24 BARRATT: MULL AND MOR DEVELOPMENT IN TEMPERATE GRASSLANDS

FACTORS OF MULL AND MOR DEVELOPMENT

IN TEMPERATE GRASSLANDS

BERYL C. BARRATT

Soil Bureau, Department of Scientific andIndustrial Research, Taita

DEFINITION OF MULL AND MOR

The humus forms of soil profiles have beendefined as the group of A horizons in whichorganic matter is concentrated (Barratt 1964).They are generally the first parts of soil profilesto respond to changes in soil-forming factors andthey are particularly worthy of study in thetemperate grasslands of Great Britain and NewZealand where grasslands have largelydeveloped on former forest soils.

Grassland humus forms have been investi-gated morphologically in the field and withthe microscope (Barratt 1960, 1964). Theyare divisible into two main groups: mulls andmors. In mulls, organic residues are finelyconllninuted and intimately mixed with thecolloidal-sized fraction of the mineral soil. Inmors, organic residues accumulate predomin-antly above the mineral soil surface, but showprogressive decomposition with increasingproximity to the mineral soil, and typicallythree layers are produced.

L layer of little-decomposed leafy resi-dues.

F layer of fragmented or partially de-composed but still recognisable resi-dues.

H layer of decomposed and finely com-minuted organic material generallywith a sharp boundary over themineral soil.

Intergrades termed mor-like mulls are recog-nised where mulls are very high in organicmatter and low in clay, so that they are diffi-cult to distinguish from mors in the field. Theirtrue nature can be determined microscopically.

Peat develops by accumulation of organicresidues to form virtually the whole of the soilprofile. It is generally distinguished from morby its great thickness, but another characterthat generally distinguishes pent from mar isits weak humification in lower layers (Taylorand Pohlen 1962).

FORMS OF MULL AND MOR UNDER GRASSLAND

Mulls and mors are further subdivided on abasis of horizon thickness, structure and con-sistence into a number of forms, each with oneor more characteristic microstructures. Theseforms are conveniently placed into four groupsin relation to the degree of litter breakdownand incorporation:-

A. Strongly Granular, Weakly Granular andMassive Mulls in which intimate organicmatter is incorporated with the mineral soilin thick At" horizons.

B. Fine and Morlike Mulls in which intimateorganic matter is concentrated largely to-wards the surface of the mineral soil.

C. Massive, Matted and Granular Mors inwhich Land F horizons are thin and com-minuted organic residues in thick H hori-zons, overlie the mineral soil.

D. Laminated Mors in which Land F horizonsare thick and contain little-decomposedorganic residues resting upon H horizonsthat are generally thin.

MICROSTRUCTURES OR FABRICS

Each of the horizons or layers of the humusform may have a different microstructure orfabric (Fig. 1). The five most commonlyencountered fabrics in Great Britain and NewZealand with specific variants in highly cal-careous or waterlogged soils are:

1. Strong mull humus. a continuous fabric ofworm-cast origin with irregular cavities. Itis dark-coloured with an intimate associa-tion of clay and decomposed organic matter.

2. Weak mull humus, a continuous fabricwith sharp-walled unconnected cavities. Itis light in colour because of a low contentof organic matter intimately associated withthe clay.

* For an explanation of soil horizon nomenclature seeTaylor and Pohlen (1962) p. 69.

CHARACTERISTIC HORIZON MICRO-FABRICS

Strong mull W~.~ mul< Mull. Io~.

humus humus mod.r

S,hu oc R.,whumus 81~«h.d A1

reod"",, m"d~r

Th," 'C"" P'"81",,,,,,os p",,", roc~

(A:C''''I)

BARHATT: MULL AND MOR DEVELOPMENT IN TEMPEHATE GRASSLANDS

STRONGLY

GRANULAR

MULL

WEAKLY

GRANULAR

MULL

"fiNE &

MOR.lIKE

MUllS

MASSII/E &

GRANULAR

MORS

FIGURE 1. Diagrammatic representation ofhumus forms under grass, showing their charac-teristic micro-fabrics.

