soils, in fact, are classifiedaccording to the amount of clay that they contain. Hilgard suggests the followingclassification:--
Very sandy soils0.5 to 3 per cent clay
Ordinary sandy soils3.0 to 10 per cent clay
Sandy loams10.0 to 15 per cent clay
Clay loams15.0 to 25 per cent clay
Clay soils25.0 to 35 per cent clay
Heavy clay soils35.0 per cent and over
Clay may be formed from any rock containing someform of combined silica (quartz). Thus, granites and crystalline rocks generally,volcanic rocks, and shales will produce clay if subjected to the proper climaticconditions. In the formation of clay, the extremely fine soil particles are attackedby the soil water and subjected to deep-going chemical changes. In fact, clay representsthe most finely pulverized and most highly decomposed and hence in a measure themost valuable portion of the soil. In the formation of clay, water is the most activeagent, and under humid conditions its formation is most rapid.
It follows that dry-farm soils formed under amore or less rainless climate contain less clay than do humid soils. This differenceis characteristic, and accounts for the statement frequently made that heavy claysoils are not the best for dry-farm purposes. The fact is, that heavy clay soilsare very rare in arid regions; if found at all, they have probably been formed underabnormal conditions, as in high mountain valleys, or under prehistoric humid climates.
Sand.--The sand-forming rocks that arenot capable of clay production usually consist of uncombined silica or quartz,which when pulverized by the soil-forming agencies give a comparatively barren soil.Thus it has come about that ordinarily a clayey soil is considered "strong"and a sandy soil "weak." Though this distinction is true in humid climateswhere clay formation is rapid, it is not true in arid climates, where true clay isformed very slowly. Under conditions of deficient rainfall, soils are naturally lessclayey, but as the sand and silt particles are produced from rocks which under humidconditions would yield clay, arid soils are not necessarily less fertile.
Experiment has shown that the fertility in thesandy soils of arid sections is as large and as available to plants as in the clayeysoils of humid regions. Experience in the arid section of America, in Egypt, India,and other desert-like regions has further proved that the sands of the deserts produceexcellent crops whenever water is applied to them. The prospective dry-farmer, therefore,need not be afraid of a somewhat sandy soil, provided it has been formed under aridconditions. In truth, a degree of sandiness is characteristic of dry-farm soils.
The humus content forms another characteristicdifference between arid and humid soils. In humid regions plants cover the soil thickly;in arid regions they are bunched scantily over the surface; in the former case thedecayed remnants of generations of plants form a large percentage of humus in theupper soil; in the latter, the scarcity of plant life makes the humus content low.Further, under an abundant rainfall the organic matter in the soil rots slowly; whereasin dry warm climates the decay is very complete. The prevailing forces in all countriesof deficient rainfall therefore tend to yield soils low in humus.
While the total amount of humus in arid soilsis very much lower than in humid soils, repeated investigation has shown that itcontains about 3-1/2 times more nitrogen than is found in humus formed under an abundantrainfall. Owing to the prevailing sandiness of dry-farm soils, humus is not neededso much to give the proper filth to the soil as in the humid countries where thecontent of clay is so much higher. Since, for dry-farm purposes, the nitrogen contentis the most important quality of the humus, the difference between arid and humidsoils, based upon the humus content, is not so great as would appear at first sight.
Soil and subsoil.--In countries of abundantrainfall, a great distinction exists between the soil and the subsoil. The soil isrepresented by the upper few inches which are filled with the remnants of decayedvegetable matter and modified by plowing, harrowing, and other cultural operations.The subsoil has been profoundly modified by the action of the heavy rainfall, which,in soaking through the soil, has carried with it the finest soil grains, especiallythe clay, into the lower soil layers.
In time, the subsoil has become more distinctlyclayey than the topsoil. Lime and other soil ingredients have likewise been carrieddown by the rains and deposited at different depths in the soil or wholly washedaway. Ultimately, this results in the removal from the topsoil of the necessary plant-foodsand the accumulation in the subsoil of the fine clay particles which so compact thesubsoil as to make it difficult for roots and even air to penetrate it. The normalprocess of weathering or soil disintegration will then go on most actively in thetopsoil and the subsoil will remain unweathered and raw. This accounts for the well-knownfact that in humid countries any subsoil that may have been plowed up is reducedto a normal state of fertility and crop production only after several years of exposureto the elements. The humid farmer, knowing this, is usually very careful not to lethis plow enter the subsoil to any great depth.
