DEFINITION OF SOIL ORGANIC MATTER

by Jerzy Weber

1. Definition of humic substances
2. Function of organic matter in soil
3. Formation of humic substances
4. Properties of humic substances
5. Properties of non-humic substances
6. Humic substances complexes with mineral components
7. Determination of content of humic substances
8. Extraction of humic substances
9. Organic forms of soil nitrogen
10. Organic forms of soil phosphorus
11. Types of humus in soils
12. Humus content in soil
13. References

1. Definition of humic substances

Agriculturist since ancient times have recognized significant benefits of soil organic matter (SOM) to crop productivity. These benefits have been the subject of controversy for centuries and some are still debated today.
Many of the benefits of SOM have been well documented scientifically, but some effects are so intimately associated with other soil factors that it is difficult to ascribe them uniquely to the organic matter. In fact, soil is a complex, multicomponent system of interacting materials, and the properties of soil result from the net effect of all these interactions.

One of the major problems in communicating in the field of humic substances is the lack of precise definitions for unambiguosly specifying the various fractions. Unfortunately, the terminology is not used in a consistent manner. The term humus is used by some soil scientists synonymously with soil organic matter, that is to denote all organic material in the soil, including humic substances. Contemporary, the term humus is frequently used to represent only the humic substances.

The term SOM is generally used to represent the organic constituents in the soil, including undecayed plant and animal tissues, their partial decomposition products, and the soil biomass. Thus, this term includes:

1.      identifiable, high-molecular-weight organic materials such as polysaccharides and proteins,

2.      simpler substances such as sugars, amino acids, and other small molecules,

3.      humic substances.

It is likely that SOM contains most if not all of the organic compounds synthesized by living organisms.

SOM is frequently said to consist of humic substances and nonhumic substances. Nonhumic substances are all those materials that can be placed in one of the categories of discrete compounds such as sugars, amino acids, fats and so on. Humic substances are the other, unidentifiable components. Even this apparently simple distinction, however, is not as clear cut as it might appear.

Distribution SOM is shown on the picture:

Organic compounds of soil - live organisms and their undecomposed, partly decomposed and completely decomposed remains as well as products of their transformation.

Living organisms alive - edaphon.

Soil organic matter - non-living components which are a heterogeneous mixture composed largely of products resulting from microbal and chemical transformations of organic debris. Soil organic matter can exist in different morphological patterns, which are the bases of the classification of so called forms and types of humus.

Unaltered materials - fresh and non-transformed components of older debris.

Transformed products - (humus) - bearing no morphological resemblance to the structures from which they were derived.These transformed components are reffered to as the humification processproducts.

Humic substances- a series of relativelyhigh-molecular-weight, brown to black colored substances formed by secondary synthesis reactions. The term is used as a generic name to describe tocolored material or its fractions obtained on the basis of solubilitycharacteristics:

• humic acids (HA)
• fulvic acids (FA)
• humins

Nonhumic substances- compounds belongingto known classes of bichemistry, such as:

• carbohydrates
• lipids
• amino acids

The chemical and colloidal properties of SOM can be studied only in the freestate, that is, when freed of inorganic soil components. Thus the first taskof the researcher is to separate organic matter from the inorganic matrixof sand, silt, and clay.
Methods for the extraction of soil organic matter have evolved from the research and thinking of many scientists.

2. Function of organic matter in soil

Organic matter contributes to plant growth through its effect on the physical, chemical, and biological properties of the soil. It has a:

• nutritional function in that it serves as a source of N, P for plant growth
• biological function in that it profoundly affects the activities of microflora and microfaunal organisms
• physical and physico-chemical function in that it promotes good soil structure, thereby improving tilth, aeration and retention of moisture and increasing buffering and exchange capacity of soils.

Humus also plays an indirect role in soil through its effect on the uptake of micronutrients by plants and the performance of herbicides and other agricultural chemicals. It should be emphasized that the importance of any given factor will vary from one soil to another and will depend upon such environmental conditions as climate and crpping history.

