Although it is the chief purpose of the author to outline and discuss the system of classification proposed by the late Dr. C. F. Marbut as adapted to American soils, it is well to briefly review the fundamentals and earlier systems as a background. Many systems which have been proposed are too technical to have much practical value, while others are too simple to have much scientific value. It seems, therefore, that if the following fundamentals are considered, both requirements may be met

Firkins, B. J.

English

Although it is the chief purpose of the author to outline and discuss the system of classification proposed by the late Dr. C. F. Marbut as adapted to American soils, it is well to briefly review the fundamentals and earlier systems as a background. Many systems which have been proposed are too technical to have much practical value, while others are too simple to have much scientific value. It seems, therefore, that if the following fundamentals are considered, both requirements may be met.</p

Firkins, B.

Digital Repository @ Iowa State University (ISU)

A  Modern Systems of SoilCiassificationBy B. I. FIRKINSAssociate Professor of Soils, Iowa State CollegeLTHOUGH it is the chief purpose of the author to outlineand  discuss  the  system  o£  classification  proposed  by  thelate Dr. C. F. Marbut as adapted to American soils, it is well tobriefly review the fundamentals and earlier systems as a back-ground.   Many  systems  which  have  been  proposed  are  tootechnical  to  have  much  practical  value,  while  others  are  toosimple to have much scientific value.  It seems, therefore, thatif  the  following  fundamentals  are  considered,  both  require-ments may be met.(1)  The system should have a reasonably permanent basis,not likely to  require  sweeping modifications  in  a  short  spaceof time.(2)  The system should be  based on fundamental  soil char-acteristics.(3)  The  system  should  express  the  genetic  relationship  ofdifferent soils to  each  other.(4)  The  system  should  not  neglect  the  environmental  fac-tors  which  are  fundamental  in  determining  the  soil  charac-teristics.(5)  The  system should  be  as  simple  as possible.   However,accuracy should not be sacrificed.T::#act.Tr;arrervoe;13S7thB:Cf.;:tatnhdatCCo::oieilnlath(e54Se£?cnS cheanjdivided  soils  into  general  groups  according  to  their  capacityto  produce  certain  plants  and  on  the  basis  of  physiographicfeatures and textural  differences.   In the  18th  and  19th  Gen-turies, German and Russian scientists in particular gave muchthought to the matter of classifying European soils.  Many di£-ferent  bases  were  established  for  grouping  the  soils  such  asvalue, character of the parent material,  chemical composition,and genesis, with emphasis on the influence o£ climate in the soilforming processes,  particularly  moisture  and  temperature  ef-[99]THE    1936    AMES    FORESTER(sedJCL)®SuOZIJOtleOeJ|nSatI}JOeJnlXelatPuOPeSBqSltunJOIequrnuaSJeTIJ{JOSe}eD(soTJeS)Tell-e}t3urtuaJt3,deTPJOeJn}t3eJIeOIuleqOPuele3ISJCtldetPtIOPeSeqSdnOJDIll(IO8elt3Op@do-IaAeP-JaAO(0)p©doleAePJ{IeleldurO3uI(q)Dado-pAeP[IeM(t3)e[H-oJdJOJ{1I-1-euIJ:OSISequOAIJCJOSett3OJOSuOtSIAIPqnSIIIJ{JOSelt3D1JeSapJCt3JDStPJeuMOJalnulS®tPSureZOuletIOAIJ{JO861t?0I     >>IOcoa)+CtiaSLd     rg     i:5Lpg:E®Oa+c6EI±=c; +L3 =coOOGAHtAi5:i:!oI+P|coO| I ±!m±I!Li-E:Jf:->>Iailjg      :a-!CJ C.)co®+CBO =. ,caLei-£ ®r|"E Etaa)a+cbCQEIII                                          EII1 P|  . rJ-I>>t^ u|Ia)CHill rII1a COcoa+ rlIIla CBng rOCe a aa all allwalSR,suovyroorfyssvlOT}OSonaNaap+O?uaWabW",JVO?xpWauC'S100THE    1936    AMES    FORESTER                          101feet.   In 1892, Dr.  Hilgard of Cali£omia suggested  the  system,adopted by the u. s. Bureau of Soils, in which he consideredboth the nature of the parent material and climate.He  first recognized  the  difference  between  the  influence  ofweathering  agencies  under  humid  and  arid  conditions  andwithin each region included:  residual, cumulose,  glacial, aelo-lan, sedimentary, and colluvial groups.   The geological groupsare recognized by the U. S. Bureau o£ Soils systems and classi-fication.In  the  early  20th  Century,  E.  Rahmann,  Glinka,  Lang  &Meyer, Vilensky,  aid Neustreuv developed  systems in  whichthe  climatic factors were particularly  stressed.   The  chief  ob-jection to such systems was that,  although  they  indicated  theinfluence of the climatic factors on the soil characteristics, theydid not distinguish between soil groups.  In 1929,  Gedrioz sug-gested a system based on the absorbing complex, grouping thesoils  into  two  closses:      (1)   Base  saturated   (chemosem  andalkaline  soils),  and   (2)   base  saturated   (podsolic  and  later-itic soils) .  