FEBRUAR.Y 1935
MEMOIR 170
CORNELL UNIVERSITY
AGRICULTURAL EXPERIMENT STATION
ECOLOGICAL PROBLEMS
OF THE HUMUS LAYER IN THE FOREST
L. G. ROMELL
ItHACA, NEW YORK
PUBLISHED BY THE UNIVERSITY
Received for publlcatlol1 July 6, 1934
CONTENTS
PAGE
'rhe
hUll1tlS
layer
it1
forest ecology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The ecological environment itl mull and
:1
ill mOl" • . . . . . . . . . . . . . . . . . .
(i
Current ideas on mu1l and mul' formation and mol' act ivntl()ll ...... .
H
Criticism of the foundations of the cnrrent
\'iclV~ .................
.
llJ
FOLtnda tiotls of a hiological theory of mu11 <lllLl mol' fom I1lH! lll •..••••
1:3
A biological explanation of the activation ('HeeLs ................. .
17
Sih'ical applications of the biologic:)l the'ory ..................... .
22
References ................................................... .
ECOLOGICAL PH.OBLEM.S
UF THE [IUMUS LAYER IN THE F'ORESTl
L. G.
HU~1l':LL
TflE HUMUS LAYER TN Frll{]!:ST ILC()LDCW
Forest-suil study i.s a separate ]lnllwh ()f scienc(" 11ist1nd from agriculLural soil tedmology simply bl'l'anse of the fundamental diffl'.l'enCe between an
extensive. form of land usc, 811eh as forestry, awl an intensiv\::\ form, such as
agriculture (Rollwll, lO;Hl) , Agricnlture, evcn in its extensive forms, is
based on some R()rt of toeJl1lic creating aml maint.aining distinctly artHicial
eonditiolls, [i'orest.ry mnst Jepund upon natuml or slightly modified eeological fac[;o1's and equilihria,
In particular, the important r,')le of krLilir.lng Htlfl soil-working is, in
forestry practice, assigned. to man only in exceptional cases, as a temporary
or an emergtmey nwasure. This fact gives special importance in fon~st­
~oilfi work to the stlldy of the humlls layer, that is, the horizon of humus
[ornntioll, the Hl:!at of tllU lwiHke~t activity of the Hoillifu. The biological
processes conl~entrated to t.he; humt1.s bycr h[wc to supply, mnong other
thing~, the nitrogen rOU{ 1 required by the forest trees for a satisfactory
prod uctiotl. 2
Humus was earlier regarded as the very essenr:L! of soil fertility. Recently the tendency is rather tllt' opposite, tlw htl1111lS having heen lal)e1eJ
as a nuisancc 01' a nceessary evil (Si1('hting, ll,:.ln a). Instead, a mpid and
complete dccomposition is t.aken as the ideal. Both the old n.llr1 the cxtreme reGent view are bascrl 011 fal::;e gcneralizations, expressed in the very
meaning of the wonl' 'humus" during different. times. To t.he early authors,
true humus mostly was htt111US incorporated in the soil. Today, the term
is often ttS(,(l. mtteh as a synonym of "Aullagelm!11us" (eover humus, unincorporated l1ttlllU:;). One or the other kind of Imt11tts predominfl tes in
cliiIen:nt types of humus byer. The real index of fertility level is less the
amount ofhu111t1S, of whatever kind, than it is the type of humus formation.
The important dilTerellccs 1ll the type of humus fOTl11ation itt. the forest
were mtrly dimly conceived by nature-ohserving foresters. Hundeshagen
(lR30), although lwsitating ahout taking a view so little scientific, distinguished t.wo "main comlitions" with respect to "the origin and eITect of
the humus." He felt that this d.ist.inction would be particularly important,
"nt least for practical rtllplicatioll," in t.he same rneaStll'c as "the theory
ahout. thefic things is d(We10PCl1." This rcmnl'kahlc forecast, which seems
('ompletely forgotten in the later litemture, came trtte Lhrough the keen
nature ohscrvcr, P. E. Mttller (1:-':i:{7), who rerliscovered Httndeshagon's
t.wo main tYPL'S of ht1l11U:-i layer nnel was the first to apply consistc11tly a
point of view of :-ioil biology 1.0 the problem, As testiGed by Ha.uch (1010),
this work came as n. ruvl'lation to Danish foresters, tapping aro1111d in the
'The present pal'''r may be t"kon as (\ scmnd und cnncll1,linll parl of an (!tIrlier article (Romell. 1932), but
it hM heen written as Ull ind"prnrh'n1. papel' and (In a slif'(htly different plan.
'The nitro!'(en of the littt", I' largelY tUmvailablc ,IHtil it: h"H become "millrmli"eu." Thu. the sttlt~mcnt
Iundc holds even where the l1itrovcn 'in fmn nnd :-.l1nw eOVCfS that frnctit)u o( the annual nitrn~,en Gommmption which is btol'ed Ill' in W<Hlrl, ~IS is still hcld hy SOIlte authors (LellliIlgcn.Wcsterht1l'g, 10al) to ho I1Dr·
maUl' the case.
.)..
4
L. G.
ROMELL
dark particularly with respect to the diniculties with Le(1Ch regeneration.
The work also laid the lasting ground £01' the biological f.orest-soil scien(,c.:1
P. E. Mtlller's two main types of humus layer, mull and mar, [l1't~ now
accepted intcrnationally, with the same designations (International Union
of Forest Research Organizations, 1\)83). As exp1ained elsewhere (RameH
and Heiberg, 10~U), they are characterized hy strl1durul and athol: differences dircctlv observable in nature.-! So nrc tlw subtypes, whIch arc
needed for fu~·thcr distinct.ion particularly in this country. This does not
mean that the types are unscient.ific makeshifts and Hre separated acconling
to "the most superficial characteristics," as judged by Bltelliing (J n20 11).
The differences in layering, structure, :md so forth, testify of important
rlifferences in soil life and i.n the cour~e of decotUpo~iticll1, as ShOl1ld he dear
after Mttllcr's classical studies. Mull and mol' have been fOlln(l usdul and
natural types also in this country. For the purpose,; of the Jlresent pa]Jel',
the broad division into these two main types, or type gl'llnps, is mostly
sufficient. Only a few of the several sl1ecifk typcs or sllhtypcs (Romell
and Heiberg, lO;n) are occasionally mentioned.
Mull is an ecologically richer type than 11101'. Its mon' luxuriant vegetation is richer in species, and inclwles nitrate plants whieh arc ahsent on
undisturbed typical mol'. With the marc requiring tree Sj)l'c1eS, mull is
silvically favorable or is even a co11tlition necessary for succ~~ss. Most of
the more valuable northeast-American broad-leaved true's, such as white
ash, hickories, tulip poplar, and basswood, arc believerl to l,e such mull
species (RomeH and Heiberg, H131). With a 11l1mbl1r of conifers mull may
be unfavorable, being too rich. Red ,;prucc does not do well 011 mull in the
Adirondacks, in competition with hardwoods (Heimburger, 10;"4). Even
direct unfavorable effects may appear, such as poor form and heavy limb::;
in Scotch pine, and heart rot and apparently reproduction ditnculties (Lincl~
quist, 1932) in Norway spruce. Otherwise at least the latter species, like
white pine (RomeH and Heiberg, 10a1) and several other conifers, call
utilize the richer conditions offered by a mull for hettl~r procitH,tion. Forest production is not necessarily higher 011 mull, however. At kast some
of the famous mixed pine and spruce stands at J()l1ftker in Sweden (8chotte,
Hl21), con:;lsponding to Schwappnch's site class I, grow on mol' soil. A
forest soil may :1lso be too shallow for good forest production, owing to
ground wa~er or some other factor, and yet have a good mull typt) of humus
layer. It IS seen that the relation between type of humUfi layer Hnd the
silvical characteristics of a forest soil is complicated. Yot the relation is
evident and has been found very important.
Varied are also thc factors favoring the development of one or the other
type of humus layer. Generally speaking, mol' is predominant in the beli
of coniferous forests on podsol soils extending through the colder parts of
the Temperate Zone, with oases of mull in localities favon.:'cl by lime in rock
'"n!at is, in the NOftl,. In (icnnall¥, P. E. Mullcr seem, to be practica.lly ("r!;totten tonal', a>l Ca.ll h~
seen m the numerous volumes of Blanck, lian.dbuGIl der Bodellle"re and elsewhere.
'In thc paper quo!ed (Romell and Heiberg, 1931), and also later (RomeH, 1082), mor waS called "duff."
Th;> arguments agamst the term "rn?r" di.sappeared in conse(j\lence of a congress resolutioll (Intemat.