3. Mull-like moder, of discrete loosely-aggre-gated faecal pellets, generally 0.15-0.2 mm.in diameter, resembling those of enchy-traeids, but coarser in some samples. Itotherwise generally resembles strong mullhumus but may contain a very high propor-tion of finely comminuted organic matterin association with the clay.

4. Silica moder and rendzina moder. Silicamoder is generally composed of smallblackish..brown organic faecal pellets, lessthan 0.1 mm. diam., such as those producedby Acarina (mites) and Collembola(springtails) but coarse moders with dis-crete pellets up to 2.0 mm. long are alsoencountered. Plant fragments are bitteninto cavities and are held loosely in themass, as are mineral grains, which arepredominantly quartz. In rendzina moderthe fabric is similar but calcite grains occurin place of quartz.

5. Raw humus consists of little-decayed leafyresidues and in places cells have orange-brown resin-like contents which appear to

25

be very resistant to decomposition. Insome samples the residues show evidenceof attack by fungi. (For illustrations ofmicrofabrics see Barratt 1964.)

OF GRASSNATUHE AND OCCURRENCE

HUMus FORMS

A. GRANULAR MULLS

1. Strongly Granular Mulls

These are characteristic of dark brown top-soils rich in organic matter, slightly to ulOder-ately acid, pH 5.5-6.0, with strongly granularto crumb structures and deep penetration of thelower boundary into the subsoils along earth-worm channels. The microfabric of their Athorizons is predominantly strong mull humus.They occur under topdressed pastures of

moderately leached southern and centralyellow-brown earths in New Zealand and brownearths in England. However, where parentmaterials are highly calcareous these mullsextend up to the cool humid zone of blanketpeat. They support fleshy, nutrient-demandingswards that usually include clover and havemoderate to high grazing value. Below groundthese swards have deep freely branching roots;active populations of burrowing earthwormsare supported which help to promote the deeporganic cycle.

2. Weakly Granular Mulls

These are generally of light brown colourand weak structure, strongly to moderatelyacid, "H 4.9-5.9, with little penetration intosubsoil horizons. The microfabric of their Athorizons is characteristically a weak mullhumus.They occur in brown-grey and yellow-grey

earths, the topsoils -of which become dry insummer. In Great Britain they have beenfound in experimental lowland hayfields thathave been cropped continuously without addi-tion of fertilisers for up to 100 years.In soils with this form grass yields are low

and rooting systems are correspondingly weak;little organic matter is supplied to the soil inthe form of plant residues and the organicregime is only weakly impressed on the mineralsoil.

B. FINE AND MaR-LIKE MULLS

Characteristically, these humus forms arerather thin, dark brown in colour, almost blackin the mor-like mulls, and have a finelygranular structure. They overlie horizons

LAMINATEO

MO'

HUMUS fORMS

26 BARRATT: MULL AND MOR DEVELOPMENT IN TEMPERATE GRASSLANDS

that are lighter in colour and coarser in struc-ture. In these An horizons the microfabricsare generally mull-like moder, generally under-lain by weak mull humus in the A" horizons.They occur under native tussock and pas-

tures of the upland and high-country yellow-brown earths of New Zealand. In the Britishuplands they have been found over brownearths from andesite and from strongly acidleached calcareous soils, pH 4.5-5.0. Theyalso occur over hard chalky limestone with anear-neutral pH of 6.7 in Southern Englandwhere the thin topsoil dries out in summer.They also form in the above mentioned hay-fields with a long-term manurial treatment offarmyard manure but are there underlaid byhorizons v\lith strong mull humus microfabrics.The soils generally support a somewhat

fibrous herbage including bent (Agrostis) andfescue (Festuca) species of rather low grazingvalue and with a tendency to form a shallowroot mat. Few earthworms oCCur in thesehorizons but enchytraeid worms and arthropodlarvae are very active.