Availability of nutrients for planth growth

Organic matter has both a direct and indirect effect on the availability of nutrients for plant growth. In addition to serving as a source of N, P, S through its mineralization by soil microorganisms, organic matter influences the supply of nutrients from other sources (for example, organic matter is required as an energy source for N-fixing bacteria).
A factor that needs to be taken into consideration in evaluating humus as a source of nutrient is the cropping history. When soils are first placed under cultivation, the humus content generally declines over a period of 10 to 30 years until a new equilibrum level is attained. At equilibrium, any nutrients liberated by microbial activity must be compensated for by incorporation of equal amounts into newly formed humus

Effect on soil physical condition, soil erosion and soil buffering and exchange capacity

Humus has a profound effect on the structure of many soils. The deterioration of structure that accompanies intensive tillage is usually less severe in soils adequately supplied with humus. When humus is lost, soils tend to become hard, compact and cloddy. Aeration, water-holding capacity and permeability are all favorably affected by humus.
The frequent addition of easily decomposable organic residues leads to the synthesis of complex organic compounds that bind soil particles into structural units called aggregates. These aggregates help to maintain a loose, open, granular condition. Water is the better able to infiltrate and percolate downward through the soil.The roots of plants need a continual supply of O2 in order to respire and grow. Large pores permit better exchange of gases between soil and atmosphere.
Humus usually increases the ability of the soil to resist erosion. First, it enables the soil to hold more water. Even more important is its effect in promoting soil granulation and thus maintaining large pores through which water can enter and percolate downward. From 20 to 70% of the exchange capacity of many soils is caused by colloidal humic substances. Total acidities of isolated fractions of humus range from 300 to 1400 meq/100g.As far as buffer action is concerned, humus exhibits buffering over a wide pH range.

Effect on soil biological condition

Organic matter srves as a source of energy for both macro- and microfaunal organisms. Numbers of bacteria, actinomycetes and fungi in the soil are related in a general way to humus content. Earthworms and other faunal organisms are strongly affected by the quantity of plant residue material returned to the soil.
Organic substances in soil can have a direct physiological effect on plant growth. Some compounds, such as certain phenolic acids, have phytotoxic properties; others, such as the auxins, enhance plant growth.
It is widely known that many of the factors influencing the incidense of pathogenic organisms in soil are directly or indirectly influenced by organic matter. For example, a plentiful supply of organic matter may favor the growth of saprophytic organisms relative to parasitic ones and thereby reduce populations of the latter. Biologically active compounds in soil, such as antibiotics and certain phenolic acids, may enhance the ability of certain plants to resist attack by pathogens.

Humic acids of peaty sandy hydro earth soil

Humic acids of haplic phaeozems

Humic acids of podzolic soil

5. Non- humic substances characteristics

Soil carbohydrates

Carbohydrates consitute 5 to 25% of the organic matter in most soils. Plant remains contribute carbohydrates in the form of simple sugars, hemicellulose, and cellulose, but these are more or less decomposed by bacteria, actinomycetes and fungi, which in turn synthesize polysaccharides and other carbohydrates of their own.

Signifcance of carbohydrates

The significance of carbohydrates in soil aries largely from the ability of complex polysaccharides to bind inorganic soil particles into stable aggregates. Carbohydrates also form complexes with metal ions, and they serve as building blocks for humus synthesis. Some sugars may stimulate seed germination and root elongation. Other soil properties affected by polysaccharides include cation exchange capacity (attributed to COOH groups of uronic acids) , anion retention (occurence of NH₂ groups), and biological activity (energy source for microorganisms).

The major groups of carbohydrates

They can be divided into 3 subclasses:

1.      Monosaccharides, which are aldehyde and ketone derivatives of the higher polyhydric alcohols.

2.      Oligosaccharides, a large group of polymeric carbohydrates consisting of a relatively few monosaccharide units.

3.      Polysaccharides, contain many monomeric units (8 or more)

The carbohydrates material in soil occurs as:

1.      free sugars in the soil solution

2.      complex polysaccharides

3.      polymeric molecules of various sizes and shapes which are so strongly attached to clay and/or humic colloids.

The cotribution of individual sugar types to soil organic matter:

Soil lipids

The class of organic compounds designated as lipids represents a convenient anlytical group rather than a specific type of compound.
They represent a diverse group of materials ranging from relatively simple compounds such as fatty acids to more complex substances such as the sterols, terpens, polynuclear hydrocarbons, chlorophyll, fats, waxes, and resins.The bulk of the soil lipids occurs as the so-called fats, waxes, resins.
In normal aerobic soil the lipids probably exist largely as remnants of plant and microbial tissues. From 2 to 6% of soil humus occurs as fats, waxes, resins.
Lipids are physiologically active. Some compounds have a depressing effect on plant growth whereas others act as growth hormones. Waxes and similar materials may be responsible for the water repellent condition of certain sands.

