SOME WESTERN PROBLEMS IN SOIL CLASSIFICATION AND MAPPING.

By Macy H, Lapham, Inspector Western Division, Bureau of Soils*

The western states embrace a wide range in climate and in conditionsof accumulation, leaching and weathering of soil materials. This givesrise to variable and special problems in recognition, differentiationand classification of soils, and necessitates certain practices in fieldmethods adapted to local conditions.

In the humid coastal belt of northwestern California, Oregon andWashington, valleys but a few miles inland are dominated by a long drysummer season during which irrigation is sometimes practiced* The aver-age annual rainfall at Port Townsend in the rainy Puget Sound Basin isbut 21 inches, At one nearby locality on the Olympic Peninsula summerirrigation is practiced while less than 50 miles distant the average annualrainfall is in excess of 14-0 inches.

In the greater part of the states west of the Great Plains theaverage annual rainfall is less than 15 inches, and in much it is under10 inches, and there are localities in which periods of one or moreyears pass without measurable precipitation.

Wide variation in average annual rainfall is accompanied by corres-ponding range in seasonable distribution and in amount of rainfall fromyear to year.

This gives rise to throe recognized types of rainfall. The Pacifictype is characterized by occurrence of most of the precipitation duringthe late fall and winter months, the summers being comparatively or en-tirely rainless. The Sub-Pacific type extends as a belt across easternWashington and Oregon. Idaho, Utah and Nevada in which a scanty rainfallie nore uniformly distributed throughout the fall, winter and springmonths. In the Arizona or desert type covering New Mexico, Arizona,southern Utah and Nevada the late spring and early summer months are com-paratively rainless', about 35 per cent of the annual rainfall occurringas thunderstorms during July and August, with a rather uniform distribu-tion during the remaining fall and winter months.

Temperature conditions owing to an irregular coastal and inter-inountain topography and - influence of westerly winds and summer coastalfogs, are likewise extremely variable. On the coast there are times whenone can go from a region of fog and overcoats to one of high summer tem-peratures within the distance of scarcely a dozen miles.

In light of recent studies on influence of climate on the develop-ment and differentiation of soils a wide and comprehensive range in soilcharacteristics under these conditions is manifest.

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Furthermore geological conditions involve a variety of rocks fromwhich soils materials are derived, and a physiographic development givesrise to land forms of pronounced contrast and to a variety of agenciesof soil accumulation. Such are the conditions under •which the westernfield man labors, and one is frequently confronted with a large numberof soil series and types varying widely in origin, mode of accumulationand in degree of weathering and leaching within the confines of a singlesnail valley and included or marginal mountains and footslopes.

One of the first really important steps in western soil classifica-tion was the recognition many years ago of two main groups of water-laidmaterials designated as Old Transported Soils or Old Valley Filling Soils,and decent Alluvial Soils. This apparently simple and obvious classifica-tion was based not alone on topographic occurrence, but upon the recog-nition of broad and fundamental differences in the material itself. Itwas soon recognized that the older modified materials of the valleyplains tended to produce soils of the more pronounced brown and redtints coincident with oxidation and weathering, that the subsoils weregenerally compact and tight, often containing an accumulation of clayin their upper part, sometimes cemented by line or iron solutions intoa firmly cemented hardpan, and frequently shrinking and cracking whenexposed giving rise to a pronounced adobe or columnar structure. Certainconditions of topography and surface configuration were also associatedwith these soil forming materials and with certain conditions in sub-soil development. Recognition of these characteristics as correlatedwith age and modification in place of the material in contrast with therecent unweathered alluvial soils of uniform or stratified profile, wasI thing, the result of experience and observation of various fieldparties arrived at more or lees independently* Little attempt wasmade however until subsequently to define or describe the basis ofdifferentiation which had been developed empirically and its true sig-nificance was not fully appreciated. Recognition of the soil profileas related to horizons of lime accumulation and especially to develop-ment of structural horizons in detail to which the term profile is nowproperly applied, has come about still later as a result of studiescarried on in the more eastern states. In fact while during the lasthalf dozen or more years close attention has been given to occurrenceand localization of horizons of line accumulation as a basis of seriesdifferentiation, the critical study of western structural soil hori-zons is at present hardly out of its embryonic stage.

