Soil Organic Matter and Biological Activity || Introduction Soil Organic Matter — A Perspective on its Nature, Extraction, Turnover and Role in Soil Fertility

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    D. VAUGHAN and B.G. ORD

    The Macaulay Institute for Soil Research, Aberdeen, Scotland.


    1. SOIL ORGANIC MATTER 1.1. Definition of terms 1.2. Early historical perspectives 1.3. Early twentieth century extraction procedures 1.4. Current extraction procedures 1.5. Nature of humic substances




    5. REFERENCES ..

    4 4 6 8

    11 14





    D. Vaughan et al. (eds.), Soil Organic Matter and Biological Activity Martinus Nijhoff/Dr W. Junk Publishers, Dordrecht 1985

  • 2


    "No man is an island, entire of itself; everyman is a piece of the

    continent, a part of the main" - John Donne (1571-1631).

    Soil organic matter is one of our most important natural resources

    and from antiquity man has recognized that soil fertility may be

    maintained or improved by adding organic manures 4 It has been

    calculated that the mass of organic carbon in soil organic matter

    (30.1 x 1014kg ) exceeds the total of all other surface carbon reservoirs (20.8 x 1014kg ) found in the world13 . Indeed, soil fertility is dependent on its organic matter content92 which may vary from less than

    1 per cent in young soils and those exhausted by over intensive 120

    cropping, to as much as 95 per cent in some deep peats Hence it is

    hardly surprising that this ubiquitous soil constituent has received

    considerable attention from many scientists of different disciplines.

    As with many other areas of research, the progress made in one

    discipline may be limited by the current techniques, knowledge, or even

    dogma, of another. Thus in the era before the concept of photosynthesis

    it was maintained that humus provided the only carbon source for plants,

    and carbon was supposedly taken up by plant roots in an elaborated form.

    Unfortunately this view continued well after it had been established

    beyond all reasonable doubt that plants could fix atmospheric carbon

    dioxidell ,12,106. But misconceptions, although not so great, continue

    to this day. This is exemplified in the concept of "organic farming".

    The proponents of this concept have quite correctly realized the fact

  • that soil organic matter plays a vital role in soil fertility.

    Unfortunately this concept has been taken to the extreme inasmuch as

    the use of "chemical fertilizers" is considered to be detrimental120

    This is reminiscent of the now settled arguments of the" "Humus" versus

    "Mineral" theories of the last century (see chapter 1). The fact is that although soil organic matter is essential for the maintenance of

    soil fertility, crop yields can only be improved dramatically to the

    levels now required to feed an increasing world population by adding

    chemical fertilizers (see chapter 12 ). On a world-wide basis we cannot depend solely on the use of organic fertilizers because

    insufficient are available for the necessary crop production.


    Today, our understanding of the chemistry of soil organic matter,

    and the manner in which it influences soil fertility, is far from

    complete. We do, however, know that is contains some 60 per cent of

    humic sUbstances45 , the importance of which have been recognized since

    the last century. Fortunately, we are now spared most of the

    antiquated terminology which persisted until comparatively recent times.

    Any student, or research worker, entering the complicated subject of soil organic matter for the first time will be confronted with a vast

    and often confusing literature. Much of the available data compares

    like with unlike, or omits adequate controls, so that it is often

    impossible to make direct comparisons between the results from

    different laboratories. All too often the results of laboratory

    experiments are extrapolated incorrectly to field situations. It would

    be too contentious to site examples, for most of us are not entirely

    blameless. The aim of this book is to present soil organic matter from

    the viewpoint of the biologist, a viewpoint which is often sadly

    neglected, particularly in studies related to humic sUbstances92 .

    However, because of the complex nature of the subject, the inclusion of some chemical data and concepts are not only inevitable but highly

    desirable. The considerable literature available necessitates that

    only a small, but hopefully representative sample, of references are

    included in this book. The diversity of the subject leads inevitably to different approaches to research being taken by researchers from

