Forest Soils

FOREST-LAND USE AND THE SOIL PROFILE1

LEWIS M. TURNER2

CiND use is necessarily a broad subject and, fur-ther, it is one which rightly should not be reduced

to any of its parts, such as forest-land use. Its verynature is inclusive; by common usage the expressioncarries the meaning of best land use whether the usefor any acre, area, or region be forest, pasture orrange, grain, fruit production, or any other use.Hence, although the topic of this paper is the relationof the soil profile to forest use, the implications arenecessarily more inclusive.

The recent increased interest in land-use planning,flood-control surveys, etc., has highlighted the needfor indicators of land potentialities. In organizing aprogram of land use for a field, a farm, a county, orlarger area, it is desirable to know more than whatthe land has been used for and what has been its rateof production of foods, clothing, and building mate-rials or other commodities. Possibly past use hasbeen misuse or, at least, not the most favorable use.It is apparent that true productive potentials of allpossible materials'afford the most reliable key to thesolution of best land use.

In a comprehensive land-use adjustment programin the South, knowledge of the role of forest produc-tion is of paramount importance. First, it is essentialto know whether any land — and in this sense landis synonymous with soil—is adapted to forest growth.Secondly, it is necessary to know what type of forestis adapted to the land and whether the type is usefulfor only watershed-protection purposes, commercialpurposes, or both. Third, if the forest is partly orwholly of a commercial type, it is essential to know itsrate of growth in terms of increment per acre peryear. Needless to say, similar information regardingthe adaptability for livestock, grains, fruits, etc.,should be known. With these data it is relativelysimple arithmetic to compare for any land the pro-ductive capacity of the various materials it will pro-duce. Assuming certain prices for, and costs of pro-duction of, the various products — and such assump-tions are usually necessary in comparisons of thistype—the comparative net returns can be established.

Investigators of forests in the South, as regardsthe above-named relationship, are fortunate in thatconsiderable natural forest remains. A relatively smallamount of forest is in virgin condition, but in manystands enough of the old growth remains, or thereproduction of the original or important species isadequate, to indicate the natural relationship offorest-species occurrence to general soil type. Ratherlarge areas are in public ownership or in the owner-ship of persons or corporations who do not objectto having studies conducted on their land. Hence,there is still an opportunity for the prosecution ofinvestigations that will amplify our present knowl-edge of the subject under discussion. A few examplesof studies of this nature will be cited as evidence thatdefinite relationships, as described, exist.

Several years past, in the progress of the ForestSurvey of the South, an attempt was made to cor-relate forest types with soil types in the loessal regionof northern Mississippi. At that time no relationshipwas observed; the same general forest type, uplandhardwoods, was found on Memphis, Loring, Grenada,and Lexington soils. It is now recognized to be sig-nificant, however, that this particular forest is pecu-liar to this soil group in that region. More recentstudies, in connection with the Little TallahatchieRiver flood-control survey, reveal that subforesttypes, dominated by specific groups of species of thegeneral upland-hardwood type, are more or lesspeculiar to each of the loessal soil types. In the sameregion the shortleaf pine-hardwoods type is confinedto the group of related soils, as Ruston, Susquehanna,and Orangeburg. As in the afore-described situations,there is some observed correlation between the occur-rence of forest subtypes and species soil types.

Similar relationships were found to exist in theTrinity River watershed in Texas. The East CrossTimbers region, dominated by soils of the Kirvin,Norfolk, Ruston, and Tabor series, is occupied by apost oak-blackjack oak-cedar-elm association. TheWest Cross Timbers region, dominated by soils ofthe Nimrod, Stephenville, Windthorst, and Renfrewseries, related to the above-named soils, is dominated

'Contribution from the 'Southern Forest Experiment Station, New Orleans, La."Forester.

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344 SOIL SCIENCE SOCIETY PROCEEDINGS 1939

by a post oak-blackjack oak type, with live oak oftenimportant on limestone — soil inclusions. Stony slop-ing phases of calcareous soils are occupied by redcedar. The effect of climatic zonation is evident, ofcourse, but soil is doubtless the dominant factor indelineating vegetative types and to considerable extentforest types.

In Arkansas, the author3 found evident relation-ships to exist between the occurrence of broad foresttypes and soil groups and in several instances betweenspecific forest and soil types, or phases of soil types.Even more definite relationships exist between slope-soil complexes and forest types.

