SOIL COLOR—ITS DETERMINATION, NOMENCLATURE, ANDSIGNIFICANCE AS A FIELD MAN'S PROBLEM1

Thomas D. Rice2

It is not necessary, at this time,to dwell at any length on the origin of col-or in soils or on the significance of coloras an indicator of soil properties. Thesesubjects have'been discussed in several text-books and official publications. The impor-tance of color as a characteristic to betaken into consideration in the identifica-tion and classification of soils is general-ly recognized. Many of our soil series havebeen established on the basis of colorswhich indicate important physical and chemi-cal properties and no one questions the val-ue of color as a criterion in soil classifi-cation.

All colors observed in soils, how-ever, are not equally useful in classifica-tion. The first problem of the soil surveyoris to determine the kind of color in anygiven soil. A soil color may be either in-herited from the parent material, or producedthrough the soil-forming processes. In somesoils an inherited color has significance; inothers it may have none. The red color ofsoils derived from the Permian Red Beds isassumed to have significance and is the dis-tinguishing characteristic of a number of se-ries. Red, black, or brown quartz sands maygreatly influence the composite color of asoil but the coloring matter of the quartzwill have no appreciable effect on the prop-erties of the soil. We know that the blackcolor of a buried soil may be due to inertcarbon which probably has no beneficial ef-fect on vegetation.

Even where coloration has been pro-duced by the soil-forming processes, caremust be taken to determine its character.Many different reactions taking place in thesoil result in the formation of colors andwidely different processes may produce simi-lar colors. A gray color may be due to pod-solization, solodization, or to conditionsof extreme acidity, strong alkalinity, or im-perfect drainage. It is necessary that soilcolor should be interpreted in the field by

the help of other features of the soil profileand by climate, vegetation, and relief.

When the soil surveyor has ascertainedthe nature of his soil colors, his secondproblem is to determine how far he can usethese colors in classification. A strikinggenetical color cannot be ignored in classi-fication and usually presents little diffi-culty. Trouble begins with variations andgradations from one color to another. Hocolor holds uniform over any large area.Changes in color are often gradual and thisis especially true with the dark color due toorganic matter, which corresponds to changesin climate and vegetation.

The charge is sometimes made that weare basing classifications on ridiculouslyfine differences in color. Perhaps in somecases, differentiations on fine color grada-tions have been carried too far, but otheruses of fine distinctions of color in classi-fication are justified. In some soils aslight change in color, not in itself impor-tant, is correlated with changes in other im-portant properties. For example, in soils ofthe semi-arid regions, a lighter color of thesurface soil may be linked with a thinner Ahorizon, and, in irrigated lands, a slight in-crease of gray, which in some soils v.ould notbe significant, may indicate an accumulationof salts.

We must also use fine color distinc-tions where they are the most consistent fea-ture by which we can draw boundaries betweentwo important soils. For example, along thenorthern boundary of the United States, an ex-ceedingly gradual change takes place betweenthe Chernozem soils of Eastern North Dakotaand the Brown soils of Western Montana. Agradation, imperceptible within many miles,takes place through a distance of more than700 miles and through three great soil zones.In such regions of gradual change, comparisonsof slightly different colors give us definitescientific evidence that one zone has beenpassed and another has been entered.

1 Contribution from the Soil Survey Division, Bureau of Plant Industry, D. S. Dept. of Agriculture,Washington, D. C.

2Senior Soil Scientist.235

236 SOIL SCIENCE SOCIETY PROCEEDINGS 1938

After the soil surveyor has classi-fied'his soils and given due weight to colorin the classification, it is then necessaryfor him to describe his soil colors. Thenaming of colors has given trouble sincesoils were first investigated in the fieldand we still have no generally accepted no-menclature.

It might be well to give a brief ac-count of the attempts that have been made inthis country to standardize soil colors andthe difficulties involved. In the earlyyears of the American Soil Survey Associationthe necessity for such a standardization wasemphasized by Professor J. G. Hutton and theAssociation appointed a Committee, consistingof Professor Hutton, Mr. A. M. O'Neal, andthe writer to consider the problem. Mr.T. M. Bushnell was afterward placed on theCommittee. Dr. Milton Yi/hitney, then Chiefof the Bureau of Soils invited the Committeeto come to Washington and placed all the fa-cilities of the Bureau at our disposal. TheCommittee had two main objectives; (1) to se-lect adequate and convenient color standardsfor use in the field, and (g) to find, ifpossible, an absolute and scientific measure-ment of soil colors.

Out of the thousands of soil samplesin the collection of the Bureau of Soils wefound that thirty-two colors could be made tocover the color range of the soils of thiscountry. The names applied to these colorswere agreed upon by the members of the Com-mittee after consulting a number of soil sci-entists and laymen. At that time it was notpossible to get the colors printed so wewere obliged to use the soil material. Werobbed the sample Jars of the Bureau ofenough material to fil] sets of small bot-tles of soils possessing these representativecolors. Forty or more of these sets were •distributed to soil surveyors and others in-terested. Neither the Association nor theBureau had the means to send men out to col-lect large amounts of the necessary soils formaking up these sets and the preparation ofthe standards was discontinued.

The Soil Survey Division of Plant In-dustry is working on the problem of having aset of colors printed that will meet the re-quirements of the soil surveyor.

Soon after the Committee started itswork we were informed that the Bureau ofStandards was investigating and correctingcolor standards devised by the Munsell Foun-dation. The Munsell system seemed to be asatisfactory system for our need. It hadlong been known that all soil colors werecomposed of black, white, red, and yellow.

By spinning Munsell disks of these four colorsand changing the exposed segments of each asnecessary, we could produce a composite colorto match any soil color; By reading the per-centages of the segments of the disks exposedwe could represent the soil color by a formulaaccording to the Munsell system. This com-posite color can be reproduced according tothe formula tomorrow, next year, or a thou-sand years from now, so, it may be said thatwe have an accurate and scientific measure-ment of soil color. Since the Committee be-gan its. investigations, work has been done byShaw, Winters, Hendrickson, O'Neal, Gemmel,and others on the measurement and. reproductionof soil color.

The Munsell system does not meet theneeds of soil surveyors with respect to no-menclature as the formulas of that systemmean nothing to the ordinary reader. As amatter of fact, there is no natural and cor-rect system of naming soil colors. Soil col-ors are measurable and can be given specificnames but any set of color terms applied tosoils must be artibtrary and agreed upon bysoil scientists.

The greatest difficulty in namingsoil colors, on any scientific basis, arisesfrom the discrepancy between the names givenon color charts and those in common use forsoils. For example the soils which appear tous as red, and are generally called red, whensubjected to color analyses or to comparisonwith standard color charts are within thedefinitions for brown. Colors in soils whichappear bluish or greenish, lack blue and.green and may be reproduced on the colorwheel by using disks of black, yellow, whiteand red.

All soil colors consist of the hues,red and yellow of varying strength or chroma,modified by varying proportions of differentshades of gray. Neither the English languagenor any other language is rich enough in col-or terms to. apply accurate and specific namesto the many gradations of soil color that arenow in use. All the principal modern lan-guages, however, possess about the same numberof color names and can supply practically thesame nomenclature. An international agree-ment on soil color designation can be reachedby applying to soil colors names that meetthe needs of the' science.

Professor C. F. Shaw has asked thisSociety to co-operate in an effort to get aninternational agreement on color terms whenCommission I of the International Society ofSoil Science meets in 1939. Such an agree-ment is highly desirable and this Societywill, no doubt, support the movement.