U~lon Fores.t Res~arqh Organ., 1033) ImplYIng that H:essclman's "mof' (for a subtype) i~ retracted. Th..
wnter heartIly lOIns III the general acceptance oE the term "mor" stnct1y in P. E. Muller's 'en"e. H .. \s
unable, however, to accent Bornebusch's definitions of mull and mor. They do not clearly refel' to types
of ~umus layer. but. in Rarnann's fashion. to "humu.... This sort of classification is open to the har~h
crittcism of Siichting (,1920 b~ •. whereas '" classification of.t}'pes.of humus l!'Lyer in Muller's Rpjrit is not.
Moreover, Bornebusch "detimtlon of mull does not fit condItIons In nature at least not in the UnIted States
(Ronteil, 1935).
'
5
lind. hi soil or by drai.nage conditions (slopes with high and yet not stagnating ground-water, even in regions very poor in lime). Mull is characteristic of the forests of mostly hroad-Ieaved trees extending in a zone with a
somcwhat milder temperate climate, mach independently of geology bat
with islands of mol' on too dry, too wet, and geologically very poor sites.
In regions with an intermediate climate, mj ill Denmark and in large parts
(If New York SLate, mull unci m{)l' may alternate very 10c8.11y, following
variations in drainage comlitiol1s, forest. sLand, exposure, and geology.
In muny cases, as ill the beech f01"ests in Europe, the mull is correlated
ltl a hrown forest-soil profile. III other cases, as in oak forests hoth in
Scandinavia Hnd particularly in Russia, it combination of a mull type of
humus layer with a podsol profile is extmlsively found (Milller, 10U;Tumm,
HJ30, ft and b). 1n northeastern U niied States, the r110re pronounced
forms of mull arc t1st1ally etJlnllined with it brown forest-soil profile. Less
pronounced mull forlllo and l1ght [01"111H of 11101' may he combined with
interesting proiile tYlll'S intermecliate betwee.n podsols and true brown
fOl·est soils, as very frequently in the Adirondack Mountains (Heimburger,
19:1-0.
Under unifo1'tll climati.c and cdaphie condi.tions, the composition of the
stand has often been SCUll to affect the type of humus layer (Miiller, 1887;
Hcsselman, Hl2!i; Plice, 1U;H). Forestry operations also may have a
striking dlect, even though not ehanging the mixttlre of species in the
stand (I-IeRfi(:'hnan, 1\.)["i1>; M¢1Jc1", JOH1). S1.1ch different measures as
thinnings, c{Jv(:'l"ing with green slash, clear cutting, light burning, ripping of
the mol' covcr or :my other [arm of fooil w01"kin~, ancll1ming, have shown a
surprisingly similar ciTcd in aetivatillg a mol', starting nitrification and
frequcntly changing tho humus byer also morphologically to approach or
even to become a 111\.t11. The sarno activation effect call be seen as a result
of the girdling practiees recently in favor in this country. It may also sct
in under an undisturhecl tree cover us a result of trenching, burning, and
other clistmbanees ('l'oumcy, 1~J2:-_S; G:u;t, l\l:m; Fivaz, In:n). On the other
hand, lack of tJrot.ecti.on llfthe ground hm; long been known to easily Htart
a degenerat.ion of a gootlmull condition (Miiller, lS01?, 1804). 'l'his lUay
lead in bad cases to the flll"II1ation of a botherst)me 11101'.
The most. striking type conversiolU; from influcnces such as those men~
tioned havc been observed in regions with intermediate climatic and other
conditions, as in south Scamlina.vb mut on the Harvard Forest in Massachusetts (Fisher, 10:28). Yct mOl" activation occurs from regular forestry
practices right in the heart of dinmtically predetermined mol' regions, as in
north Scantlinavia. However, it takes more drastic measures to start the
activatiun u11<ler severer cond,itions. Clear catting tUay be ofticient where
tilinnings are l1UL. The last rCRonrce among the cheaper, more extensive
measures is a light hurning of the deaN~ut areas. This is much used in
northern Swerhm to activate the more refrac'tivc forms of mor. All these
type-conversion and mor-aetivaiioll efTeets arc of the greatest importance
silvieultttrally. The Dunish beech l:iilvkulture is one continuous tight for
the mail1tcnanee of a good mull, which iH (;$sential11ot only for reproduction
of beech but also for its good growth. In norLh-Scandinavian forestry,
bascd on IJine and spruee, a sufikiellL aclivation of the mol' is essential for
a Sltccessful regeneration (Hesselman, I\l17 b). Examples could he quoted
as well from other countries.
13
L, G,
ROMEL!.
A cnrious fact, and one not I:w.tisfactorily explained, is that the same
measure may affect a mol' not only to a different degree, but also in oppo·
site directions, under different conditions, A selective diameter cutting
may he fairly good silviculture with the beLLE'!' forms of 11101' in central
Scandinavia, but in the North it has producer! a mol' worse than the
average. Clear cntting is universally good in north Sweden, although 1]ot
u,lways in itself sufficiently activating; but it is feared by forestt'l's in the
Black Forest in southwestern Germany, as aggravating instem1 of imp1'ov·
ing a mor condition.
Even very heavy amI pronounced northeast-American forlllS of nlCJl'
(bdonging to or approaching the greasy type) n~act with an I!ase surprising
to a European observer. They seem to be activated lry the slightest dis·
tUl'baI1ces. The reason {or theil' grcater reactivity has not heen explaillcd.
THE ECOLOGICAL ENVIRONMENT IN MULL AND IN
M~l{
Variations in the degree of intermixing between humus and minoral soil,
in aeration, in acidity and related characteristics, and inllitl'ugull 11101Jiliziltion, have heen proposed as ecologically significant envirolll11l'ntal (liCeer·
ences between mull and mar.
The extensive mixing of humus wi t11 miuerul soil has been regarded, both
by older and by 11101'e recent authors, as a chief iactor favorahle both to
higher plants growing on a mull soil and, to the bacterial soil life. It cannot, however, in itself be regarded as constitut.ing a group rliffcrencc between mull and mar. It is charaetel'istic of the ertlll11J mull and relater!
forms, rather than of thc 111ull group. Moreover, there are uxi'cl'uingly
poor mol' soils, for example those of the Danish heathR, whkh may contain
just as much, if not more, humus intimately mixed with 111ineml Roil as do
rich mull soils. The difference hetween mull and mol' lies rather in Lhe
way in which a mixture of htunus witl1 mineral soil is formed.
P. E. Miiller made an exceptional mistake in assuming the aeration to be
generally poor in and under a mol' cover. He used this assumption in ex·
plaining the properties of the mol' am'! its etTer.t 011 the underlying mineral
soil. This idea seems still to hc held hy some anthors, although it bas been
t'oneiusively disproved (RomeH, 1\J2~), Apart fro111 these results, the lWtO·
1'ious abundance of fungi in mol' spetLkK distinctly against the idea of Tloor
aeratioll, since very few anaerobic fungi nre known.
If any kind of mol' is characterized by poor aeration, it should be the
pronounced greasy types, containing few living hyphae in their heavy and
compact H-layer (highly humifled horizon). In order to test this point,
the oxidation-reduction potential of the hest type of mull (crumb mull from
Entield, New York) was compared with that o( thl! heaviest and most
typical hemloek mol' which could 1)l) procnre(l in the vicinity (from El11s
Hollow, neftr Ithaca, New York). Both wt;l'e used fresh.
The equilibriulll to be measured takes a long- time to establish itself
(Wurmscr and Geloso, Ul28), and during that time [tir must he carefully
excluded. Thus, highly un1latural conditions an~ cn~ated which may
start processes foreign to the Roil in its natu1'al state. As a t~oI11pl'omise
between clifferent eonRiderations, the temperature was maintained at GOo C.
to exclude most of the biological activity and i>horten the time lleedecl without too greatly hastening unnatural, purely chemical, changes of the mate-
PROBLEMS OF THE HUMUS LAYER
7
rial. A 3-per-cent dextrose solution was included in the test, for comparison.
All three samples were <lujusted to pH 6.u, phosphate bl,.lffers being used.
Potentials were measured between either of tWCl 15-cm-long finc platinum
wires in each Hask with ~0i1 suspen~ion or dextrose solution, and another
platinum wire in phosphate )Jl,.lf[Ul' of pH (Ui kept at fiOo C. <md saturated
with quinhydrone hefore measuring. Phosphatc buffer was used as a
bridge between the half-cells. The potentials wcrc approximately constant after foul' tu live duy:-; at GO° C. At the cnd of the experiment, quinhydronc was added to thc flasks and another reading was taken as a check
on the pH. The figures recorded are as follows:
Potential (volt~)
Potential with lluinhydronn (volts)
Mol'
-0.28
+0.025
Mull
-0.51 to - 0 ..?2
+0.025
Dextrose
-0.31 to -0.32
+0.020
Thus the mull su~pension wns found to be rather strongly reducing, but
the mol' suspension was less rcdudng than dextrose. When the flasks were
opened, the 1111111 suspension smelled sLrongly like horse manure ju:-;t excreted, while the mol.' suspension had the normal smell of boiled 11101'.