Amino acids

Amino acids exist in soil in several different forms, like:

1.      As free amino acids

o        in the soil solution

o        in soil micropores

2.      As amino acids, peptides or proteins bound to clay minerals

o        on external surfaces

o        on internal surfaces

3.      As amino acids, peptides or proteins bound to humic colloids

o        H-bonding and van der Waals' forces

o        in covalent linkage as quinoid-amino acid complexes

4.      As mucoproteins

5.      As a muramic acid

Amino acids, being readily decomposed by microorganisms, have only an ephemeral existence in soil. Thus the amounts present in the soil solution at any one time represent a balance between synthesis and destruction by microorganisms.
The free amino acids content of the soil is strongly influenced by weather conditions, moisture status of the soil, type of plant and stage of growth, additions of organic residues, and cultural conditions.

6. Humic substances complexes with mineral components

Connection of humic substances with mineral fraction

Extensive studies have shown that not much of the humic substances in soil is in free state but much is bound to colloidal clay. The ways in which humic substances are combination with mineral portion of soil as follows:

1.      As salts of low - molecular organic acids (acetate, oxalate, lactate and others).

2.      As salts of humic substances with alkaline cations - humate, fulvate.

3.      As chelate with metal ions.

4.      As substances held on clay mineral surfaces.

Salts of low - molecular organic acids

Salts of low - molecular acids forms in result of action of acids (acetic acid, oxalic acid, fumaric acid, lactic acid) on minerals (magnesite, calcite, siderite and others) or salts of mineral acids with Ca, K and others cations.

Salts of humic substances with alkaline cations

Salts of humic substances with alkaline cations comprehensive of compounds:

• humate (salts of humic acids)
• fulvate (salts of fulvic acids)

They are the most characteristic compounds of soil humic substances. The alkaline cations (Na⁺, K⁺, Ca²⁺, Mg²⁺) are held primarily by simple cation exchange with COOH groups (RCOONa, RCOOK etc.). The humate and fulvate occur in soil largely as mixture with hydroxide of Fe and Al.

Chelate with metal ions

A chelate complex is formed when two or more coordinate positions about the metal ion are occupied by donor groups of a single ligand to form an internal ring structure. In soil role of ligands fulfilment simple organic compounds and functional groups of humic substances. The order of decreasing affinity of organic groupings for metal ions is approximately as follows:

-O- > -NH₂ > -N=N- > =N > -COO- > -O- > C=O

The order of decreasing ability of metal ions to chelating is as follows:

Fe³⁺ > Cu²⁺ > Ni²⁺ > Co²⁺ > Zn²⁺ > Fe²⁺ > Mn²⁺

The complexing ability of humic and fulvic acids results largely from their content of oxygen-containing functional groups, such as COOH, phenolic OH and C=O group. Soil organic constituents form both soluble and insoluble complexes with metal ions and thereby play a dual role in soil. Low - molecular - weight compounds (biochemicals, fulvic acids) bring about the solubilization of metal ions and affect their transport to plant roots. In contrast, high - molecular - weight compounds (e.g. humic acids) function as a "sink" for polyvalent cations. Natural complexing agents are of considerable importance in weathering processes and in the movement of sesquioxides into the subsoil.

Clay - organic complexes

The interaction of organic substances with clay has a multitude of consequences that are reflested in the physical, chemical and biological properties of the soil matrix. Several mechanisms are involved in the adsorption of humic substances by clay minerals, the main ones being:

• van der Waals' forces
• bonding by cation bridging
• H - bonding
• adsorption by association with hydrous oxides
• adsorption on interlamellar spaces of clay minerals

Van der Waals' forces

Van der Waals' forces operate between all molecules, but are rather weak. Essentially, these forces result from fluctuations in the electric charge density of individual atoms. An electrically positive fluctuation in one atom tends to produce an elecrically negative fluctuation in a neigh-boring atom and a net attractive forces results. Attractive forces resulting from these fluctuations are every pair of atoms or molecules.

Adsorption caused by van der Waals' forces can be of considerable importance in the adsorption of neutral polar and nonpolar molecules, particulary those which are high in molecular weight.