In the arid regions or wherever a deficient rainfallprevails, these conditions are entirely reversed. The light rainfall seldom completelyfills the soil pores to any considerable depth, but it rather moves down slowly asa him, enveloping the soil grains. The soluble materials of the soil are, in partat least, dissolved and carried down to the lower limit of the rain penetration,but the clay and other fine soil particles are not moved downward to any great extent.These conditions leave the soil and subsoil of approximately equal porosity. Plantroots can then penetrate the soil deeply, and the air can move up and down throughthe soil mass freely and to considerable depths. As a result, arid soils are weatheredand made suitable for plant nutrition to very great depths. In fact, in dry-farmregions there need be little talk about soil and subsoil, since the soil is uniformin texture and usually nearly so in composition, from the top down to a distanceof many feet.
Many soil sections 50 or more feet in depth areexposed in the dry-farming territory of the United States, and it has often beendemonstrated that the subsoil to any depth is capable of producing, without furtherweathering, excellent yields of crops. This granular, permeable structure, characteristicof arid soils, is perhaps the most important single quality resulting from rock disintegrationunder arid conditions. As Hilgard remarks, it would seem that the farmer in the aridregion owns from three to four farms, one above the other, as compared with the sameacreage in the eastern states.
This condition is of the greatest importancein developing the principles upon which successful dry-farming rests. Further, itmay be said that while in the humid East the farmer must be extremely careful notto turn up with his plow too much of the inert subsoil, no such fear need possessthe western farmer. On the contrary, he should use his utmost endeavor to plow asdeeply as possible in order to prepare the very best reservoir for the falling watersand a place for the development of plant roots.
Gravel seams.--It need be said, however,that in a number of localities in the dry-farm territory the soils have been depositedby the action of running water in such a way that the otherwise uniform structureof the soil is broken by occasional layers of loose gravel. While this is not a veryserious obstacle to the downward penetration of roots, it is very serious in dry-farming,since any break in the continuity of the soil mass prevents the upward movement ofwater stored in the lower soil depths. The dry-farmer should investigate the soilwhich he intends to use to a depth of at least 8 to 10 feet to make sure, first ofall, that he has a continuous soil mass, not too clayey in the lower depths, norbroken by deposits of gravel.
Hardpan.--Instead of the heavy clay subsoilof humid regions, the so-called hardpan occurs in regions of limited rainfall. Theannual rainfall, which is approximately constant, penetrates from year to year verynearly to the same depth. Some of the lime found so abundantly in arid soils is dissolvedand worked down yearly to the lower limit of the rainfall and left there to enterinto combination with other soil ingredients. Continued through long periods of timethis results in the formation of a layer of calcareous material at the average depthto which the rainfall has penetrated the soil. Not only is the lime thus carrieddown, but the finer particles are carried down in like manner. Especially where thesoil is poor in lime is the clay worked down to form a somewhat clayey hardpan. Ahardpan formed in such a manner is frequently a serious obstacle to the downwardmovement of the roots, and also prevents the annual precipitation from moving downfar enough to be beyond the influence of the sunshine and winds. It is fortunate,however, that in the great majority of instances this hardpan gradually disappearsunder the influence of proper methods of dry-farm tillage. Deep plowing and propertillage, which allow the rain waters to penetrate the soil, gradually break up anddestroy the hardpan, even when it is 10 feet below the surface. Nevertheless, thefarmer should make sure whether or not the hardpan does exist in the soil and planhis methods accordingly. If a hardpan is present, the land must be fallowed morecarefully every other year, so that a large quantity of water may be stored in thesoil to open and destroy the hardpan.
Of course, in arid as in humid countries, itoften happens that a soil is underlaid, more or less near the surface, by layersof rock, marl deposits, and similar impervious or hurtful substances. Such depositsare not to be classed with the hardpans that occur normally wherever the rainfallis small.
Leaching.--Fully as important as any ofthe differences above outlined are those which depend definitely upon the leachingpower of a heavy rainfall. In countries where the rainfall is 30 inches or over,and in many places where the rainfall is considerably less, the water drains throughthe soil into the standing ground water. There is, therefore, in humid countries,a continuous drainage through the soil after every rain, and in general there isa steady downward movement of soil-water throughout the year. As is clearly shownby the appearance, taste, and chemical composition of drainage waters, this processleaches out considerable quantities of the soluble constituents of the soil.