The setting-in of anaerobic proce~ses in water-Ioggedl11ull can be demonstrated by simpler means at ordinary tcmpemtnres.
The result shows that tho mar did not contain uny strongly reducing
substances indicating poor aeration. The writer is of the opinion that a
good crumb mull, rather than any form of mol' occurring on normally
drained land, is likely to contain more or lcss anaerobic centers during wet
periods (in the interior of its mtlwr large crumbs). 'l'his may be of imporiance, for instmlce by creating conditions for nitrogen fixation by Clostridium. At any ratl\ thero are no indications of a group difference i11 the
other directiun, as has been supposed.
A difference il1 acidity IJctween mull and mol' was noticed long before the
modern technic was developed to measure it (MiHler, U:\87). Generally
speaking, the aeirlity increases rathcr regulur1y from the best mull to the
worst very acidmor, as i~ shown on American material by a rather extensive set of dat:L (Ramell anci Heiberg, H131; Hcimlmrger, 1084). There is
considerable variation, however, in inclividual samples, so much so that
hard and fast boundaries as to acidity can hardly be drawn even between
extreme types.
It was eady noticed (Emds, lR7fi; Me'tller, U';87) that 1111.111 and mol' give
extracts o[ different co1ot'!-\ either with distilled water or with weak ammonia (Schiltze's te!it). The less intense humus color of mull extracts
would indicute n. greater base saturation of the hUllluS eolloids in the mull,
according to ulOdcrn ideas on the su1Jject. This may not be the whole explanation, since the "h1.1111t18 colloids" in mull and those in mol' may be
entirely different (Plate 1). Yet the simple extraction test would be
worthy of a rC1]ewed investigation. Recently attempts have been made,
by ion-exchange methods, to determine base saturation or ullsaturatioll,
either as such or as exehangc aeidity. For various reasons, however, the
data thus far appear to be insufficient for judging whether differences of
this order are more consi~tcnt1y chm'actt;;ristic of different types of humus
layer than 41l'e the aetual acidities. The eonsicier['l,hle work done in the
writer's laboratory on this problem did not advance further than to a
critical study of methods. Promise was given by the barium-acetate
8
L. G.
ROMEL1.
method even with difficult samples from greasy typcs; whereas the ammoniLtl1~-acetate method broke down completely on samples of this kind.
Related to base satumtion are content of "available" lime and nntacid
Guttering capacity, although the latter does not seem to be determined by
the base content to the extent which might have becn anticipated (Plice,
ID:H). In a material of very dilIerent tYPCH of humus layer, from the
Adiro11llacks (Heimburger, 1U:H) , the acid-soluble lime, expressed on a
humus basis, ran remarkably closely parallel to the pH values. The
rather SCarce available data on antacid buffering hardly permit any definite conclusion to be drawn. One difficulty is the impossibility of correcting data obtained for equal weights of soil, to equal amounts of humus.
Mull has been fonnd to he remarkably low ill antacid as well as in acid
buffers (Hesselman, 192()); only in part could this be explained llY a lower
humus content.
As might be expected from the vegetation, there is a tlistinet difference
between mull and 11101' with respect to nitrogen mobilization. Mull has
practically always been found to he nitrifying, typical mor as a rule 'not
nitrifying unless it is aeti.vated by some disturbance. This difference is
seen also in material from this country (Romell am1 Heiberg, Imn; Heimburger, 103-1). However, in the American matt!rial, nitl'ification was found
to be not strictly confined to mull. Light forms of mol' rather frequently
nitrified. Even heavy mol', belonging to 01' appronching the greasy type,
nitrified occasionally. These somt!what unexpectod tlndings are paralleled
by the ul1usnull'eactivity of the same American mol' forms, as mentioned in
the preceding section.
To summarize, a clear averLLge difference in ecological ellvironment has
been found between mull and mar in two main respects. Ono is in acidity,
and characteristics related to or governing acidity such as base saturatiol1
or content of available lime expressed on a humus basis. The other is in
nitrogen mobilization.
The typical lack of nitrification in mol' has frequently been l~xpl<Lined by
acidity and by scarcity of bases. In fact, the nitrifying organisms, even
those thus far studied from forest soil, seem to be remarkably little tolerant
against acidity (Winogradsky and Winogradsky, 19;3:1). On the other
hand, nitrification has been found to occur occasionally in forest humus
of almost maximum acidity and poor in available lime (RomeH and Heiberg, 19:H; Heimburger, 1084). This mayor may not be owing to the
occurrence of centers less acid and higher in lime (Winograd sky and Winogradsky, U)3a), as first suggested by Gaarder and Hagem.
Whether or not the differences in nitrogen mobilization are directly connected to those obtaining in acidity and. related characteristics, both appear
as fundamentally important. All the 1110re puzzling is the fact that, as far
as present knowledge goes, both arc statistical differcnces only, and that it
is not possible in either case to draw a clear dividing line even between
pronounced forms of mull and of mol'. The efforts made in Germany to
s~parate "humus types" according to some sort of measured acidity do
v101ence to nature, and can hardly be explained otherwise than as an expression of. a curious modern tendency to regard everything recorded by
apparatus 111 the laboratory as being more scientific than visual observa"
tions in nature.
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CURRENT IDEAS ON MULL AND MOR FORMATION AND MOR
ACTIVATION
Following Ramann's lead, and more ana more influenced by ideas o[
Glinka, Gec1roiz, Ehrenberg, Wicgner, ana others eminent in geneml soil
science, forest-soil scientists have tcnrled to abandon Miiller's (lSi-I7) original biological concept of the primary diHcrences between mull and mar.
Gradually a common agreement has heen reached on a body of doctrines of
a more physico-chcmical than biological pattern. According to this view,
the differences in "microcosm" between mull and mor, strongly emphasizcd
by M(iller, arc secondary. They afC generally considered to bc of importance only in so far as they inthlcnce the rate of decomposition of the litter
material. Mull has come to be regardcd as a l'e!.olult or a sign of rapid
decomposition, mor as re,mlting from unfavorable conditions not permitting the decomposing organisms 1.0 do their work as rapidly and as completely as in mull (Romcll, 10;32).
Not only the soil life, hut also the typcs of humus layer as such, have
come to be looke<l upon as symptoms of the geneml conditions of soil
(ormation rather th<1.n as being primarily important. Even most northern
workers have identified l11Ltll soil with brown forest snil, forgetting the
existence o[ the wieldy distributed mnll podsol. The ocC'urrcncc or nOlloccurrcnee o[ nitritication in the humus layer was referred hy Hcssl'1m<ll1
(l!n 7 a) to assumerl dill'crcllceS in the "electrolyte" supply fro111 soil and
ground-water, rather than to the directly observable differences between
mull and mar. The charaetcristic structure of mull was interpreted
largely in Ramann's way as a ftocclllation phenomenon due to the electrolytes held responsiblc for nitrification also.
The characteristic higher acidity und lower buse content of mor, as compared with those of mull, are partly accounted for by assuming primary
differences in base supply and by n. varying acidity and base content of
forest litter (llesselman, l02()). What can110t be explained 1n this way is
ascribed to differences in leaching 01' in rate of decompositiol1, or by an
interplay of these factors.
The ash constituents will naturally tend to
ftccn111ulate in the humus formed (htring decomposition. This is counteracted by the leaching eff~)cL of rain a.nd snow. With sufficiently slow decomposition, the latter effect will be the stronger one. This is taken to
account for the formation of an ariel mol' from a less acid litter materia!, as
whcn mol' is formed under beech, a very common occurrence in Denmark.
Hesselman (Un7 b) has uscd his "electrolyte" theory t.o explain also the
effect of different J'Ol'efltl'Y measures in activating a mol'. For example, the
effect of a clear cut was explained largdy as a result of illCreas(;)d eva.poration from the surfacc, with a resulting more ullwanl trend in the wandering
of the electrolytes. The good effect of green slash was in part referred to
an aSHmued higher mineral-nutrient content ill the green needles. The
activating effect of a burn was taken to depend mainly on a concentration
of bases in the remaining humus. As n. partial explanation of the effect of
disturbing the humus layer mechanically, an increased supply of mineral
nutrients cltiC to mixing with .mineral soil was suggested first by Mftllcr
(1910). Hesselman (1917b) takes that as the main explanation.
The favorable soil effect of thillnings has been generally mther dimly explained hy increased amOul1ts of "light and air," or more specineally by a
10
L. G.
ROMELL
more rapid dec()mpos~tjon due to illCl·~a~L\d. Ro~l temper'lt.nn' (IT moisture, or
both; hence again a hIgher conccutra tlOn 01 111ll1cwlllutncuts 111 the humus.