In some <jf the more recent surveys a start has been made in thisfertile field for study and typical profiles have been submitted forpublication with the reports. (Exhibit profiles of Twin Falls siltloam and Mesa clay loam, Twin Falls, Ida, and Unita River Valley,Utah, Areas),

Before this can be developed fully we must meet a pressing necess-ity for a system of nomenclature and designation for the predominatingwestern horizons and profiles as has already been done in some of the

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Middle and Great Plains states.

While typical western .profiles rjay be relatively few, there aremany modifications and degrees of transition coincident with loacl dif-ferencas in elevation,, rainfall, drainage, and degree and rate of weather-ing and leaching. At present an unknown number of immature or variantprofiles would appear as well worthy of study from the standpoint ofresearch in soil science, and as important in differentiation of soilseries as the more mature profiles which are reduced to simplicity innumbers.

Differentiation of soils based on detailed study of structuralprofiles new demanded in soil science as well as on other dominant in-herent characteristics, entails under western conditions certain diff i-culties! These are concerned mainly with recognition of visible dif-ferences in color, texture, structure, lime content and topography*

1t Color.

i'^ost of the mature soils are of pronounced brown, red or yellowishcolor. Color s.s indicative of degree of weathering and modified sub-soils necessitates determination as to whether the color is inheritedfrom parent materials or is ^n acquired characteristic. If inherited,differentiation between old e.nd young soils rests entirely upon othercharacteristics evident in the soil profile.

The true color of the soil material is frequently masked by in-cluded gravels or fine rock particles, or by nodules and small frag-ments of gray lime cemented hardpan or caliche material brought up byburrowing animals ani insects and intimately mixed with the surfacematerial, or by thin surface veneer of wind blown sands, Color ofmany of the western soils, particularly those of the richer brown andred tints, often changes materially with variation in moisture content.This is pronounced under lie Pacific and Sub-Pacific types of rainfall,rich tints bein^ brought out under influence of the winter rains whileduring the rainless summer the color fades into duller brown and grayishtints. There are in the Great Interior Valley of California extensiveareas of soils of the San Joaquin and related series. During the winterseason they are distinctly red or dull red in color. They are recog-nized and are locally known as red soils or "red lands", but during therainless summer they fade to a dull reddish brown or brown. Repeatedattempts to. Collect typical samples of these soils at this time havealways resulted in failure to satisfy any., of the field men familiarwith these soils. In another case two associated series of soils weremapped in the same area. Air dry samples were in both cases of darkcolor, nearly black, and hardly distinguishable. Under moist fieldconditions one of these became decidedly black, the other decidedlybrown. Correlation of these two series 6n basis of color of the drysamples alone would have led to error.

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Highly calcareous soils of high organic matter content which occurin some western areas are black under moist field conditions and dryout to a light gray or white chalk like color. Subsoils of high limeaccumulation always dry out much lighter in color than when freshly taken.

Another potent factor of error in judging color in the field isintroduced by variation in angle with which the light strikes the fieldsurfaces, the relative dif f icul ty varying from day to day and with thehour of the day and the relative intensity of the light, being most pro-nounced in desert areas. The incessant glare from reflection of lightfrom desert surfaces is very trying, to some of us intolerable withoutresort to colored sun glasses which introduces further difficulties indifferentiation of soil colors.

