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5SOME ASPE'CTS OF SOIL CLASSIFICATIONBY P. H. GALLAGHER, D.Sc.The classification of soils must necessarily be of very considerableinterest to the forester if only for the reason that he is so often expectedto make some practical use, not only of an assortment of crumbs, butalso of the more or less indigestible bones that may, so to speak, fallfrom the agriculturist's tableNumerous as are the soil problems of ordinary farming, it is incases where the farmer is fully convinced that these problems are economicallyinsurmountable that you are apt most frequently to find landon ofler to the forester. The latter, we will admit, has at his disposala crop capable of giving an economic n '!turn on many classes of soilwhich may reasonably be considered unsuited for normal farming use.It is the character of the soil which ultimately decides whether a particulardistrict be economically plantable, just as it is the character ofthe soil which chiefly decides the tree species which will likely give themost profitable return.Vegetation as an Indicator of Soil Conditions.Many foresters rely to a considerable extent on evidencederived from existing vegetation when assessing the value of a plantationsite. There is unquestionably a relationship between the vegetationestablished on a given site and the locai soil conditions. Experience andcaution are" of course, necessary if the inferen.ces to be derived fromone type of vegetation are to serve as a universal guide in forecastingthe growth of another. Consideration of the whole plant community orassociation serves as a more reliable guide to soil conditions than doesthe presence or absence of any single plant. Some plants require fairlyspecific s0il conditions in order to establish themselves. but the absenceof a plant is no proof that the soil is unsuited to it. The general plantassociati'Jn, on the other hand, will sometimes be found to vary withchange of environment, even where soil conditions may not be materiallydifferent. Elevations and wind exposure have a notable influencein this respect. We have examples of plant associations in the Surveyof the Vegetation of South Co. DublillJ carried out by Petherbridge andPraeger some year,'; ago. Fraser. of the Macaulay Soil Research Institute,has in recent years given considerable attention to the planta ssociations as a means of assistance in forestry development in Scotland.Relationship of Soils and Geology.In the course of the many attempts which have been made to formulatea comprehensive system of soil classification in the past, geologyhas figured very prominently. So much was this the case that at onetime the scientific study of the soil was frequently regarded as beinge xclusively within the province of the geologist and the mineralogist.yrhen the Irish Geological Survey was established over a century ago,It was set the task of accomplishing a soil survey in conjunction withthe solid 'Or rock geology. The project of soil classification was abandoned,however, at quite an early stage. the reason stated being that itwas considered preferable to devote the full rpsources of the organisationto the more fundamental solid geology.L.o0king back at the position which existed a century ago, one mayque~tlOn whether a useful survey .of Irish soils was at that time reallyfeasible, and whether a realisation of tl',e true position after some littleexperience may not in fact have determined its abandonment. For somereasons which I shall refer to at a latpr stage, no direct relationship canbe said to exist generally in this country between the soil and the solidgeology. The theory of glaciation, which goes far in explaining the displa~ement:>of our soils from their parent rocks. was not formulateduntil conSiderably l~ter than the period at which the Irish GeologicalSurvey commenced Its work. Moreover. there did not exist, a centuryago, any proper realisation of the climatic or weathering processes whichhave a. fundamental influence on the character of a soil.The Climatic Soil.forming Process.The study of the weathering processes which impart to the soil mostof its individuality was first developed in Russia towards the dose ofthe nineteenth century. Makbg due allowance for the ability and initia-