The "electrolyte" theory thus applied locally as well as l'L~giol1ally has fl
great appeal by connecting the efIed~ ob?erved uncl.er om\ common point
of view dominatin(T also in general soll seIcnCl'. [t 1R no wunder that the
theory has been ratllCl' generally accepted, 1Iy fon~stcrs mld hy soil scientists.
Sporadic attacks only hav(;; been mach: ngainst parts of tl;(\ modern
elcctrolyte theory. The most lmportant 01 thcA(\ attacks [Ire behcved to be
the following. Milller (I D24) pointed out that the existcnce of a widely
distributed mull-podsol type presents a difficulty to [I('cellLancc of the
theory. Hesselrnan (192G) found it impossihk to account fot' the scarcity
of antacid buffers in 111ullmel'ely by a lower hUI1111s <,ontunt. He hinted
the importance of a differcnt type of r1l'l'omjJositiOll, as 1m,; been done recently also by several other northern authors (Cbal'der [111(1 Hagem, L921;
Glq'>111me, 1021:;; Brenner, lD:~O). The Jewish mycologist, Richard Falck
(H)28, HJ2H, 102.'1, In;~O), has, even 1110re speciflc~ll1y 1han 1,ho young P. E.
Mt"lller, referred the formation of humus with difTl'rent aridity amI different
fertility level to the activity of definite organisms. A strong attack
ag,lillst the chemical conception of TIlll!l bas ulwxpcc·tcdly l'OllW from Ran1ann's pupil and ~uccessor, Lang (1\);~1). He givl's runsuns to show tl1at
the crumb-mull stmcture cannot he a result of flocculati(ln by lime. He
also has l'ecliscoverccl several facts stressed by P. E. M11l1l!1' and pointing to
the importance of the soil fanna, particularly carthworms.
CRITICISM OF THE FOUNDATIONS OF THE CURRENT VIEWS
The foregoing rapid review of current irleas i11Clientcs that these arc very
largely based on the assumption of a consistent difIel'Cl1Ce in rate of decomposition between 1111111 an(l mol'. It hm, been S110wn earlier (Romen,
1fl32) , in some detail, that tl1h; aS~tllnption is not Sttppol'tccl hy facts alld
very likely cloes not correspond to the reality. Spceilical1y, it comparison
between two pairs of carefully selected localities in Now York imlicated
tha.t the total dccompositionls slower in the hest mull «(~l'\1lnb mull) than
in a light mol' (root mor), not faster as is currently taken for granted.
The following new arguments may be added. The currcnt idea of a
general slower decomposition ill mol' seems to be based simply on the fact
that a greater number of yearly litter crops can he recognized in mol' tha11
in mull. The amount of decomposition may he VC1'y considerable, however, even though the plant remains arc stil1n.,C'ognizuhle as st,ch (Rnmann,
1911). When the litter is c1iRintcgrated and buried in tJ1l' mull, this cloeR
not necessarily mean a greater loss of matter hy decomposition.
In fact there are experimental re8ults indicating not a more rapid decomposition, but a slower one, when the soil fauna is morc efficiently engaged in the process (Falck, 1\)28; Ulrich, H)8:5). The total respiration of
the soil fauna was found sometimes higher, sorneLimes lower, in hoech mull
than in beech mol' (Bornebuscb, H)30) , with the mull avcr:.\.)!;ing only 4 per
cent higher; this In spite of the fact that probably more of the mol' fauna
than of the mull fauna was lost in sampling (Tragardh and l1'orsslund,
1932). Since it cannot be denied that the mull fauna is more active than
the mar fauna in making the plant remains unrecognizable, the result
quoted seems to give additional evidence that the current conception about
PROBLEMS OP TilE H.UMUS LA YEl{
11
the rates of decomposition ill mull and in rnor is simply the result of an
optical illusion.
In reasonings all the current 1msis, it is at least very frequently understood that the hU111U~ accumulated is an undecomposed remainder of the
original litter. It is furthe1' reafionecl as if the humus colloids were much
the same everywhere and differed in val"ious types of humus layer essentially in base saturation. r, Both views seem to be rather conclusively disproved by a (lired microscopical study of the humus layer, using suitable
staining methods. In t]w H-layer of very pronounced roor as well as of
mull, the bulk of the hU111l1S appears to consist of living amI dead organic
material huilt 'liP hy the microflom active in uecornposition. The mieroflora and thc synthesized material are strikingly different when extreme
types are compared, as in Plate 1.
In the cn1mb mull (Plate I, A), jelly caps1.1lc~ of bacterial colonies apparently form a cOllsi(kmhle part of what has earlier Leen reg~lrlled [.lS "ftoccuIatell humus colloids." In the greasy mar (Plate I, B), fragmcnt.s of brown,
coarse hyphae are equally dominant. Remains of litter 111'e rare, in the
well-decompused hUlllus. 11, Seems likely that the jelly cnpsules and the
wall sulJstance of the brown hyphae differ in chemical natnre, 1101, merely
in base saturatioll. Thi~ points again to the importance of the type mther
than of the raLe of decomposition.
Strung objections can be raised against the cnrrent explanations of mol'
activation 1JY forestry measures. A wandering of eketrolytes upward fronl
or through the pnrlsol byer, sufficient t.o markedly influence acidity and
base saturation of the n101' and start a development towards a nitrifying
humus layer, seems difficult to conceive in a humid climute, even on a c1carcut area. The good effect of green slash can hardly be due to a highet base
content of the green needles, since the content of lime (the most important
base) has heen found to be lower and not higher in greelll1cedles anclleaves
than in ripe fall Etter (Woll1' , lR80). As to bUl"1ling, the best effect is ob~
tained by a very light hum, little more being done than to kill off the mor
vegetation, and most of t.he increase in available lime content. of the humus
has been found to ocettt· later and not as an immediate result of the burning
(Elleroth, 102:-;). Neither of these findings would be expected [rom the
electrolyte theury o[ activation. Finally, the activating effect of mechall~
kal disturbances is by no means confined to cases ill whieh the mol' is ttC~
tually mixed with or covered by mineral soil, us would be required by the
electrolyte theory. Where actual soil-working has occurred, the reaction
is frequcntly noticed on the nwr around t.he wodced spots rather than 011
the areas actually worked. This was pointed out speci-fically to the writer
by the emincnt Swedish forester, E. Rouge.
Decreased interception of light and ruin is frequently taken as the cause
of mol' activation following thinnings. However, Danish foresters particularly experiencecl in thinning practices have pointed out. that it is
darker under stands propcrly managed by thinnings than under comparable
un thinned stands. Yet it is in Denmark where thinning practices have
shown the most remarkahle effects on humus layer and forest production.
Moreover, the gcnerall'ule that the condition of the humus layer tends to
''I'hi" view has'p:!lined apparent suppnrt hy recellt findings (Mattson, 1032; Waksman and lyer, 1932.
[l.. "htnul.ls conlplt.!:x" or "hunluS ntlC1eUf;" c1nSicly corre!';ponc1ing to the most important fraction of
l!l!l~·n on
the humus which elln be extracted with alkHli from different l<ind~ of nutura! humus or peat.
12
L. G.
ROMEI,L
be better under well-closed Btands (Hesselman, 1\)20) doe;; not speak for a
dominating role of the interception of RUllshine and ruin on the part of the
stand. Finally, activation efIects much like those produced by cuttings
can be observed under undisturbed stands, as on trenched (Toumey, 1928),
burnt (Gust, 1930), or raked (Fivaz, 1m 1) spots. A re[lction similar to
that following cuttings is indicated, for example, hy the germination of
seeds of Ribes (Fiv[lz, H)31) or of fire cherry (trenched plots at Dummcr,
New Hampshire, kindly demonstrated to the writer hy Dr. Henry I.
Baldwin).
FOUNDATIONS OF A BIOLOGICAL THEORY OF MULL AND MOR
FORMATION
The current theorics being unsatisfactory, it is time to look for new view"
points. The studies of Mi'tller and of Falck clearly indicate where to find
building stones for a new theory. In fact, it is most curious that an evidently biological problem suell as that of mull and 11101' formation has been
commonly treated from chemical rather than from biolugical points of view.
In a pronounced mol', the suil life is completely domilHtted by fungi
(Milller, IR87; Frank, lkR~; Jensen, 18:31). The J-ihroufi type of mol' is
woven together by roots and by the hyphae of mycorrhizal und other fungi,
~o that the whole forms a tough felt or mat. This is not so mnch seen in
greasy mol', but H. microscopical examination shows the dominating 1'()le of
fungi also in this type (Plate I, B). The mull harbors a relatively scarce
fungous vegetation, and insteacl a more cfficient animal life. This is evidenced by direct observation, by inference from the morpholugical characteristics of the mull, and by comparative analyses of the fauna in beech
mull and beech mol' (Borneb1.tsch, H130). In pronoullced mnll, bacteria
appeal' very largely or almost entirely to take the place of fungi (Plate I, A).