C. MASSIVE, MATTED AND GRANULAR MORS

In all of these forms little litter remainsunfragmented and their thick black H horizonsmay have massive to medium granular struc-ture, with silica moder or rendzina modermicro fabrics.They occur in soils liable to become either

very wet or very dry, such as the podzolisedsouthern yellow-brown earths, or in peats thatare beginning to decompose after drainage, orin skeletal soils where the thin black soil pene-trates between fissures in the rock. These soilscan be extremely acid, pH 4.0, as in the podzolsor neutral to alkaline as in skeletal soils overlimestone.The vegetation may contain fibrous, acid-

tolerant grasses such as Nardus stricta (mat-grass) over the podzols, or prostrate mosses andcushion plants over the skeletal soils. In thesesites the only fauna capable of tolerating thelimiting soil conditions are surface-feedingforms adapted to the habitat, such as mites(Acarina) with their chitinous exoskeletonsand springtails (Collembola). Where condi-tions are not so limiting coarser moders con-taining arthropod larvae and s_mall earthwormstend to occur.

D. LAMINATED MORS

In these humus forms organic residues, pre-dominantly leafy, accumulate at the soil sur-face to form thick Land F horizons, extremely

,

acid, pH 3.5-4.5, with raw humus microfabrics,that generally overlie thin H horizons withsilica moders. The H horizons pass in somesites to mull-like moders and weak mull humusfabrics in A" and A" horizons, or bleachedmineral soil in A'J, horizons.They characterise untopdressed British up-

lands with their cool moist climate but arealso found in acid and gleyed soils of low-lands. They have been induced in hayfieldsby long-term treatment with acid ammoniumsulphate on soils where mulls develop in theabsence of the acid treatment. In New Zea-land they have so far been found under pas-tures on the coarse~textured pumice soils suchas Taupo sandy loam where few earthwormsare present to mix the soil.The humus forms develop generally under

fibrous acid-tolerant grasses such as matgrass(Nardus stricta) with their roots in the humuslayers, so that a shallow organic cycle isfavoured. As their micro fabrics might indi-cate, they are almost devoid of fauna in theirLand F layers and the little decompositionoccurring appears to be fungal, although meso-fauna may be active in the H and An hori-zons.

DEVELOPMENTAL FACTORS

Humus forms are part of the soil profile andare thus subject to the same soil-forming factors,set out by Taylor and Pohlen (1962) as:Climate, topography, soil parent materials,organisms (including the vegetation) and ageof the site (including modifications by man).Condensation of the organic cycle towards

the soil surface and the transition from mullto mar are associated with definite trends insite factors.

Climate and TopographyThere is an increasing tendency for mor

development" as climate becomes cooler andmore humid throughout the year with increasedaltitude and latitude, and this is well shown inuntopdressed British uplands below the zoneof blanket peat (Fig. 2a). At the lowest alti-tudes granular mulls are found over brownearths of high base status. but with rising altiMtudes these pass progressively to fine, mattedand leafy laminated mulls and laminated marsover podzolic brown earths and podzols. Cromp-ton (1958), Stapledon (1936). In New Zea-land, with its wide climatic range and widerange of zonal soils, all gradations from weaklygranular muns to laminated mars occur (Fig.2b).

BAltRATT: MULL AND MOlt DEVELOPMENT IN TEMPERATE GRASSLANDS

FIGURE 2a: Idealised diagram showing thebroad distribution of humus forms in relation toclimate, soils and vegetation, in Great Britain.

<i

FIGURE 2b: Idealised diagram showing thebroad distribution of humus forms in relation toclimate, soils and vegetation, in New Zealand.

Soil climate is severe at alpine elevations, atsubantarctic latitudes and on skeletal soilslopes, so that plant growth and chemical rockweathering are minimised. Soils survive windaction mainly in the protection of crevices, andthey are thin, with massive mors. In the drierclimate of the brown-grey and yellow-greyearths, by contrast, organic activity is sup~pressed more than rock wea thering andaccumulation so that the organic cycle has arather weak effect on the mineral soil. Thisis reflected in their weakly-developed mulls.

Parent Material and Drainage

In many places the broad climatic sequencefrom mull to mor is greatly modified by parentmaterial and drainage. On lirne~rich parentmaterials mulls have been found almost up tothe blanket peat zone in the northern Pennines;conversely, in uncultivated wastelands or heathsof the lowlands, laminated rnors have beenfound over acid, coarse~textured and badlydrained profiles.