Attempt is aade to check and correct discrepances between fieldand sample color and inaccuracies introduced by the above mentionedcauses of error by studying the soils in the field from various angles,by repeated moistening and drying of hand samples, and by describingthe range of color observed under varying conditions. Description ofsoils as given in the reports is later checked by the inspector withsubsamples, a complete set of which is submitted for ee.ch area to theBerkeley off ice ,

2. Texture*

Texture is often masked or influenced by conspicuous though rela-tively unimportant mineral materials. Occurrence of sharp angularmineral particles, most frequent in soils of granitic origin underdesert conditions in which processes of weathering are less rapid thanthose of disintegration, often renders it very diff icult to judge withaccuracy the texture of the predominating fine material of hand samples,either in a wet or a dry condition. The tendency under such cases isusually to judge the texture lighter than it really is. Upon the otherhand subsoil textures in the more mature desert and semi-arid soilswhich are compacted or slightly cemented are almost consistently judgedby the field man to be heavier than is revealed by mechanical analysis*These are f requent ly exceedingly tight and are bored or dug out withdiff icul ty and become plastic when wet. Such conditions are most pro-nounced in the more maturely weathered and oxidized soils of red or redbrown color with which it appears is probably associated a large pro-portion of colloidal materiels* In order to guard against misjudgmentof texture in the field, mechanical analysis of a large number of sam-ples is called for with which the soil descriptions are checked in theinspector's reading and revision of the report.

There is however a question as to whether the field man is notafter all right in field judgment against laboratory data as to physi-cal behavior of the soil under irrigation and cultivation, A simplemethod of determining the proportion of colloidal materials in soils,and revision of the basis of textural classification with which this

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may be correlated ig greatly to be desired,

High accumulation of lime also frequently increases plasticity andsimulates physical properties of a heavier textured material.

The field man is sometimes misled in desert areas by accumulationof gravels thickly strewn over the surface ,-and known as "desert pave-ment", (Exhibit specimens and photographs), below which the surface andsubsoils nay contain but an occasional pebble. Such accumulations haveresulted through a long period of selective removal, often involvingmany feet, of the finer soil material by surface run off and by winderosion, and is indicative of but little as to gravel content below thesurface one or two inches,

3. Structure*

Where pronounced horizons of compaction or cementation, or ofporous materiels occur they are vital in soil differentiation as con-cerned with irrigation, internal drainage, water holding capacity andunimpeded development of fruits, alfalfa and other deep rooted crops.Such factors determine the duty of water in irrigation, adaptation towestern agriculture, ;jsd the necessity of use of explosives in plantingfruit trees on hardpan lands which is frequently successfully resortedto* Upon the other hand development of pronounced surface crust andmulch horizons may occur in very recent soils of high alkali contentand is alone of little index to the age of the material or to stage ofweathering and subsoil conditions.

Field mapping in the western areas outside of the comparativelylimited humid northwestern coastal region, necessitates a study of bothupper and deeper subsoils in which the three foot soil auger is as muchout of place as an Esquimo in an orange grove. Field mapping in allthe arid and semi-arid areas is based upon a study of the 5 foot pro-file and of the deeper material where practicable,

Conditions of structure in the surface and subsoils involving com-paction -?.nd columnar structure usually associated vrith weathered soils,often results in recent alluvial deposits f rom the intense desiccationand baking effects of but a f^? months of desert heat -i-nd sun. (Exhibitsample and photographs of Holtville clay, Palo Verde Area, Cal.)*y*

4. Lime content*

While the soils of the arid and semi-arid regions are predominantlycalcareous, the older and raore mature soils nay be entirely leached oflime, others occurring under still more restricted rainfall or of lesspermeable character may have leached surface soils with subsoils havingone or more horizons of lime accumulation, and some are calcareousthroughout• This nay be evident as well defined horizons of light coloror as lumps, nodules, concretions, seams or mottlings. If of still more

pronounced development firmly cemented lime carbonate or caliche hardpanhorizons may occur. These may be of fragmental or intermittent occurrenceor as a mass of concrete like hardness extending to the depth of 10 or morefeet* ^Weathered soils of recent accumulation may or may not be calcar-eous. Wien calcareous the lime is typically of uniform distributionthroughout the soil section though conditions occur in which slight ac-cumulations of lime indicate the beginning of a stage of transitiontoward the modified profile of the older materials*