The fungi and baeteria important in mol' and in mull are, with few exceptions, unidentilied, owing to shortcoming s of the technic used in analyzing the soil microfiora. Neither the visibly particularly important hymenomycetes nor the "autochthonous" (Willogradsky, 1025) bacteria
develop in the petri dishes, with the methods useu (Brierley, In2:3; Falck,
Hl23; Waksman, UJ30). Nevertheless, certain conclusions about the
ecological importance of the different organisms seem to he permitted by
the experience at hand.
Fungi attacking cellulose or lignin-cellulose often produce acid substances. Some of these may convert cellulose "quantitatively" to oxalic
add (Falck, ] 1):30). Fungi of the brown-rot type, decomposing ce11ulose
but not lignin, all seem to produce acidity (Falck, 192G, H);~O), at least when
growing 011 wood (Wehmer, 1025), becuuse of the formation of free lignin
if not of organic acids. Fungi of the white-rot type, decomposing lignin
as well as ~e~lulose, may (Glcpmme, 1028) or ml1y not (Falck, 11)26, 1930)
produce aCIdIty. Hyphae of some fungi have been found to give lignin re"
action or to contain a solublt; acid fraction resembling certain "humous"
matter (Czapek, 1922; Rippel, 1931). Thus, when the bulk of the dead
plant material is broken down by fungi, there are chances for a considerable
production of acidity as a direct result of the decomposition or of the synthetic activity of the decomposing organisms.
Scattered observations with stored samples (Suchting, 1925; Hesselman,
PROBI,BMS OF
nm
HVll'JUS LAYER
12
1U~(j; GI¢l1l1UC, 1~~12; Pike, 1D34) indicate that a biological acidity forma~
tion actually occurs in mar. Severnl samples of very acid (greasy) mol'
changed. their reaction to less acid in electrodialyzing experiments conducted in the writer's laboratory. This indicates that their natural acidity
waS due, not merely to extreme ullsalnration of the humus colloids as a
result of the leaching-out of bases, but also to actual production of acids.
The aerollic ce1lulose-ciccOlnposing haderia, on the other hand, do not
form acic1s (WakSll1lLll and Reusser, Hl:m) , and the slime produced by some
of the most important forms is so weakly acid that the reaction changed, in
the cultures, in one or the other direction depending upon whether ammonium salts or nitrates were used as a source of nitrogen (Winogradsky,
1929). The jelly capsules of hacteria cot1RisL, so far as is known, of carbohydrates or sometimes p1'Oteins, not of distinctly acid colloids (Czapek,
1\)22). Anaerobic cdlulose'lleco11lpusing baetcria pro(lucing; organic acids
may occu]' ill t'rtllnb mull in the interior (If the (·nunhs. Under the conditio1ls obtaining, llOWeY(~r, they woulrl n()t b() able to contribute much towards making the aci<lity higher, as is explained latt>r.
With a bacterial type of mieroHora, as in mull, therc seems to be only one
chaHce left for any GOllsiderahle biological production of acidity-namely,
the liberatiun of lignin from lignin-cellulns(!. The hactel'ial decomposition
of lignin has hee1l little st.urJied, hut the micl'oscopical VictUl'C offered by
a c1'tlll1b mull illllieates a ]Jl'<Lctieally complete decompositiun of the origi11(11 plant 1'esidues in this nmst j)1'llllllUnce<1 hacterial type. In an experiment with strawy manurl~ (Falck, I !J:2:~» lignin was broken down as well as
cellulose. This was a mixl'(l type of (kcol11position much resembling natural l1mll forma1.ioll, with bacteria and insect larva(' acting together on the
material. Also, the hummi formed was neutral (later acid only because of
an accmtl1.l1ation of nitric acid in the strongly nitrifying material). A
heech 111u11 was found tu change its reactioll to less acid as a result of decomposition of litter rich in nitrogen (Bornebusch, 1925), and this in spite
of a s1;rong formation of nitrie acid. One-year-old beech litter was found
to have changed its reaction to less acid ill a mull locality, although the
same kind of liUer haril'hangcd its rc~acti()n to more arid in it mor locality
(BoI'llcfmseh, I mlO).
The fads cliseussL~d above iurlicatc all important group difference between tIl{' baet(>l'ial mull type and the fungal mol' type of decomposition.
NoL only is the 111ull equilibl'iu1ll1'dativ!'ly nctltl'Opltilotls, as compareu with
thc acidophilotls m<ll' equilibrium; t.he former rtpp(~[lrs also us neutrogenous,
the latter as aciclogcnotlR, as postulated by Bl'cnller (10:30). One reason
for the mull being in hiological equilibrium appears to bf:! that acidity is not
produced to any clallgt>rotlS ext.ent during decomposition. In mol', acidity
is very likely llro[hl<~(>.<1 as a ciireet result of decomposition, quite apart from
leaching dler:ts.
From this point of vlL'w, thl~ more efikicllt animullife so characteristic of
Uw mull must playa very important l'(lle in the maintenance of a mull
equilibrium hy keeping llOWll add-producing fungi. If the latter were left
alone, they would easily start a progressivt~ change towards more acid conditions, inimical to th~! mul1life adaptt~d to lower acidity, and a mor formation woulcll)e all tho way. A mull condition thus maintained by the constant activity of animal guardians against biologically produced acidity
tnw:;t represent a rutIler delicate biological equilihrium. Anything weaken~
L. G.
ROM ELL
ing the soil fauna, even though it were only temporarily, might irreparably
throw the equilibrium out of balance. A mor appears to represent an
equilibrium incomparably stronger biologically, self-stabilizing to such a
degree that a mor type would tentl to become more and more pronounced
once it had started to form.
The tendencies thus to be expected are in excellent agreement with a
wealth of silvtcal experience. Danish foresters, and thOfic of other countries, know all too well how easily a 11101' formation may be started aH a result of some little carelessness, and what drastic meanR are usually requh'ed
to revert the development so as to get back [L good mull condition. An
active formation of acidity by the humus life in the mol' also makes it easier
to understand how the mar can have such a striking and relatively rapid
podsolizing effect on the underlying soil (Ml'tller, 1887).
The role of the animal life in a mull equilibrium, and of the fungi in mol'
formation, was emphasized by Muller. More recently, Falck has claimed
for fungus-devouring larvae und worms, and for certain fungi, a rMe still
more specific than that suggested here. To Falck, mull formation alwaYH
rcpresents a white-rot type of decomposition interrupted by larvae and
worms, and mol' formation is ascribed to brown-rot fungi only, c'ausing a
progressive accumulation of <,eid humus from the undecomposecl lignin of
the litter. For several reasomi, the writer prefers a less specific theory.
The most pronouncecl mull does not have an F-1nyer, although it must have
Recorcling to Falck's views. Even the heaviest (greasy) 11101' which the
writer has examined seems to be built up chiefly by dead fragments of
brown hyphae, and not by ttndeeomposedlignill remains from the litter.
In the preceding reasoning, the differenee between 111ull and mol' in nitrogen regimc has not been used and has not been accounted for. NO!' has it
been really accounted for by any previous theories, so far as the writer is
aware. The difference, according to all indications, is not merely in presence or absence of nitrification, but in level of available nitrogen. The
usual lack of nitrification in 11101' scelllS to be due less to acidity than to
other factors. In Danish beech woods, Weis (192,1) found the strongest
nitrification in a district with very acid 1111.111 on acid soil. Mol' samples
among the most acid found may nitrify (Romell and Heiberg, HJ31~ Hcimburger, 1034). The simplest explanation of the findings is that the occurrence of nitrification is gove1'1led principally by the level of available raw
material (am1l10nia), as hinted in an earlier work (Gaarder and Hagem,
1921). In other words, nitriflcation would be a symptom of the level of
available nitrogen more than anything else. This not only is in line with
generalmicl'obiological experience, but also is indicated by the generally
greater luxuriance on mull, both of forcst growth and of ground vegetation.
The current ideas bascd on differences in rate of decomposition could not
well account for the apparently better nitrogen supply in mull, even if the
decomposition were actually consistently 11101'e rapid in this type. According to rules weU established by microbiologists, excess nitrogen occurs
in the form of amlllonia or nitrates when the carbon-nitrogen ratio of the
decomposing mass has decreased to a critical value, this differing somewhat
a.ccording to the nature of thc active organisms. This valuc may be
higher or lower, and may take a shorter or a longer time to be reached, but
it is reached sooner 01' later. Unucr balanced natural conditions, the only
difference which would result from variations in rate of decomposition
PROBLEMS
OF THI<: HUMUS LAYER
15
would be in the lltllllhcr of yearly litter crops which would be furnishing
available nitrogen. Theil' combined annual production of ammonia pluB
nitrates would be the S[lme, if nitrogcn fixation and denitrification are
excluded. Nor would any difference be expected between mull and mol'
in level (concentration) of available nitrogen, since there is hardly any
group difference as 1,0 the volume (depth) of the active strata.