As regards the relationship of rate of growth tosoil characteristics, relatively little has been done inthe South. Several investigations of this subject havebeen made in the Northern and Northeastern States.Most of these, as is doubtless well known, have indi-cated that rate of growth cannot be related to singlefeatures of the soil, but definite relationships do existbetween growth rate and certain groups of soil-profile characters. As in the instance of forest-typeoccurrence, even greater correlation exists betweenslope-soil type or soil-type phase complexes and rateof growth of trees. Study of the rate of growth of pinein Arkansas4 and North Carolina5 indicates the exist-ence of such relationships. Coile found that the siteindex of pine is related to the silt and clay content,or the colloidal content of the BI horizon, and thedepth at which this layer is encountered. The authorfound somewhat similar conditions to obtain in Ar-kansas as regards the rate of growth of shortleaf andloblolly pine. Apparently, except in the instance ofthe steeper-sloping soil phases, profile features mak-ing for loose structural A and B horizons were thosemore conducive to higher growth rates. For instance,the higher the clay content of the BI and B2 horizons,the lower the growth rate — a fact more evident asregards the 62 horizon. There was a considerablyhigher degree of correlation between the rate ofgrowth and depth of the BI horizon. In both theNorth Carolina and Arkansas studies, the most evi-dent relationships were between combinations of soilprofile and topographic factors and rate of growth.

In some land-use planning, attempts have beenmade with varying success to use soil-productivityinformation. Much land planning, however, is broad,generalized planning. This is useful in pointing upregional problems and in showing broad areal rela-

tionships, but it is relatively ineffective in solving theland-use problems of more restricted areas. Usually,generalized soil maps are used, and many of theseare old. Some county land-planning studies whichhave made intelligent use of good county soil mapsmore nearly approach the ideal of satisfactory plan-ning. Oftentimes, however, little or nothing is knownof the relative production potentials of the soils, ashave been previously described.

Current land-use adjustment studies, of which theYazoo River backwater investigation is an example,offer an excellent opportunity to employ the principlesoutlined. The Lower Yazoo basin is subject to bothheadwater and Mississippi backwater flooding. Head-water floods can probably be controlled at a nominalcost, but backwater floods are another problem.Although the region doubtless includes considerablehigh-grade agricultural land, it is questionable if itcan be protected from Mississippi flood water at a costcommensurate to benefits. The economic and socio-logic implications are many and significant, but thebasic determining factor is doubtless the productivityof the soil. The productive capacity of its soils willlargely determine whether it is or is not economicallyfeasible to protect the area from floods for farm-cropproduction, or devote it to forest production andrecreational use. Obviously, the proper solution ofthe problem cannot be attained with anything shortof a comprehensive, comparative soil-productivitysurvey.

Future forest surveys, to serve their most usefulpurpose, should encompass at least a suitable recon-naissance soil study. It is important to know thevolume, growth, and drain of our timber, but it isalso important to know how these fit into the pictureof over-all land use. Possibly forest is growing whereother crops should be grown, or vice versa. The soilprofile is an important clue to the solution of thisunknown.

What is needed is broader investigations of thetype that will relate soil-profile characteristics toproduction of various materials. State or regionalstudies should be conducted. As is well known, suchlarge-area studies lend themselves to sketchiness andinconclusiveness, and proper precautions should betaken to prevent slipshod methodology. As regardsforest growth-soil studies, earlier technique should bebroadened to include better analysis of all the moresignificant features of the soil profile. In studies cor-

3TuRNER, LEWIS M. Some soil characters influencing the distribution of forest types and rate of growth of trees in ArkansasJour. Forestry, 35:5-11. 1937.

4TuRNER, LEWIS M. Growth of second-growth pine on the coastal plain soils of Arkansas. Ark. Agr. Exp. Sta. Bul. 342. 1937.6CoiLE, T. S. Relation of site index of shortleaf pine to certain soil factors. Jour. Forestry, 33:726-730. 1935.

ABSTRACTS 345

relating occurrence of forest types with soil, in addi-tion to more intensive study of the soil, better analysisshould be made of the species content of types. Doubt-less, improved statistical analysis would reveal rela-tionships between species-concentration and soilphases hitherto unrecognized.

Much remains to be learned of the relationship ofsoil-profile characters to production-potentials, asregards production of forest and other products.

Since such relationships are the principal physicalbasis for determining best land use, it is difficult toconceive of a successful land-use readjustment pro-gram which does not have such a foundation. This isto say, land-use adjustment programs should giveprimary consideration to comparative production-potentials, but what is better, such information shouldbe secured in advance of such programs by soundinvestigation.