    different disciplines. This diversity is also reflected in the manner

    in which the authors have approached the writing of their individual


  • 4

    This introductory chapter is designed to place into perspective

    those aspects of soil organic matter which are individually dealt with

    more thoroughly in later chapters. By necessity, however, it also

    'considers briefly the preparation, nomenclature and nature of humic

    substances which are referred to frequently throughout the book. For

    more detailed information on the formation, stability and chemistry of

    these humic materials, the reviews of Kononova68 , perssonlOO ,

    Schnitzer and Khanl12 , Bonneau and souchier14 , and stevenson127 are



    1.1. Definition of terms

    Strictly speaking the term soil organic matter should encompass

    all the organic components present in the soil. Such a notion

    presents obvious difficulties when considering living organisms,

    particularly the macrofauna or roots of higher plants. The difficulty

    in producing an adequate definition for soil organic matter to satisfy

    all the authors contributing to this book, let alone all scientists,

    stems from the heterogeneous nature of organic matter found in soil, its

    origin and state of decomposition. In fact as MCKibbin87 emphasized

    over 50 years ago "in an average soil there will be found organic

    matter existing in an almost infinite number of stages of decomposition".

    Because soil organic matter is composed of broadly three fractions 92

    (a) the living microflora and fauna that constitute the biomass, (b) fresh debris and readily decomposable material and (c) the humic substances which are fairly resistant to biological attack, most

    definitions of it are a somewhat arbitrary and subjective compromise. stevenson127 defined soil organic matter as the total of the organic

    components in soil exclusive of undecayed plant and animal tissues,

    their "partial decomposition" products (the organic residues) and the soil biomass (living microbial tissue). He used the terms soil organic

    142 matter and humus interchangeably as suggested by Waksman These

    definitions are perfectly acceptable, but because of difficulties

    encountered in removing plant remains, particularly fine roots, from

    the soil prior to analysis we have preferred to use the term soil

    organic matter in a more practical sense to encompass all the organic

    components in the soil including the biomass but excluding the

  • macrofauna and macroflora. Like Stevenson, we use the terms soil

    organic matter and humus interchangeably.

    Humus is sometimes classified into two broad types which are

    characteristic of the environment in which they are formed. Mull

    humus is characteristic of Cambisols and Rendzinas while mor humus is

    produced where acidity inhibits humification such as in Podsols. It

    is now generally agreed that humus can also be conveniently divided


    into two groups designated as non-humic and humic substances 35 ,68,I08,127.

    The non-humic group comprise organic substances which are well defined

    chemically, are usually uncoloured, and which are not unique to the

    soil. Most of these substances are simple, low molecular weight

    compounds which can be utilized as substrates by soil microbes and as

    such may have a transient existence in the soil. Among these substances

    may be found carbohydrates, hydrocarbons, alcohols, auxins, aldehydes,

    resins, amino acids and aliphatic and aromatic acids. Gases such as

    ethylene and hydrogen sulphide may also be present. In contrast, the

    humic substances comprise yellow to black coloured, relatively high

    molecular weight polyphenolic components which are distinctive to, and

    synthesized in, the soil.

    Humic substances can be very stable and may persist in agricultural

    soils with a mean residence time (MRT) of over a thousand years 22 ,23,59,I07 There is no certainty,however,that they remain unaltered during chemical

    extraction, but the method of extraction is used to distinguish three

    classes of humic materials:- (a) Fulvic acid which is soluble in -----

    alkalis and acids, (b) Humic acid which is soluble in alkalis but insoluble in acids and (c) Humin which is insoluble in both alkalis and acids. Alkaline extractions followed by acidification do not of course

    result in a clear-cut fractionation of humic substances because the

    solubility of a given fraction can vary with experimental conditions.

    In addition each humic fraction will contain non-humic compounds

    having the same solubility because the extraction procedures designed

    for humic substances will also release non-humic material. The

    problem then arises as to when an apparently non-humic substance is an

    integral part of the humic substance or merely an "impurity". In this

    context Nannipieri and sequi92 have emphasized that fresh organic

    carbon added co soil appears in the humic fraction before humification

    could possibly have occurred.