The preceding reasoning applies, whether or not fungus-eating animals
are engaged in the tlccomposition. For an isolated batch of decomposing
material, an attack of larvae or of worms will thoroughly change conrlitions
hy rapidly mobilizing nitrogen slorcd up in hyphae, as in Falck's (102:1)
experiment. Under balanced natural conditions, however, this speedingup of the nitrogen turn-over can be of no 1110re importance thHn other
vLlri<Ltions in the decomposition rate. With the rcservations mude in the
precelling paragraph. a consittntly active 111ull fauna and flora coulcl 110t
be expectcd to furnish more available nitrogen yearly than would a soi11ife
of any other type. Thl~ 111ull fauna cannot directly be a condition 11000::;sat'Y for a liutisfncto1"Y mobilization of the nitrogen contained in. the litter,
or even for nitrification. According to general microbiological mles,
nit.rification will set. in as soon as excess nitrogen is mobilizc(l as am111onia,
provided only that the development of the nitrifying organisms is not in-·
hibitccl 1>y an excessive acidity or by any other factor.
The solution of the puzzle, according to the writer's views, lies in the
structural differences between mull and UH)r. Characteristic of the luttcr
is the abundance of fungus mycelia, rnHUY of whir.h helong to the mycorl'hizas of the trees unO. of the other higher vegetation. Practieul1yeverywhere in the whole mass of a mol', partieularly in the fibrous types, there is
a 1'001" a mycorrhiza, 01' a hypha ready to absorb available nitrogen. The
average dilitance hctwecll the points of production and of consumption of
available nitrogen n'i.t1st be very short. In a typical mull, the roots are
spread much like those of garden plants in a hed of good soil, I1nd mycotrophy occurs to a much less t:xL(::lnt. The path of diffusion of the available
nitrogen to the absorbing organs must he considerably longer than in a mor.
It foHowli that, with the same tum-over (production and consumption) of
availahle nitrogen, the nitt'ogCl/ leval (average concentration) will be considerably higher in the mull.
The above reasoning is in line with ideas on the function of the mycorrhiza expressed by Stahl (1900) and utilized hy later authors (Miil1er,
190:3; Mclin, 1025; Hesse1man, 1020). It requires that the mycorrhiza
hyphae shall be keen c01l1petitors for available nitrogen. This Seems to be
evident both from physiologiml (Mclin, I H2fi) und from ecological (Hesselman, 1027; Melin, 1H27) experiences, the latter huving shown good mycorrhizas to develop only in humus samples where the nitrogen mobilization
was good without mycorrhiza fungi.. Such a result indieates that the
mycorrhiza fungi arc benciactors to their hosts, not by attacking ordinat'ily
unavailable Rources of nitrogen, lmt by serving us adc1itionall'oots helping
in the general keen competition for availahle nitrogen food.
With the viewpoint lwre proffereo., the animal life of the mull actually
has a role in creating a higher nitrogen level, but an indirect one. Its importance is to keep down the fungi whieh create hy their omnipresence the
low nitrogen level of the mol'. The explanation allows the possibility of
much the same nitrogen ttt1'Jl-ov{~r in mol' al1d in mun. This scems to be an
16
L. G.
ROMELL
advantage of the theory, since forest production has not been found consistently higher on mull.
Acidity and nitrogen level have been treated separately in the preceding
reasoning. These factors must, however, influence eaeh other in several
ways. If organic acids are formed, their effect upon acidity, and hence
upon leaching and base saturation, must depend very much on the nitrogen
level. With a high nitrogen level. their effect must be nil or very low,
since they are necessarily short-lived in the soil under such cOll(litions,
being excellent nutrients for a great many microorganisms. If available
nitrogen is lacking, the organic acids can be utilized ouly by a few nitrogenfixing organisms, and thus are likely to persist longer and exert a greater
effect (Suchting, 1928). Their lifetime can be expected to be particularly
long if the nitrogen level is low and at the Same time acidity is high, since
at least most nitrogen-fixing organisms seem to be little acid-tolerant. Relationships such as those pictured cannot be without importance for stabilizing a mol' condition.
In the more acid mol' forms, most 01' all of the nitrogen-fixing organisms
are probably excluded. Under less acid conditions, nitrogen fixation is
likely to occur. Nitrogen-fixing organisms have, in fact, been found in the
better types of humus layer in the forest (Brenner, ]028), and fixation has
been found to occur during the decomposition of leaf litter but only when
the acidity was not too strong (Olsen, 10:32). On the other hand, more
nitrogen is likely to be lost as water-soluble humus from an acid mol' poor ill
bases, than from a less acid type with more saturated humus. These
losses may be considerable. If Aschan's (1932) figures are correct, Finland
loses 1 kg of nitrogen per ha yearly in this way. This is probably about
one-fourth of the combined nitrogen carried clown by rain and snow.
Finally, nitrogen may he stored away (in the form of a "humus nucleus" 01'
otherwise) in the walls of the "almost indestructible" (Mti.ller, 181m brown
hyphae forming a great part particularly of greasy mar, for so long a time
"that a regime is never reached and so this nitrogen has to he regarded as
lost. All this makes it rather probable that not only the nitrogen level,
but also the yearly supply of nitrogen, averages higher in mull than in
mol', as is in fact indicated by the usually, although not always, higher
productivity of mull soils.
The biological theory here outlined seems, better than do current COllceptions, Lo account for the facts observed, and particularly also for the
very puzzling diversity of conditions favoring mull or mol' formation. The
theory does not minimize the importance of either climate or lime content
of the soil. It is evident that scarcity of lime and a very humid climate
conducive to strong leaching must very generally favor acid-tolerant mol'
types of soil life, and make it morc difficult for a mull type to establish and
maintain itself. At the same time, the theory accounts, better than do
current ideas, for a number of local variations and silvical experiences, such
as the favorable effect of good, even, moisture conditions due to seepage
water (even though very poor in lime) and of soil protection. The theory
also makes it possible for one to understand why 111ull ill poorer regions is
frequently confined to moister situations, while in richer regions it occurs
even on rather dry sites. Furthermore, the theory may settle controversies
such as those between the young P. E. Miiller on the one hand, and Ramann
(1888) and Emeis (1893) on the other, with respect to the soil fauna as a
PROBLEMS OF THE HUMUS LAYER
17
condition for mull formation. The bacterial mull equilibrium can evidently be maintained unuer good conditions, such as a sweet soil and a
mild climate, with less activity on the part of the animal soil life than is
required under less favorable soil and climatic conditions.
A BIOLOGICAL EXPLANATION OF TI-m ACTIVATION EFFECTS
It is shown in a preceding section that current ideas centering RfOUlld the
electrolyte theol'Y must be very much strained to account for the striking
effects on the humus layer abundantly observed as a result of various forestry measures. Biological explanations have been offered as supplementary, particularly for the effects of burning and of green-slash manuring
(Hesselman, 1917 b; Eneroth, 1928; jtt11(!kcr, 1030), but never as a common
explanation of the activation phenomena. It has even bccn stated repeatedly that no biological explanation seemed possible (Ht'ssdman, I!) 17 b:
\)75; Milller, 1924:7(J). In the writer's opinion, this is a fundamental misjudgment. A starting point fol' a biologieal theory is offered by the very
remarkable high nitrogen mobilization in isolated lots of mol' material.
Except for the most acid samples from the poorest types, the nitrogen
mobilization ill stored humus samples from mol' has proved amazingly high,
higher than in samples from better types of humus layer, including mull
(Hesselmau, lO2G). This result is hard to reeollcile with the ecological
cllaracter of the eOl1esponding types of humus layer. Hesschnan (Hl2G:
3:31, 528) suggests, as an explanation of the discrepancy, that nitute nitrogen is a better nutrient than ammonia, This is not borne out by physiological data. The secondary l.mfavorahle influence on certain crops, clue
to the acicl ion of ammonium salts used as fertilizers (Kostytschew, 192(j:
1·15), is an effect belonging entirely to the domain of agricultural chemistry.
It cannot be of any importance in natural soils, where the a111monia is not,
added in the form of ammonium salts but is formed from proteins.
Besselman also hints at the possibility of nitrogen less availahle being
unintentionally determined as ammonia. This would be a very serious
source of error, if nitrogen stored in living hyphae in the 1110r were included
in t.he ammonia determination. Such does not seem to be the case, however, to any gre<lt extent, since t.he tests frequently hnve shown no ammo~
nia present in fresh mar samples.