Again, in 1930, Stebutt proposed a scheme based oncharacteristics of pedogenic processes recognizing undevelopedand  developed  soil formations.   These  are  not  sufficiently  de-failed to be regarded as true classifications.DR. MARBUT, in explaining his scheme, maintained that aclassification  of  bodies  consists  o£  their  ar'rangement  insome kind of order and is not concerned with their creation;  itassumes the fact of creation.  The creative function must havebeen  performed  before  classification  can  take  place.   One  iscreative and constructive, the other mechanical.   Recent writ-ings  reveal  the  fact  that  Marbut's  scheme  is  generally  usedthroughout  English  speaking  countries  and  the  Russian  andother systems are being closely correlated with it.The accompanying map shows the distribution of the majorsoil groups in the United States which fall under the  severalcategories  in  Marbut's  system.   A  schematic  arrangement  ofthe seven categories recognized in his complete system appearson the opposite page.PEDALFERIC  GROUPSThe  Pedalfers  are  non-lime  accumulating  soils.   Throughthe processes of physical and chemical weathering,  there hasbeen an accumulation  o£ aluminum  (A1)  and  iron  (Fe),  andit  will  readily  be  observed  that  this  combination  of  symbolshas given rise to the name.   The  soil formations  in  this  great102                          THE    1936    AMES    FORESTERgroup  occur  west  of  a  line  from  northwestern  Minnesota  toCorpus Christi, Texas.There are five major subgroups in this pedalferic  group,  asfollows:(a)   Podzols(b)   Gray-brown forest(c)  Red and yellow earths(d)  Ferruginous laterites(e)  PrairyerthsThe first four form a series o£ east and west belts from north tosouth in the eastern part o£ the major pedalferic zone, while thePrcLirLe  cover.prairyerths comprise a north-south belt adjacent to the  west-ern  boundary.   The  boundaries  are  based  on  mature  profiledevelopment for each subgroup.PodzoZs.-   The  major  areas  of  the  podzolic  soils  in  NorthAmerica  are  to  be  found  in  southem  Canada,  but  severalsmaller areas are indicated in the humid northern part of theUnited States.  They are characterized by:(1)   A sur£a,ce layer  (1''-6;')  o£ raw humis or vegetable matter.(2)  An acid gray to nearly white silicious; highly leached up-per mineral horizon.THE    1936    AMES    FORESTER                          103(3)  A darker brown or coffee brown sub-soil with a relativelyhigher  organic   content  than  the   characteristic   leachedlayer.The surface horizons have a platy or single grained structurewhile that of the sub or "B'' horizon is angular.GrcLgr-brOco7L Jot-eSt.a   These  soils,  where  undisturbed,  usuallyhave an upper leaf litter layer  (1''-3'')  arid a layer of leaf mould(1''-2'')  overlying the  true  soil.   The  two  chief  characteristicsof this subgroup are as follows:(1)   They  are  non-lime  accumulating  and  have  "B"  horizonsrelatively high in iron and aluminum.(2)   They are forest soils, with the natural vegetation consist-ing of associations of deciduous trees such as maple, birch,beech, oak, and yellow poplar.In  the  eastern  belt  they  have  been  formed  under  30''-45''  ofrainfall and with the exception of the prairyerths, are less acidthan any of other of the pedalferic group.Red cmd e/eZloco eclrths.a  The yellow soils are developed in thesouth  Atlantic  and  gulf  coastal  plains  and  the  Appalachianplateau.   The  red soils  occur  chiefly in  the  Piedmont  and  theTennessee  section  of the  valley  and  ridge  province.   The  twotogether are recognizd as transition soils in the Mississippi up-land  loess  region  and  the  more  rolling  areas  o£  the  coastalplains.   Compared to  the  gray-brown forest  soils,  the  red  andyellow soils are more thoroughly bleached, their minerals aremore  completely  decomposed,  and  their  structural  character-istics are inferior.  Consequently, they are rated as poorer soilsfrom  a  fertility  standpoint.   The  names  are  derived  from  thefact  that  the  "B''  horizons  of  rederths  are  red  and  for  theyellowerths are yellow.Fe7'7'ttg¢7LOtLS  ZcLter6tes.I    These  red  soils  are  not  extensivelydeveloped in the United States, the principal areas being foundin Gorgia and Florida.  They are characterized by their contento£ indunated bodies of impure iron oxide in the form of pebbles,seams, ledges and surface coatings or crusts about the granulesor aggregates.  As a rule such soils are moderately to stronglyacid and are thoroughly leached.  