  • 6

    To understand some of the difficulties encountered in applying

    precise definitions to the organic constituents of the soil it is

    essential to consider the manner in which the extraction procedures

    have evolved. The word evolved instead of planned is used quite

    deliberately because in many ways biologists are still limited in their 92,100

    thinking by the traditional extraction procedures

    1.2. Early historical perspectives

    The first extraction of humic substances is usually accredited to

    Achard (1786)1. He extracted peat with an alkaline solution which on acidification yielded a dark, amorphous precipitate which subsequently b k h ' 'd 1 1 l' 141 , ecame nown as umlC aCl . E even years ater, Vauque lne uSlng alkaline solutions extracted humus-like substances from elm wood

    infected with fungi. On acidification these extracts produced a , , , h 11 d l' b Th 135 822 preclpltate whlc was ca e u mln y omson In 1 ,

    .. 28 --Dobereiner names the dark coloured component of the soil organic

    matter humussaure or humus acid, although in 1837, wiegmann145

    referred to a similar fraction of peat as humic acid or ulmin. At

    this time the terms humus acid, humic acid and ulmin were used

    synonymously thus introducing confusion especially between the two

    former terms. Indeed it was not until 1936 that waksman142 emphasized

    that humic acids referred specifically to the precipitate formed after

    acidifying alkali extractions of soils, whereas humus acid comprised

    the whole alkali soluble material.

    During the first part of the nineteenth century considerable

    attention was paid to the chemistry of humic substances. A detailed

    description of the analysis of humic acid with particular reference

    to its acidic properties was given by sprengel125 ,126 One of his

    innovations still in vogue today was the pretreatment of soil with

    dilute mineral acid prior to its extraction with alkali. Sprengel also

    observed that after removal of all the mineral acid, freshly

    precipitated humic acid partially dissolved in cold water, but after

    drying, the humic material was converted into a less water-soluble

    form for which the term humus coal was proposed.

    The first serious attempt at classifying humic substances was that

    of BerzeliuslO ,11,12, who suggested two main groups depending on

    solubIlity in alkaline solutions. Thus humic acid was soluble whereas

  • mull coal, later called humin, was insoluble. Later, Berzelius added

    a third group consisting of crenic and apocrenic acids which were

    obtained from mineral waters and slimy mud rich in iron ochre. The 1 "f" "b I" d d b h" "lId 88,89,90,91 c ass~ ~cat~on y Berze ~us was exten e y ~s pup~ Mu er


    who took into consideration colour in addition to solubility in water or

    alkali. This classification may be represented as: (a) brown bodies comprising either alkali-soluble ulmic acid or alkaline-insoluble ulmin,

    (b) black bodies comprising alkaline-soluble humic acid or alkaline-insoluble humin and (c) water soluble crenic and apocrenic acids. Although Mulder's contribution was important his adherence to the view

    that humic substances were chemically discrete molecules was unhelpful

    to the further development of the subject. But the ultimate in unhelpfulness must surely go to the

    51,52 classification devised by Hermann , He, in common with his

    contemporaries, divided humic substances into" three main groups

    depending on solubility but each group was then sub-divided to give

    an array of bewildering terms (Table 1). Let us consider how Hermann arrived at the classification of humins which he based on nitrogen

    content. Anitrohumin was obtained artificially from sugar and hence

    was nitrogen-free whereas nitrohumin was produced from the soil and so

    contained nitrogen. On the other hand, nitro lin was obtained from wood

    and contained as much as 12% nitrogen. At least Hermann considered

    that humic substances could contain nitrogen, a view which was not

    universally accepted at the time. However he persisted in giving

    specific formulae for most of the substances in his classification.

    Table 1. Hermann's classification of humic substances.

    Name of fraction

    Humic acids - anitro-humic acid, sugar-humic acid, ligno-humic acid, meta ligno-humic acid. Apocrenic acids - torfic acid, apotorfic acid, arvic acid, apoarvic acid, porIa apocrenic acid

    Humous extract, crenic acid

    Anitrohumin, nitrohumin, nitro lin

    Solubility status

    Soluble in alkali and insoluble in acids

    Soluble in water

    Insoluble in alkali and water

  • 8

    Despite the fact that Mulder criticised Hermann's classification, 7

    even as late as 1909, Baumann remarked that Hermann's work should be

    accorded greater emphasis than that of Mulder in the classification of

    humic substances. As Kononaa68 has emphasized, the misconception that

    humic substances were chemically individual compounds arose from the

    absence of detailed investigations on their nature, structure and

    properties. It is not surprising, therefore, that by the end of the

    last century there was a prodigious increase in the number of discrete

    humic substances isolated from the soil including the mudesous acid of

    Johnson61 , the carbo-ulmic acid of Herz 53 and the hymatomelanic acid of 54 Hoppe-Seyler Interestingly, this last name (ethanol-soluble humic

    acid) survives to the present day. Before passing harsh judgements,...


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