That the high nitrogen mobilization in most mOl' samples is real, is distinctly shown by cultme experiments with mol' mat.erial (M61ler, 1902,
H103; Mi'tller and Weis, 1900; Gyllenhammar, HI09; Holmgren, 1(11) indicating a very good supply of o.vailabll~ nitrogen in isolated lots of such
material (not only when mixed with mineral soil, as the generalrcsult of
these experiments has repeatedly l)cen misquoted under the influence of the
electrolyte theory).
The very high nitrogen mohilization in most mol' hU111us isolated from
its natural connection cannot well be explained simply by stopped root
competition, since the values not only equal, hut considerably exceed, those
found with mull samples, even though the values are expressed on a humus
basis. There must be a real increase lnllitrogen mobilization as compared
wi th the na tuml stu teo
In fact, stirring, mixing, and shaking has been found to enhance microbio.l activity, not only with various soils, but in general in disintegrated
18
L. G.
ROMELL
solid material where microbial processes are guing un (Petersen, 1871;
Schloesing, 1897; Van Suchtelen, 1(20). Tho cff ect has been explained as
being (lue to improved aeration, but tho fallacy of this explanation was
clearly demonstmted by Schloosing (1897). He suggested instead that
bacteria and unutilized deposits of food may be brought togethcr as a result of the mechanical disturbance (Demolon and Barbier, ]\):il).
The general effect studied by Schloosillg is not sufficient, however, to
account for the higher nitrogen mobilhmtion in mol' samples, since mull
samples would be likely to be affected in the same way. There must be
some factor at work which affects the mol' samples morc than it does the
mull ::;amples. The structural differences between mull and mol' point
clearly to one such factor. It is simply the disruption of natural connections which inevitably accompanies the sampling of a mar. A mar has a
de±lnite and a more or less permanent architecture. Sampling a humus
layer of this type nccessarily means tearing off a great number of roots,
and of mycorrhizal or other hyphae, which unclcr natural conditions hold.
the mass together. Much of this material will be present in the sample
in a state of necrobiosis. Thii.i sampling effect must be incomparably less
pronouncecl with a typical mull, naturally worked by soil animals and consisting of crumbs or grains in loose layering.
The changes brought about by sampling a 11101' imply a radical alteration
of the natural conditions of competition, In addition, the sample is fertilized with material formerly unavailahle as microbial food because it wai.i
living; the sample is "green-manured." As a whole, the sampling effect
could he compared to partial sterilization of soil (Waksman and Starkey,
HJ2a) , which has been shown to heLVe a most ~triking effect in bcech humus
rich in fnngi (Stabl, 1!)00).
The fungus flora of the mol' is atIected by the sampling ill much the ~ame
way as when di~turbed by soil animals. The microbial food made available is "green," relatively rich in nitrogen. When combined, this must
strongly enhance microl)ial activity, and is also likely to change the soil
Hfe mOl'e 01' less in the direction towards a mull type. An abnormally
strong nitrogen mobilization in the isolated sample can be expected, both
from the increased activity, from the ullt1stUllly high nitrogen content of
the fresh raw material, and from the chunge to a more bacterial type of
decomposition which is likely to occur (Waksman, HJ:30;4fl; Waksman
and Starkey, H);31: 94 ff.; Waksman, lOB:2: 4-L9-'150, 744). This reasoning seems to give for the first time a reasonable explanation of the curiously strong nitrogen mobilization in stored mol' humus. It has its princiVal interest, however, as applied to activation effects observed in forestry
practice. Such all application is diredly invited by the close correlation
found (Hesselman, 1926, 1927) between nitrogen mobilization in stored
humus, and the response of the corresponding 11101' to activating forestry
measures.
The measures known to have an activating effect must, in fact, influence
the life of the mol' more or less in the same way as docs sampling. This
seems to be self-evident in the case of all kinds of soil-working. Burning
and cuttings both have the effect of killing of[ great amounts of roots and
their mycorrhizas. There must result both a green-manuring [mel a change
in the competition conditions, such as is explained for the sampling. In the
case of activation by green slash, the green-manming effect is evident.
PROBLEMS OF THE HUMUS LAYER
19
The slash cover also frequently kills off the mar vegetation 11101"e or less,
resulting in an additional green-manuring <tnd in freeing the mol' from the
strong grip of the mycorrhiza fungi belonging to the 11101' vegetation. (This
is discussen further below.) The same points of view may in part be applied to liming practices. Curiously enough, a biological explanation of
the activating effect has earlier been surmised in this very case, where, as
an exception, the electrolyte theory appears satisfact01'Y. Discussing
some experiments with liming of heath soil, M('tller ([ ma) suggested as a
possibility that the principa1r6le of the lime was to check the overwhelming competition fro111 fungi (molds). Similur points of view on the effect
of lime have been expressed more recently (Waksman, 10:30:4F;). Lime
(mfers, however, from other activating agenLs, except burning, 11y its favorahle action being 110t only an indirect one. It also eliminates directly
more or less of the acidity and low base saturation inimical to better types
of humus life. This naturally explains its mpid and strong effect even
though there muy be little 01' no gl·een-mauuring.
The preceding reasoning seems to satisfactorily explain the curiously
similar activation effects caused by very different mensures and disturbances. The explanation offered is in line with the theory of mol' formation given in a preceding section, but independently of t.his theory it suggests itself naturally from observational facts and experimental data. It
is free fro111 the highly hypothetical and partly improbable 01' even disproved assumptions upon which current explanations are based.
No more than current theories, can the present theory foretell the strength
and dumtion of the activati011 dfed. Thus it might be olJject.ed that,
although a green-manuring and a weakening of the fungi must occur as a
result of different activating measures, thcse eiIects may not explain the
activation phenomena actually observed. New research, along lines in(Heated by the biological theory, is needed in order to answer any questions
of a quantitative nature. It is known, however, that thc addition of organic material is one of the most important factors influential in changing
the microbial life of the soil (Waksman, l!X~O: 415). The remarkable effect
of immature plant material, in particular, is well known in agriculture.
Green-manuring has been found, by stimulating tIll' soil life, to mobilize
uitrogen in amounts grC'ater than are contained in the gl'een-manuring material (L6hnis, HJ2G).
Green-manuring of a mol' can be expected to be a part.icularly powerful
factor, under favomrlle circllDl:-Jtances, for two r(:~asons. Not only does it
mean a quite exceptionally heavy manuring of the forest soil, but also it
may induce a change in type of decomposition just Ul-l important as the
pure fertilizing dIed, or more so. As to the latter point, it'seems obvious
that one of the 1110St radical means to secnre a type change would be to lot
the humus formation start from the heginning on a sufficient amount of
fresh raw material, under conditions favoring a bt,tter type of soi1life in its
competition with a present poorer type. Grocn-manuring not only furnishes the raw material, but also f~LV01'S a hettel', more bacterial, more mulllike type of decomposition, as is eX]llained il1 the discussion of the sampling
effcct. In addition, the green-manuring as effected by the different measures known 'Lo activate Ii mor, is almost jnvariahly accompanied by influences directly weakenillK the fungus flora of the mol' and thus further
favoring a change in the direction towards mull.
20
L. G.
ROMELT.
It can be foreseen, from the stanc1poi11:t of the theory, that different forms
of mor will react differently, according to variations in strnctuTe, vegetation, acidity, and other factors. The greatest reactivity can be expected in
humus layers of a decided mar character, but where acidity and other conditions do not too strongly predetermine a mor type of decomposition.
Among the factors Elw1y to interfere with the desirable reaction, one is a
luxuriant mycotrophic ground vegetation. If this vegetation is 110t killed
as a result of the activating measures, its mycorrhiza fungi may offset the
activating ciIect by maintaining a low nitrogen level, and a normal 11101'
condition may soon again be stabili.zed.
The role, in 11101' formation, of dwarf shrubs such as blueberry bushes and
heather, has been the subject of 111ueh controversy. Practical foresters,
basing their judgment on observation and cxperience, have extensively
rcgarded those plants, which are characteristic of many forms of mol', as
harmful forest weeds directly contributing to 1110r formation. The opinion
of :;;oie11tists has been and siill is divided. Recenily, under the influence of
the electrolyie theory, there seem:;; to be an increasing tendency, even
among forcsters, to look upon the dwarf shrubs, like the other 1110r vegetation, as purely secondary, merely indicating a mol' condition. Yet some
foresters maintain the old view in opposition to scientific authorities
(Hassenkamp, 1928).