Bonding by cation bridging

Since organic anions are normally repelled from negatively charged clay surfaces, adsorption of humic and fulvic acids by clay minerals such as montmorillonite occurs only when polyvalent cations are present on the exchange complex. Unlike Na⁺ and K⁺, polyvalent cations are able to maintain neutrality at the surface by neutralizing both the charge on the clay and the acidic functional group of the organic matter (e.g. COO^-). The main polyvalent cations responsible for the binding of humic and fulvic acids to soil clays are Ca²⁺, Fe³⁺ and Al³⁺. The divalent Ca²⁺ ion doesn't form strong coordination complexes with organic molecules. In contrast Fe³⁺ and Al³⁺ form strong coordination complexes with organic compounds. The polyvalent cations act as a bridge between two charged sites.

For a long chain organic molecule, several points of attachment to the clay particle are possible.

H - bonding

This is a linkage between polar groups of the organic molecule and adsorbed water molecules or oxygens of the silicate surface through bonding with a single H⁺ ion. The strengh of an individual bond is small, but they are additive thus total adsorption energy can be appreciable. Rigorous drying, such as by desication at the soil surface or consumption of available moisture by plant roots, will tend to increase the bonding between humic material and clay by eliminating hydration water and bringing the humic matter in closer contact to the clay.

H - bonding

This is a linkage between polar groups of the organic molecule and adsorbed water molecules or oxygens of the silicate surface through bonding with a single H⁺ ion. The strengh of an individual bond is small, but they are additive thus total adsorption energy can be appreciable. Rigorous drying, such as by desication at the soil surface or consumption of available moisture by plant roots, will tend to increase the bonding between humic material and clay by eliminating hydration water and bringing the humic matter in closer contact to the clay.

Adsorption by association with hydrous oxides of Fe and Al.

For many soils, hydrous oxides are equal in importance to mica-type surfaces in sorbing humic substances. When clay minerals are coated with layers of hydrous oxides their surface reactions are dominated by these oxides rather than the clay. Organic anions can be associated with the oxides by simple coulombic attraction. Anion associated with clay in this manner are readily removed by increasing the pH or by leaching with NaCl or NH₄Cl. The fact that very little humic material can be recovered from soils by these treatments suggest that most of the adsorbed organic matter is retained by supplementary machanisms. Coordination or ligand exchange occurs when the anionic group penetrates the coordination shell of Al or Fe and becomes icorporated with the surface OH layer. The sorption of fulvic acid on oxide surfaces is accompanied by displacement of OH groups by COO^- ions. The organic anion is not easily displaced with simple salts, although adsorption is pH sensitive. As was the case with organic cations on clay mineral surfaces, a very strong bond will result if more than one group on the humic molecule participates.

Adsorption on interlamellar spaces of clay minerals

An important mechanisms for retention of proteins and charged organic cations by expandable-layer silicates is through adsorption on interlamellar spaces. Considerably contrversy exist as to whether humic and fulvic acids are bound in this way in the natural soil. Evidence for interlamellar adsorption of fulvic acid by montmorillonite at pH < 5.5 has been given by Schnitzer and Kodama and Theng. The high - molecular - weight humic acids may be too large to penetrate interlamellar spaces.

8. Extraction of humic substances

Extraction of Soil Organic Matter

The properties of soil organic matter, and especially humic substances, can be studied only in free state, that is, when freed of inorganic soil components. Therefore, the first task of the researcher is to separate organic matter from the inorganic matrix of sand, silt, and clay. A variety of techniques have been employed, depending upon the nature of the material to be examined. Thus nonpolar compounds like fats, waxes, resins, etc. can be extracted with such organic solvents as hexane, ether, alcohol-benzene mixtures, and others. Hydrolysis prosedures have been used for isolating individual monomers, such as amino acids and sugars.
The ideal extraction method is one which meets the following objectives:

1.      the method leads to the isolation of unaltered material

2.      the extracted humic substances are free of inorganic contaminants, such as clay and polyvalent cations

3.      extraction is complete, thereby insuring representation of fractions from the entire molecular-weight range

4.      the method is universally applicable to all soils

Reagents used for extraction of organic constituents from soil (Stevenson 1982)

Alkali extraction

NaOH solution of 0.1 to 0.5N concetration in water and a soil to extractant ratio of from 1:2 to 1:5 (g/ml) have been widely used for recovering organic matter.Repeated extraction is required to obtain maximum recovery. Leaching the soil with dilute HCl, which removes Ca and other polyvalent cations, increases the efficiency of extraction of organic matter with alkaline reagents. As a general rule, extraction of soil with 0.1 or 0.5N NaOH leads to the recovery of approximately two-thirds of the soil organic matter.