5. Topography*

Careful observation of topographic features isr-t all times of valuein soil differentiation in conjunction with color and profile character-istics. The older soils tend to develop certain characteristics of sur-face configuration such as hogwallows, correlated with compacted andcemented subsoils* Topography can not be relied upon as a basis ofdifferentiation between unmodified and weathered soils or between in-dividual series owing to the vagaries of soil accumulation through theagency of transient desert streams* Areas of friable, permeable recentstream deposits may occur high upon alluvial fans or valley slopes inassociation with or at an elevation above maturely weathered soils ofcompacted or cemented subsoil horizons, and of apparently similar to-pography. Conversely soils occupying low lying stream bottoms may haveaccumulated so slowly that rate of accumulation has been outstripped byprocesses of weathering giving rise to a nature profile. Differentia-tion between recent river flood plain and weathered terrace soils con-fidently based on topographic features such as occur in the eastern andmiddle states has little place in the west.

The classification and napping of arid and semi-arid soils basedon characteristics of surface and subsoils to the depth of 6 feet in-volves ticie and frequently much arduous labor owing to limitations inthe use of the soil auger* wuch of the time the soils are either ex-tremely hard, baked and difficult to penetrate, or are reduced to a con-dition of dry powdery dust which can not be withdrawn upon the auger*In such cases water carried for the purpose assists in softening thematerial and in its removal. Special soil augers adapted to boring infine textured dry soils of flour like consistency sometimes prove useful.

When gravels or rock fragments occur as they do abundantly in desertregions, or it is desired to carefully study profile development, recoursemust be had to pick and shovel. (Exhibit photographs)* One never knowshow deep a 6 foot hole is until one has dug one under a dazzling, broil-ing desert sun. This sometimes discourages the study of desert profilesbut examination of the soil material by this method is much- more pro-ductive of dependable data than by the usual soil borings*

Frequent recourse is had to the canteen in moistening at> samplesin judging texture, as well as for other obvious purposes, the materialis tested throughout for lime with dilute hydrochloric acid* The de-

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sirability of having at least one set of samples showing the completeprofile development for each soil series is urged upon the field parties*

Special equipment essential in effective study and mapping of soilsunder the conditions and difficulties indicated above is simple andeasily acquired* The essentials include first a car of reliable thoughit may be of popular make, - strong, tried and true. The most of thesehave been severely tried but not all are true. Accessories include a6 foot soil auger, a pick,-preferably of man size-, a long handledshovel, several gallons of water, extra tires and tubes, and plenty ofgasoline-, oil, and grease, the later mainly of the elbow variety.Dilute hydrochloric acid is carried in a small dropping bottle with lipand ground glass stopper. Care is taken to keep the stopper lightlygreased to prevent sticking and breakage* A less pretentious out per-haps more convenient arrangement for carrying acid in the field consistsof an empty pop bottle fitted with stopper in which a small glass tubeor quill i& inserted. This is conveniently carried in a case consistingof an empty tin can of convenient size attached by screw or nailthrough the bottom to the floor boards or running board.

If the survey entails the determination and mapping of alkaliadditional equipment in the form of an electrolytic bridge, mixing cups,spatulas, thermometers and a supply of distilled water is necessary.Under such conditions it is sometimes necessary to provide for thecarrying of as much as 8 or 10 gallons of water in desert areas, andin at least one such area the field thermometers which register to only100 degrees were packed in ice each morning to prevent breakage yrhennot in use.

Tire chains, nails, screws and a few feet of flexible wire shouldbe carried at all times. There are occasions when the long handledshovel is useful for purposes other than the study of soil profiles, forthe desert is a land of surprises and strange contrasts in which a fewraindrops or seepage from an irrigation ditch may render the slickclay surface of a desert playa or sink almost impassable, and in whichone can become bogged down, almost in a mirage.

Field equipment should at all tines be maintained in good repairand should be ample to meet emergencies* The efficiency with whichthis may be employed will depend on the industry, application, thorough-ness, and sometimes upon the ingenuity and resourcefulness of the manin the field.

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