The present bi010gical theory i:lupports the old interpretation of the blueberry bURhes (Miiller, IS/:(7 :HH, U)24 :lOR) and other dwarf shrubs as actively important in creating or maintaining a mor condition. In doing so,
the theory seems to aecomit for some puzzling facts. One of these is the
opposite effect of clear-cutting in northern Sweden and in the Black Forest
in Germany, mentioned in a preceding section. This difference is paralleled hy an opposite behavior of the blue1>erry bushes (Hesselman, 192(i:
361). In north Sweden, these degenerate and gradually disappear when
fully exposed; in the Black Forest, they luxuriate on clear-cut areas. The
modernists taking this difference as a secondary effect must explain the
progressive mol' formation on clear-cut areas in the Black Forest by poorer
conditions for decomposition there than in Lapland. This is an utterly
improbable assumption. It is more likely that the activation effect is
offset and revcrsed by the mycorrhizas of the luxuriating blueberry bushes
which the SUmmer droughts in the Black Forest are not sufficient to kill,
even on clear-cut areas.
The unfavorable effect of selective diameter cutting 1n north-Swedish
spruce forests iR also accompanied by a greater luxnriance of the blue1>e1'1'Y
vegetation in treated than in untreated stands. This fact may again very
well be the primary one. Apparently the sunshine is not strong enough in
the selectively cut stands to kill the blueberry bushes, although it does
kill them on clear-cut areas in the same region.
In fa.ct, many observations by foresters indicate a very direct harmful
influence of the dwarf shrubs accompanying pOOl' forms of 11101'. One of
the most striking and best-studied instances is the relation between heather
and spruce in the heaths of western Jutland. A", S0011 as the heather closes
over the ground around the young spmce, a characteristic stagnation of
the latter, with chlorosis of the needles, very regularly sets in. This can be
remedied by renewed soil-working, but is likely to repeat itself at any time
until the plantation is closed and the heather is definitely crowded out.
PROBLEMS OF THE HUMUS LAYER
21
The exact reason for the heather injury has been a mystery: "Here we
stand before an x, an unknown factor; it would be of the greatest importance to find out whether it is physical or chemical in nature 01' belongs to
the domain of sol1 physiology" (Mi'tller, 1~97).0 The modern tendency,
most pronounced with Silchting, has been to solve the mystery by denying
the effect, taking the heather as a simple indicator of poor conditions. This
docs not do justice to a wealth of experience from westcrn Jutland, nor, in
particular, to the very interesting exceptions from the general course of
events. One of those exceptions is that the stagnation is not noticed on
hcath land previously farmed, even though thc heather may close completely over the ground (Mflller, lk97).
From the standpoint of the biological thcory, largely developed earlier
by Mi.mer (HlO:3, lOW, HH3) in this particular case, both the generally
observed heather effect and the mentionc(l exception to the rule seem easy
enough to understand in principle, as follows. The heather, through its
111ycorrhizas, lays a dead hand over the huml1s formation, which suffers also
fro111 lack of raw material since the young spruces shed very few needles.
The plant ation suffet·s if the new h ltl1lUs forma tinn domina tcd hy the hea ther
is the only nitrogen SllUl'ce. Yet, if there is an old culture horizon, this
may harbor for a while a 1110re favorable humus life furnishing the necessary
nitrogen to the young trees. The well-known favorable influence exerted
hy pilles (Mil11er, 1nO:3) may be due simply to their furnishing 1110re raw
material for the new humus formation.
The following ohservation, mude by the writer when visiting one of the
lat.e Dr. F. Weis' experimental plantations 011 heath land in western Jutland, apparently illustrates how radically conditions arc changed hy simply
killing t.he heather, without any soil-working. Outside the plantation,
some straw used as packing material had been left in a pile on the raw
heath. The straw had settled considerably and had taken a grayish color,
but it appeared very little decomposed. Yet it had been there long enough
to kill the natural heather vcgctation which it covered. A st.rong reaction
was the result, as was evidcnr.erl by tall. luxurilltlt, flowering specimens of a
nitrate plant, Chamael1erion <lllgustijo[£1I11l, breaking through the straw
cover.
The abundance and vitality of dwarf shrubs is very likely a factor of
general importance in influencing the reactivity of different forms of mar.
The commoner occun'enCe of Vaccinia or Callunn in the flora of most of the
heavy forms of European mol' may be a clue to the ttnderstunding of their
much lower reactivity as compared with American mol' forms of a more or
less greasy type, even extremely heavy aud acid ones. The more hel'tmceotls vegetation of the latter, with Vaccinia and similar dwarf shruhs
missing 01' playing a subordinate r61e, cannot dominate the 11"101' by itl:)
mycorrhiza hyphae to nearly the same extent as does a blueberry or a
heather vegetation. Thus the felling of a tree means a relatively more
extensive weakening of the symbiotic fungi in the surrounding mar. An
activation effect due to green-manuring will be easier to start and will not
so easily be offset by remaining mycorrhiza fungi as in rl.105t European
forms of heavy mol'.
'Transl(ltion from the origina.1.
22
L. G.
SILVICAL APPLICATIONS
Ol~
ROMELl,
THE BIOLOGICAL THEORY
The theory which the present paper cuntributes, like the older ones which
it supplements, will be useful in eorrelating :-;i1vical experiences and in
guiding silvical thought rather than by furnishing directly new practical
recipes. It suggests, however, experimentation alung partly new lines,
and, if further corroborated by experience, it should inHuence forestry
practices to some extent. Significant Hew experimental evidence can best
he ohtained in the [on.~st. The d'fect of killing a luxuri<tnt mar vegetation
should be studied. New methods of soil preparation, effeeting a trenching
rather than a soi1-worki ng in the ordinary sense, should be tried for experimental purposes.
In j1..1dging the experirnental results, it will in most cases be difficult to
distinguish between the effects (liscussed in this paper and the effect of
lessened root competition in the ordinary sense of that expression. From
a practical point of view, this is of little importance in extJcrimenting with
different activ[Ltion measures. In other practices the case may be different, as in judging the most fnvorable space of planting under different
conditions. If root competition between the trees themselves is the more
important fodor, a wiele spacing would. be generally indicated on dry and
poor soil, as actually recommended by Si.'tchting (1929 ",,1931). If thoinfluence of the symbiotic fungi in one direction, ancl of green-manuring in the
other, is particularly important, as suggested by the biological theory, close
planting would be indicatcd in many cases, even on poor soil, in order to get
quickly a closed stand and crowd out a harmful ground vegetation. Close
planting followed by frequent thinnillgs would, in general. be expected to
yield a beLter result than would wide spacing from the beginning. Contrary to current ideas un root competition, the new theory distinctly
militates against the very wide spacing ill forest planting recently advocated in this country. Only in certain cases-as, for example, if the site is
very dry and poor and yet does not tend to be inv:tded hy a hm:mful
ground vegetation-can a wide spacing be expected to be bettcr.
Probably the principal practical use of the biolugical theory, if it proves
to be sound, will be in helping to find new, cheaper, or hetter methods of
soil preparatio1l for use in fOl'estry practice. If the most important effect
0:[ soil-working and burning is to kill off the mol' vegetation, these measures
may advanta.geously be replaced by cheaper or less hazardous methods, as,
for example, by some spraying technic.
One achievement of the theory is to explain the remarkable soil etiect
of the modern frequent and strong Danish thinnings, as practiced particularly on Frijsenborg by E. Moldenhawer. The amazing results obtained,
with primary conditions of soil and climate not particularly favorable, are
a challenge to the forestry profession in general. From the standpoint of
the biological theory, it can be hoped that the Danish exporienceg will
have a rather general application in principle, if the thinnings are supplemented, where needed, by some suitable treatment controlling a harmful
ground vegetation. Thus the theory encourages foresters in other countries to try to meet the Danish challenge, and it may help them to attain
similar apparent site-improvement effects and remarkable yields where
conditions permit the use of intensive methods.
PROBLEMS Ol~ THE I-IUMtTS LAYER
23
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Memoir HJ4, Aquatic Diplera. Pari I. Nemocera. Ex</u,ive of Chi"o'101I!idae and CeratopogOllidac. the
preceding number in this series of pub1i(·ations. was mailed on November 20, 1934.
Memoir Hi5, For.,e·T-YPe Studies in the Adi,'ondack Region. was mailed 011 Janua.ry Hl, 19~.i.
Memoir 1M, Acidity. A,.lacid Buffering. aml Nfltrimt COIII.llt of Jlorest Liller ;11 Relation to [fum,.s and
Soil, was mailed on October 20, lIJ34.
Memoir 107, Faclorlii A:DecUnfl: Ihe Dt.elopmelll of ehe Cotyled01!"'Y BlIds of liz. COIIIIIIOII Bem!. Phasen/fI'
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Memoir 158, Studiesc! I)M Eil~ctj of Storage Temporatltre 0'1 the Propagatiol! Value o! Potato 1't,b,",',
was mailed on November 14. 193+.
Memoir HlO, A Physi%Rica/ SIully of Dorma/lcy i" Tilia Seed, was mailed on November IlB, 1034.
si~th