When the soil contains decomposing organic matter,such as roots, leaves, stalks, the gas carbon dioxid is formed, which, when dissolvedin water, forms a solution of great solvent power. Water passing through well-cultivatedsoils containing much humus leaches out very much more material than pure water coulddo. A study of the composition of the drainage waters from soils and the waters ofthe great rivers shows that immense quantities of soluble soil constituents are takenout of the soil in countries of abundant rainfall. These materials ultimately reachthe ocean, where they are and have been concentrated throughout the ages. In short,the saltiness of the ocean is due to the substances that have been washed from thesoils in countries of abundant rainfall.
In arid regions, on the other hand, the rainfallpenetrates the soil only a few feet. In time, it is returned to the surface by theaction of plants or sunshine and evaporated into the air. It is true that under propermethods of tillage even the light rainfall of arid and semiarid regions may he madeto pass to considerable soil depths, yet there is little if any drainage of waterthrough the soil into the standing ground water. The arid regions of the world, therefore,contribute proportionately a small amount of the substances which make up the saltof the sea.
Alkali soils.--Under favorable conditionsit sometimes happens that the soluble materials, which would normally be washed outof humid soils, accumulate to so large a degree in arid soils as to make the landsunfitted for agricultural purposes. Such lands are called alkali lands. Unwise irrigationin arid climates frequently produces alkali spots, but many occur naturally. Suchsoils should not be chosen for dry-farm purposes, for they are likely to give trouble.
Plant-food content.--This condition necessarilyleads at once to the suggestion that the soils from the two regions must differ greatlyin their fertility or power to produce and sustain plant life. It cannot be believedthat the water-washed soils of the East retain as much fertility as the dry soilsof the West. Hilgard has made a long and elaborate study of this somewhat difficultquestion and has constructed a table showing the composition of typical soils ofrepresentative states in the arid and humid regions. The following table shows afew of the average results obtained by him:--
Partial Percentage Composition
|Source of soil ||Number of samples analyzed ||Insoluble residue ||Soluble silica ||Alumina ||Lime ||Potash ||Phos. |
|Humid ||696 ||84.17 ||4.04 ||3.66 ||0.13 ||0.21 ||0.12 ||1.22 |
|Arid ||573 ||69.16 ||6.71 ||7.61 ||1.43 ||0.67 ||0.16 ||1.13 |
Soil chemists have generally attempted to arriveat a determination of the fertility of soil by treating a carefully selected andprepared sample with a certain amount of acid of definite strength. The portion whichdissolves under the influence of acids has been looked upon as a rough measure ofthe possible fertility of the soil.
The column headed "Insoluble Residue"shows the average proportions of arid and humid soils which remain undissolved byacids. It is evident at once that the humid soils are much less soluble in acidsthan arid soils, the difference being 84 to 69. Since the only plant-food in soilsthat may be used for plant production is that which is soluble, it follows that itis safe to assume that arid soils are generally more fertile than humid soils. Thisis borne out by a study of the constituents of the soil. For instance, potash, oneof the essential plant foods ordinarily present in sufficient amount, is found inhumid soils to the extent of 0.21 per cent, while in arid soils the quantity presentis 0.67 per cent, or over three times as much. Phosphoric acid, another of the veryimportant plant-foods, is present in arid soils in only slightly higher quantitiesthan in humid soils. This explains the somewhat well-known fact that the first fertilizerordinarily required by arid soils is some form of phosphorus:
The difference in the chemical composition ofarid and humid soils is perhaps shown nowhere better than in the lime content. Thereis nearly eleven times more lime in arid than in humid soils. Conditions of aridityfavor strongly the formation of lime, and since there is very little leaching ofthe soil by rainfall, the lime accumulates in the soil.
The presence of large quantities of lime in aridsoils has a number of distinct advantages, among which the following are most important:(1) It prevents the sour condition frequently present in humid climates, where muchorganic material is incorporated with the soil. (2) When other conditions are favorable,it encourages bacterial life which, as is now a well-known fact, is an importantfactor in developing and maintaining soil fertility. (3) By somewhat subtle chemicalchanges it makes the relatively small percentages of other plant-foods notably phosphoricacid and potash, more available for plant growth. (4) It aids to convert rapidlyorganic matter into humus which represents the main portion of the nitrogen contentof the soil.
Of course, an excess of lime in the soil maybe hurtful, though less so in arid than in humid regions. Some authors state thatfrom 8 to 20 per cent of calcium carbonate makes a soil unfitted for plant growth.There are, however, a great many agricultural soils covering large areas and yieldingvery abundant crops which contain very much larger quantities of calcium carbonate.For instance, in the Sanpete Valley of Utah, one of the most fertile sections ofthe Great Basin, agricultural soils often contain as high as 40 per cent of calciumcarbonate, without injury to their crop-producing power.