Undesirable features of alkali extraction are as follows:

1.      Alkali solutions dissolve silica from the mineral matter and this silica contaminates the organic fractions separated from the extract.

2.      Alkali solutions dissolve protoplasmic and structural components from fresh organic tissues and these become mixed with the humified organic matter.

3.      Under alkaline conditions, autoxidation of some organic constituents occurs in contact with air both during extraction and when the extracts are allowed to stand.

4.      Other chemical changes can occur in alkaline solution like condensation between amino acids and aldehydes or quinones.

The more alkaline the solution and the longer the extraction period the greater will be the chemical changes. The amount of organic matter extracted from soil with caustic alkali increases with time of extraction.

Mild extractans

Several milder and more selective extractans have been recommended in recent years as alternatives for the classical extraction with strong alkali. Included are salts of complexing agents (Na₄P₂O₇ and EDTA), organic complexing agents in aqueous media (acetylacetone), and organic solvents of various types.Whereas less alternation of organic matter may result, these extractans are much less effective than alkali hydroxides in removing organic matter. The main exepction being the illuvial (B) horizon of the Spodosol. As was the case with alkali extraction, recovery of organic matter frequently can be increased by pretreating the soil with mineral acids to remove carbonates (HCl) or silicates (HCl-HF mixtures).
For certain ivestigations, a mild extractant is definitely preferred; for others, a more complete extraction is caustic alkali. At now, many investigators are using a sequence of extractants in which part of the organic matter is recovered by a mild reagent before alkali extraction.

Na₄P₂O₇ and other neutral salts

In many soils, Ca and other polyvalent cations (Fe, Al) are responsible for maintaining organic matter in a flocculated and insoluble condition. Accordingly, reagents that inactivate these cations by forming insoluble precipitates or soluble coordination complexes lead to solubilization of the organic matter. Such reagents as ammonium oxalate, sodium pyrophosphate and salts of weak organic acids have been used for this purpose.
Of the various neutral reagents, Na₄P₂O₇ has been the most widely used. As note earlier, the amount of organic matter recovered (<30%) is considerably less than with caustic alkali, but less alteration occurs.To minimize chemical modification of the humic material, extraction should be carried out at pH 7.0.

Formic acid - HCOOH

Extensive research on the extraction of soil organic matter with formic acid shows that under certain circumstances up to 55% of the organic matter in mineral soils and as much as 80% of that in composts can be extracted with formic acid containing LiF, LiBr or HBF₄ (see picture).
Advantages of anhydrous formic acid for extraction of organic matter is that it is a polar compound that neither exhibits oxidizing nor hydrolytic properties. Furthermore, formic acid is a good solvent for a wide variety of compounds, including polysaccharides. Large quantities of Ca, Fe, Al and other inorganic components are dissolved from the soil along with the organic matter and thus far it has not been possible to remove the inorganic material completely.
Formic acid is most efficient with soils where much of the organic matter is only partially humified.

Organic chelating agents

Organic compounds such as acetylacetone, cupferron and hydroxyquinoline, which are capable of forming chelate complexes with polyvalent metal ions, have been used for extracting illuvial organic matter from Spodosols. The organic matter in the B horizon of these soils occurs as complexes with Fe and Al and the complexing of these metals by chelating agents releases the organic matter to soluble forms.Organic chelating agents are rather ineffective for extracting organic matter from other soil types.

Outline of extraction procedures in IHSS method

Step 1. Equilibrate the sample to a pH value between 1-2 with 1 M HCl at room teperature. Adjust solution volume with 0.1 M HCl to provide a final concentration that has ratio of 10 mL liquid/1 g dry sample. Shake the suspencion for 1 hour.

Step 2. Separate supernatant from the residue by decantation after allowing solution to settle (or by low speed centrifugation). Save supernatant for XAD-8 isolation.