Vegetation

In England hill soils which now carry untop-dressed pasture once supported oak (Quercus)and birch (Betula) forest over brown earthswith mull. However, under grass, with itsshallower organic cycle, there is an inevitabletendency towards mor development. This isaccentuated with rising altitude, or on poorersoils, by changes in sward composition. Thesward becomes progressively more fibrous,lower in feeding value, shallow-rooting andacid~tolerant in a sequence from bent(Agrostis) with Festuca ovina over the brownearths, to matgrass (Nardus) or commonheather (Ca/luna vulgaris) over podzols andflying bent (Molinia caerulea) on the morepoorly drained sites (Stapledon 1936). In thealpine and skeletal soils however, prostrate andcushion plants are characteristic, with lowherbage production and thin soils tending toform under the little protection afforded by thevegetation.

In much of New Zealand, by contrast, grassesseem to be promoting mull in son1e soils thatformerly supported mor-forming forest. In thealpine and subalpine tussock country of theTorlesse range of Canterbury, for example,Molloy (1964) has shown buried beech forestprofiles with mor where fine mulls have sincedeveloped under tussock. In North Auckland,original kauri mors now appear as mflssive

27

28 BARRATT: MULL AND MOR DEVELOPMENT IN TEMPERATE GRASSLANDS

mulls under grass, although their mixed com-minuted raw humus with bleached clay micro-fabrics give evidence of their former condition.

Soil Organisms

Soil organisms occur in a definite sequencefrom mull to mor that is closely related to themicrofabrics they help to produce. Stronglygranular mulls are characterised by an activepopulation of burrowing earthworms (VanRhee 1963) such as Allolobophora longa, A.caliginosa and Lumbricus terrestris whose needfor adequate moisture, and association withfleshy herbage and lime have been shown bySatchell (1955). Smaller and shallower-working fauna such as small earthworms e.g.LumbriClls rubellus (Lee 1959), arthropodlarvae~ potworms (Enchytraeidae), mites andspringtails, promote finely-granular mulls, roOT-like mull intergrades and massive mors. Inthese, conditions are too acid, periodically toowet or too dry, or textures too coarse to sup-port burrowing earthworms. Under the moreacid and generally wetter conditions inlaminated mors with a fibrous herbage of lownutrient value, only fungi appear to be active.Comparisons of mull and mor have showngreater microbial activity, as measured by C01evolution, and higher bacterial numbers in mull(Kirkwood 1964).

Time and Modifications Caused by Man

Generally speaking, in the course of timethe tendency in temperate latitudes is for soilsto become leached with the eventual develop-ment of podzols with mor humus. Man's actionis to retard or accelerate this process throughhis influence on the soil-forming factors.By replacing some of the mor-forming

forests by grass, man is promoting mull. Thisis probably because of increases in soilmoisture, ground flora and soil faunal activitywhen the canopy is opened. In most districts,however, the shallower organic cycle lnain-tained under shallow-rooting grasses tends topromote lnor.This is offset by topdressing in most parts

of New Zealand and in the intensively farmedBritish lowlands. However, in the unfencedcommon grazings of the British uplands andmoorlands or heaths, soils which once mainlysupported oak forest are becoming leachedunder a fibrous herbage of low grazing value.Forest regeneration in these rough grazings isprevented by light grazing at rates of the order

of one sheep to five acres. On the poorest soilsCalluna heath is maintained by repeated firingfor grouse shooting and sheep grazing.

These upland and heath soils support morhumus and exhibit podzolisation in variousstages, but examination of the soil profiles inJnany districts reveals the original forest orcultivated mull humus form beneath the pre-sent mor humus (Crompton 1953). Theoriginal forest brown earth profile has alsobeen revealed beneath Bronze Age and morerecent earthworks in various parts of the coun-try now occupied by heather podzols (seeDimbleby 1962).

In many places the trend from mull to morappears to be reversible. This is shown by thesharp contrast between swards within sheltersor topdressed paddocks close to farm buildingsand surrounding moorland or heath. In suchplaces close-grazed swards of better grazingvalue with their associated mull humus formscontrast with poorly grazed moorland speciesof low grazing value and their associated morhumus forms.

CONCLUSION

Humus forms can give a rapid and reliableindication of biological changes occurring inthe soil. When these changes are unfavour-able they can be reversed before the profileas a whole becomes seriously affected. Thisstudy shows that neither vegetation nor anyother factor should be considered in isolationwhen attempting to account for mull and mordevelopment. All five soil-forming factorsmust be considered when assessing their rela-tive importance at a given site.