In the table are two columns headed " SolubleSilica" and "Alumina," in both of which it is evident that a verymuch larger per cent is found in the arid than in the humid soils. These soil constituentsindicate the condition of the soil with reference to the availability of its fertilityfor plant use. The higher the percentage of soluble silica and alumina, the morethoroughly decomposed, in all probability, is the soil as a whole and the more readilycan plants secure their nutriment from the soil. It will be observed from the table,as previously stated, that more humus is found in humid than in arid soils, thoughthe difference is not so large as might be expected. It should be recalled, however,that the nitrogen content of humus formed under rainless conditions is many timeslarger than that of humus formed in rainy countries, and that the smaller per centof humus in dry-farming countries is thereby offset.
All in all, the composition of arid soils isvery much more favorable to plant growth than that of humid soils. As will be shownin Chapter IX, the greater fertility of arid soils is one of the chief reasons fordry-farming success. Depth of the soil alone does not suffice. There must be a largeamount of high fertility available for plants in order that the small amount of watercan be fully utilized in plant growth.
Summary of characteristics.--Arid soilsdiffer from humid soils in that they contain: less clay; more sand, but of fertilenature because it is derived from rocks that in humid countries would produce clay;less humus, but that of a kind which contains about 3-1/2 times more nitrogen thanthe humus of humid soils; more lime, which helps in a variety of ways to improvethe agricultural value of soils; more of all the essential plant-foods, because theleaching by downward drainage is very small in countries of limited rainfall.
Further, arid soils show no real difference betweensoil and subsoil; they are deeper and more permeable; they are more uniform in structure;they have hardpans instead of clay subsoil, which, however, disappear under the influenceof cultivation; their subsoils to a depth of ten feet or more are as fertile as thetopsoil, and the availability of the fertility is greater. The failure to recognizethese characteristic differences between arid and humid soils has been the chiefcause for many crop failures in the more or less rainless regions of the world.
This brief review shows that, everything considered,arid soils are superior to humid soils. In ease of handling, productivity, certaintyof crop-lasting quality, they far surpass the soils of the countries in which scientificagriculture was founded. As Hilgard has suggested, the historical datum that themajority of the most populous and powerful historical peoples of the world have beenlocated on soils that thirst for water, may find its explanation in the intrinsicvalue of arid soils. From Babylon to the United States is a far cry; but it is onethat shouts to the world the superlative merits of the soil that begs for water.To learn how to use the "desert" is to make it "blossom like the rose."
The dry-farm territory of the United States maybe divided roughly into five great soil districts, each of which includes a greatvariety of soil types, most of which are poorly known and mapped. These districtsare:--
1. Great Plains district.
2. Columbia River district
3. Great Basin district.
4. Colorado River district.
5. California district.
Great Plains district.--On the easternslope of the Rocky Mountains, extending eastward to the extreme boundary of the dry-farmterritory, are the soils of the High Plains and the Great Plains. This vast soildistrict belongs to the drainage basin of the Missouri, and includes North and SouthDakota, Nebraska, Kansas, Oklahoma, and parts of Montana, Wyoming, Colorado, NewMexico, Texas, and Minnesota. The soils of this district are usually of high fertility.They have good lasting power, though the effect of the higher rainfall is evidentin their composition. Many of the distinct types of the plains soils have been determinedwith considerable care by Snyder and Lyon, and may be found described in Bailey's"Cyclopedia of American Agriculture," Vol. I.
Columbia River district.--The second greatsoil district of the dry-farming territory is located in the drainage basin of theColumbia River, and includes Idaho and the eastern two thirds of Washington and Oregon.The high plains of this soil district are often spoken of as the Palouse country.The soils of the western part of this district are of basaltic origin; over the southernpart of Idaho the soils have been made from a somewhat recent lava flow which inmany places is only a few feet below the surface. The soils of this district aregenerally of volcanic origin and very much alike. They are characterized by the propertieswhich normally belong to volcanic soils; somewhat poor in lime, but rich in potashand phosphoric acid. They last well under ordinary methods of tillage.
The Great Basin.--The third great soildistrict is included in the Great Basin, which covers nearly all of Nevada, halfof Utah, and takes small portions out of Idaho, Oregon, and southern California.This basin has no outlet to the sea. Its rivers empty into great saline inland lakes,the chief of which is the Great Salt Lake. The sizes of these interior lakes aredetermined by the amounts of water flowing into them and the rates of evaporationof the water into the dry air of the region.