6tive of the Russian investigators concerned, it will be seen that a countryso vast and possessing such diversity of climate, offered a umque opportunity'for the study of a problem of this kind. Throughout most ofCentral and Western Europe the climatic soil-forming pr()cess IS substantiallyso uniform that it was perhaps, pardonable that persons studyingthe soils in these regions tended to forget the eXIstence of any ,specIficweattering process at all. In Central and Western Europe, attenhon wasin great measure confined to factors connected WIth local geology. TheRussian investigations to which I have referred, shewed that where youhad sufficient differences in climate, the effects of climate superimposedthemselves on local!:(eology in different ways. A particular rock situatedin one climatic region would, for instance, give rise to the relatively infertilesoil-type known as the "podsol," in another climatic region thesame rock would yield the very fertile " black earth" or chernozem.The same geological formatiun in the tropics would give rise to a lateriticsoil,. which is totally cifferent from the other two. These interestingand important facts have given rise to a somewhat deplorable consequence.They have led nowadays to a rather common beli-ef that thegeological parentage of a soil is of little or no practical significance. Fromyour own practical experience of Irish soils, I doubt very much if youwill be disposed to' agree' with this view. At the same time it is truethat each species cf climatic SOlI-type occurring b this country is to befound on quite a variety of geological materials. The essential point isthat the climatic factor can be superimposed on a geological materialwithout entirely obliterating the individual qualities of that material.There are, moreover, qualities inherent in our several geological formationswhich have determined the extent to which each has reacted inthe past to climatic influence.Clim'atic Factors Influencing Soil Formation.To gain an understanding of the essential characters of Irish soi1-types it is necessary first to consider the climatic factors which haveaffected their formation. Soils, of cour,e, originate from the crumblingdown of .the rocks which form the solid core of the country. Mere disintegratedrock, however, does not in the true sense constitute a soil. Itis merely the raw material from which, through the combined operationsof climate and vegetation the soili6 fashioned. The features of climatewhich are of importance from the point of view of soil development aretemperature and humidity. These Olre interdependent, since for a givenTainfall, the higher the temperature, the greater wIll be the surfaceevaporation;, and consequently the le::'ser will be the effects of rainfall asa leaching agent in the soil. The higher the temperature, the morerapidly and completely also will the residues of vegetation tend todecompose. Where you have excess of rainfall over evaporation, andwhere also the temperature is such that growth is good, but the decompositionof vegetation residues is comparatively slow-in such climaticcircumstances soil formation becomes dominated by a leaching processof acid character, resulting: ultimately in the soil-type known as thepodsol. The climaFc circumstance ~ nt>cessal'Y for podsolisation arefound in what the geographers term the moist temperate zones, whichis precisely the type of climate that we occupy.Sources of Soil Acidity,I have referred to the process which gives rise to the pod sol as aleaching process of acid character. Although the geologists speak ofcertam rocks, "uch as granite, as being acidic, very rarely does one find afreshly disintegrated rock to be actually acid. Even quartz, thoughchemIcally acidic, is not an active acidifier. The primarY source of acidification in the soil lies in t.he residues of vegetation. This is notnecessarily the so-called "humic acid ,., that one, frequently hearsabout. 'Plant tissues themselves are acidic. They cont~in various comparativelysimple. soluble, well-defined organic acids, such as oxalic andtartaric. Plant r~sidues reaching the soil not only contain such acids,b~t .they further produce them on decomposition. Simple organic acidsof tms kmd, however, are somewhat rc
Page 1 and 2: IRISIHIIFORESTRYPublished byTHE SOC
Page 3 and 4: SOCIETY OF IRISti FORESTERSPRESIDEN
Page 5: 4journal, to remain popular, must .
Page 9 and 10: 8Brown Eartha.1 wish now to refer t
Page 11 and 12: 10wherever it may occur, soils of p
Page 13 and 14: 12NISBET'S OBSERVATIONS ON IRISH FO
Page 15 and 16: 14of Silver Fir and any species of
Page 17 and 18: Samples of Costs and Revenues.16Var
Page 19 and 20: 18Whalley Abbey, both in Co. Wicklo
Page 21 and 22: 20heather shoots are stimulated and
Page 23 and 24: 22~ee a patchwork of grey stoney gr
Page 25 and 26: 24in these cups that the seeds deve
Page 27 and 28: 265. They reprrJduce themselves fre
Page 29 and 30: 28able are those which are cut from
Page 31 and 32: 30of gr
Page 33 and 34: •32takes the seed with. it and co
Page 35 and 36: 34~ which have been used agricultur
Page 37 and 38: 36The Planting Material.Planting of
Page 39 and 40: !38financial assistance were define
Page 41 and 42: ~oposal to increase the forest area
Page 43 and 44: 42Annual General Meeting, 1945The t
Page 45 and 46: 44survive. They do not appreciate t
Page 47 and 48: 46He stressed the desirability of m
Page 49 and 50: 48point of view. I feel that the fi
Page 51 and 52: 50'" Diver, Neil, Rathdrum, Co. Wic

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