In spite o£ these characteris-tics, however, they are classed as strongly desirable soils underproper management.PprcLirt/erthS.-   The  chief relationship  of this  sub-group  to  thepeda1£ers lies in the fact that they are non-lime accumulating.]|ly]J:m9nC,IV]9ndd3lSllO¬mOm3^aWO3tlS7lQSJ_S]gojNMOyg~Ny9S7,Of7OSOOJ`-\``\?sl`i------:+----®®®®®I®|--..®®®I|®J       I-_  -        _  -_-  -_  -I               _          -®|.-......I®.rT-  I --i- - - r--.7=...I.-+ ---.-.-..............-.....IIST'oS]ldlVddSJIOgN3SOrvy3HJS7nSJ.rIN|SJHJS7lOSWMOg8S7JOS`/lytyOO]J:HIJ_N3d3J]lCINr,g:-:-==:-`:-I\`                              \I..I_...o.....%¬.*t=;.--.....ITHE    1936    AMES    FORESTER                           105Unlike  the  other  members  they  have  been  developed  undergrass cover and have acquired dark-colored "A" horizons, richin  organic matter.   The  "B"  horizons  are  brounish  to  yellowbut  are  not  much  heavier  textured,  are  not  so  extensivelyleached, and are fairly high in soluble salts.  They have a pro-Forest  soil  freshly  turned  in  a,  firelane.nounced granular structure and although low in lime are notexcessively acid in reaction.The black, waxy, Houston-clay soils o£ Texas are similar insome respects to the normal prairyerths, but would be classedas immature soils and designated as  "Rendzinas."   They havea higher natural lime content than the parent material.PEDOCALIC  GROUPSThe pedocals derive their name from the fact that they arelime  (calcium)  accumulating,  As shown on the map there arefour major  subgroups  of  soils  extending  in  north-south  beltsand two  areas o£ undifferentiated  soils  bordering  the  area  o£gray desert soils.   The subgroups are:106                          THE    1936    AMES    FORESTER(1)  Blackerths-chernozems(2)  Chestnuterths-arid brown soils(3)  Brownerths-arid brown soils(4)  Grayerths-gray desertBZcLCkert7LS-C7Ler7LO2;emS.a   This  group  has  many  features  incommon  with  prairyerths,  but  may  be  differentiated  by  thefollowing characteristics:(a)   Blacker color(b)  More perfect granulation(c)   Higher content o£ mineral matter(d)   Neutral to alkaline reaction.The prairyerths are,  as  a rule,  acid  in reaction,  as  the  resultof  more  extensive  leaching  action.     The   chernozems  wereformed  under  insufficient  moisture  and  the  carbonates  haveaccumulated.  In some cases, they have accumulated in exces-sive amounts and have given rise to alkali or saline soils.Chest7t%te7'thS.I  These soils formed in the central part o£ thebelt vary from dark brown in the north to dark reddish brownin the south and the  characteristic  subsoils  are  brol^m to  red-dish brown.  They are lighter than the chemozems, due to thewestward decrease inprecipitation and density of vegetable cov-ering.   The  zone  o£  lime  accumulation  is  shallower  and  theupper horizons have not  been  leached  as much.   Their  struc-ture is not as granular as the chermozems, being more columnaror cloddy,  but the  aggregates break  down  easily  under  culti-vation.Broco7LerthS.'   These  are  the  highest  colored  of  all  the  greatplains soils and the carbonate zone is the shallowest.  The lightbrown  colors  have  been  developed  under  g-rass  cover,  but  isless dense than in the other belts.  The surface soils are struc-tureless  or  loosly  deflocculated,  and  the  "B"  horizons  showcolumnar breakage.  They are wholly unleached with the lowprecipitation, and only a small part, if any, o£ the soluble saltshave been removed.G7'CLt/ert7LS.a   These soils vary in  color from brounish-gray inthe  north  to  reddish  tinted  soils  in  the  southern  part  of  theregion.   The  typical  surface  horizons  forms  a  thin  "desert-crust" or "desert pavement" with a substructure of loose, por-ous material called "desert mulch."  This zone is underlain bya  zone  of  lime  accumulation  which  may  often  form  a  firmlycemented hardpan.  The organic content is very low and therehas been little or no leaching of mineral matter.THE    1936    AMES    FORESTER                           107The broken lines extending` from east to west as indicated onthe map, mark off the north-temperate, mid-latitude, and south-temperate  zones.   The  soils  are  somewhat  darker' colored  inthe northern zones and become increasingly lighter colored asone  goes  south.   Temperature  is  a factor in  addition  to  vege-tation in influencing the soil characteristics.  Later studies mayresult in furthel- separations in the regions of undifferentiatedsoils.

Modern Systems of Soil Classification

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Modern Systems of Soil Classification

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