REFERENCES

BARRATT. B. C., 1960. An investigation of the morphologyand development of some grassland humus forms.Ph.D. thesis. Univ. of Durham.

BARRATT, B. c., 196+. A classification of humus formsand microfabrics of temperate grasslands. J. SoilSei. 15: 34-2-56.

CROMPTON, E., 1953. Grow thE': soil to grow the grass.Agric. 60: 301-8.

MARPLES: ECOLOGY OF CANDIDA ALBICANS

CROMPTON, E., 1958. Hill soils and their productionpotential. J. Brit. Gr{J$Sl. Soc. 13: 229-37.

DXMBLEBY,G. W., 1962. The development of Britishheathlands and their soils. Oxford Forestry Memoir23. Clarendon Press.

KXRKWOOD,R. C., 1964. A summary of factors causingmat formation of reseeded pasture swards. J. Brit.Gr{J$sl. Soc. 19: 387-95.

LEE, K. E., 1959. The earthworm fauna of New Zealand.D.S.l.R. Bull. 130. Govt. Printer.

MOLLOY,B. P. J., 1964. Soil genesis and plant successionin the subalpine and alpine zones of Torlesse Range,Canterbury, New Zealand, Part 2. Distribution,characteristics and genesis of soils. N.Z. J. Bot. 2:143-76.

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SATCHELL, J. E., 1955. Some aspects of earthwormecology in Soil Zoology, ed. D. K. McE. Kevan, p.180-201. Butterworths.

STAPLEDON,R. G., 1936. A survey of the agriculturaland waste lands of Wales. 1st edition. Faber &Faber.

TAYLOR,N. H., and POHLEN, 1. J., 1962. Soil surveymethod. Soil Bur. Bull. 25. N.Z. Govt. Printer.

VAN RHEE, J. A., 1963. Earthworm activities in thebreakdown of organic matter in agricultural soilsin Soil Organisms, ed. by J. Doeksen, and J . Van clerDrift 1st edition. North Holland Publishing Co.

SOME OBSERVATIONS ON THE ECOLOGY OF CANDIDA

ALBICANS, A POTENTIAL MAMMALIAN PATHOGEN

MARY J. MARPLES

Department of Microbiology, University of Otago

INTRODUCTION

Candida albicans is a yeast-like organism,belonging to the Family Cryptococcaceae. Itsnormal habitat is found in the alimentary tractof warm-blooded vertebrates, where it lives asa budding yeast on the mucosal surface and inthe mucosal secretions. The organism is, how-ever, an opportunist pathogen. When altera-tions occur in its environment, through changesin the host tissues or in the microbial com-munity exploiting them, Candida albicansbecomes invasive and penetrates the livinglayers of the substrate. This invasiveness isassociated with a change in morphology, sothat branching filamentous hyphae to a greatextent replace the budding yeast-like form.

Although C. albicans must be constantlyinoculated onto the skin and its appendages itonly rarely establishes itself in this habitatand, when it does so, usually behaves like apathogen. In the following pdragraphs onlythe colonisation of the skin is considered. Theresults of invasion of the intestinal wall andthe deeper organs are much more serious forthe host, but the factors involved in the

pathogenesis of systematic candidiasis are too. complex to be summarised in this paper.

HEADQUARTERS AND RANGE

Candida albicans is found as a member ofthe normal flora of a large number of mammalsand possibly of birds, and it seems reasonableto designate the alimentary tract of mammalsas the headquarters of this yeast. Van Uden(1960) has divided the intestinal yeasts into 3groups:(1) obligate saprophytes(2) facultative saprophytes( 3) passers-byHe includes C. albicans in the first group,

defined as organisms which have a naturalhabitat exclusively in the mammalian gut.Table 1 shows the distribution of C. aZbicansin the intestinal tract of some New Zealandmammals, other than man. It is clear that anumber of mammals can be carriers of thisyeast. It may not however be present in thealimentary tracts of all species, for Parle (1957)failed to recover it in samples taken from cows,guinea pigs, mice or rats.