In recent geological times, the Great Basin wasfilled with water, forming a vast fresh-water lake known as Lake Bonneville, whichdrained into the Columbia River. During the existence of this lake, soil materialswere washed from the mountains into the lake and deposited on the lake bottom. Whenat length, the lake disappeared, the lake bottom was exposed and is now the farminglands of the Great Basin district. The soils of this district are characterized bygreat depth and uniformity, an abundance of lime, and all the essential plant-foodswith the exception of phosphoric acid, which, while present in normal quantities,is not unusually abundant. The Great Basin soils are among the most fertile on theAmerican Continent.
Colorado River district.--The fourth soildistrict lies in the drainage basin of the Colorado River It includes much of thesouthern part of Utah, the eastern part of Colorado, part of New Mexico, nearly allof Arizona, and part of southern California. This district, in its northern part,is often spoken of as the High Plateaus. The soils are formed from the easily disintegratedrocks of comparatively recent geological origin, which themselves are said to havebeen formed from deposits in a shallow interior sea which covered a large part ofthe West. The rivers running through this district have cut immense canons with perpendicularwalls which make much of this country difficult to traverse. Some of the soils areof an extremely fine nature, settling firmly and requiring considerable tillage beforethey are brought to a proper condition of tilth. In many places the soils are heavilycharged with calcium sulfate, or crystals of the ordinary land plaster. The fertilityof the soils, however, is high, and when they are properly cultivated, they yieldlarge and excellent crops.
California district.--The fifth soil districtlies in California in the basin of the Sacramento and San Joaquin rivers. The soilsare of the typical arid kind of high fertility and great lasting powers. They representsome of the most valuable dry-farm districts of the West. These soils have been studiedin detail by Hilgard.
Dry-farming in the five districts. --Itis interesting to note that in all of these five great soil districts dry-farminghas been tried with great success. Even in the Great Basin and the Colorado Riverdistricts, where extreme desert conditions often prevail and where the rainfall isslight, it has been found possible to produce profitable crops without irrigation.It is unfortunate that the study of the dry-farming territory of the United Stateshas not progressed far enough to permit a comprehensive and correct mapping of itssoils. Our knowledge of this subject is, at the best, fragmentary. We know, however,with certainty that the properties which characterize arid soils, as described inthis chapter' are possessed by the soils of the dry-farming territory, includingthe five great districts just enumerated. The characteristics of arid id soils increaseas the rainfall decreases and other conditions of aridity increase. They are lessmarked as we go eastward or westward toward the regions of more abundant rainfall;that is to say, the most highly developed arid soils are found in the Great Basinand Colorado River districts. The least developed are on the eastern edge of theGreat Plains.
The judging of soils
A chemical analysis of a soil, unless accompaniedby a large amount of other information, is of little value to the farmer. The mainpoints in judging a prospective dry-farm are: the depth of the soil, the uniformityof the soil to a depth of at least 10 feet, the native vegetation, the climatic conditionsas relating to early and late frosts, the total annual rainfall and its distribution,and the kinds and yields of crops that have been grown in the neighborhood.
The depth of the soil is best determined by theuse of an auger. A simple soil auger is made from the ordinary carpenter's auger,1-1/2 to 2 inches in diameter, by lengthening its shaft to 3 feet or more. Whereit is not desirable to carry sectional augers, it is often advisable to have threeaugers made: one 3 feet, the other 6, and the third 9 or 10 feet in length. The shortauger is used first and the others afterwards as the depth of the boring increases.The boring should he made in a large number of average places--preferably one boringor more on each acre if time and circumstances permit--and the results entered ona map of the farm. The uniformity of the soil is observed as the boring progresses.If gravel layers exist, they will necessarily stop the progress of the boring. Hardpansof any kind will also be revealed by such an examination.
The climatic information must be gathered fromthe local weather bureau and from older residents of the section.
The native vegetation is always an excellentindex of dry-farm possibilities. If a good stand of native grasses exists, therecan scarcely be any doubt about the ultimate success of dry-farming under propercultural methods. A healthy crop of sagebrush is an almost absolutely certain indicationthat farming without irrigation is feasible. The rabbit brush of the drier regionsis also usually a good indication, though it frequently indicates a soil not easilyhandled. Greasewood, shadscale, and other related plants ordinarily indicate heavyclay soils frequently charged with alkali. Such soils should be the last choice fordry-farming purposes, though they usually give good satisfaction under systems ofirrigation. If the native cedar or other native trees grow in profusion, it is anotherindication of good dry-farm possibilities.