Step 3. Neutralize the soil residue with 1 M NaOH to pH=7.0 then add 0.1 NaOH under an athmosphere of N₂ to give a final extractant to soil ratio of 10:1.

Step 4. Extract the suspension under N₂ with intermittent shaking for a minimum of 4 hours. Allow the alkaline suspension to settle overnight and collect the supernatant by means of decantation or centrifugation.

Step 5. Acidify the supernatant with 6 M HCl with constant stirring to pH=1.0 and then allow the suspension to stand for 12-16 hours.

Step 6. Centrifuge to separate the humic acid (precipitate) and fulvic acid (supernatant - FA Extract 2) fractions.

Step 7. Redissolve the humic acid fraction by adding a minimum volume of 0.1 M KOH under N₂. Add solid KCl to attain 0.3 M (K⁺) and then centrifuge at high speed to remove suspended solids.

Step 8. Reprecipitate the humic acid as in step 5. Centrifuge and discard supernatant.

Step 9. Suspend the humic acid precipitate in 0.1 M HCl/0.3 M HF solution in a plastic container. Shake overnight at room temperature.

Step 10. Centrifuge and repeat HCl/HF treatment(step 9), if necessary, until the ash content is below 1 percent.

Step 11. Transfer the precipitate to a Visking dialysis tube by slurring wiyh water and dialyze against distilled water until the dialysis watr gives a nagative Cl^- test with the AgNO₃.

Step 12. Freeze-dry the humic acid.

Step 13. Pass the supernatant from step 2 through a column of XAD-8 (0.15 ml of resin per gram of initial sample dry weight at a flow rate of 15 bed volumes per hour). Discard the effluent, rinse the XAD-8 column containing sorbed fulvic acid with 0.65 column volumes of distilled water.

Step 14. Back elute the XAD-8 column with 1 column volume of 0.1 M NaOH, followed by 2-3 column volumes of distilled water.

Step 15. Immediately acidify with 6 M HCl to pH=1. Add concentrated HF to a final concentration of 0.3 M HF. Solution volume should be sufficient to maintain fulvic acid solubility.

Step 16. Pass the supernatant from step 6 through a column of XAD-8 (1.0 mL of resin per gram of initial sample dry weight).

Step 17. Repeat steps 14 and 15

Step 18. Combine the final eluates from steps 15 and 17 and pass this solution through XAD-8 resin in glass column (column volume should be 1/5 of sample volume). Rinse with 0.65 column volumes of distilled water.

Step 19. Back elute with 1 column volume of 0.1 M NaOH followed by 2 column volumes of distilled water. Pass eluate through H⁺- saturated cation exchange resin (Bio-Rad AG-MP-5) using three times the mole of Na ions in solution).

Step 20. Freeze-dry the eluate to recover the H⁺- saturated fulvic acid.

9. Organic forms of soil nitrogen

Organic form of soil nitrogen

Over 90% of the nitrogen N in the surface layer of most soils occurs in organic forms, with most of the remainder being present as NH₄^- whichis held within the lattice structures of clayminerals. The surface layerof most cultivated soils contains between0.06 and 0.3% N. Peat soils have high N contents to 3.5%. Plant remains and other debris contribute nitrogen N in the form of:

Amino acids

Amino acids exist in soil in several different forms, like:

1.      As free amino acids

o        in the soil solution

o        in soil micropores

2.      As amino acids, peptides or proteins bound to clay minerals

o        on external surfaces

o        on internal surfaces

3.      As amino acids, peptides or proteins bound to humic colloids

o        H-bonding and van der Waals' forces

o        in covalent linkage as quinoid-amino acid complexes

4.      As mucoproteins

5.      As a muramic acid

Amino acids, being readily decomposed by microorganisms, have only anephemeral existence in soil. Thus the amounts present in the soil solutionat any one time represent a balance between synthesis and destruction by microorganisms. The free amino acids content of the soil is strongly influenced by weatherconditions, moisture status of the soil, type of plant and stage of growth,additions of organic residues, and cultural conditions.

Amino sugars

Amino sugars occur as structal components of a broad group of substances, the mucopolysaccharides and they have been found in combination with mucopeptides and mucoproteins. Some of the amino sugar material in soil may exist in the form of an alkali-insoluble polysaccharide referred to as chitin. Generally the amino sugars in soil are of microbial origin.From 5 to 10%of the N in the surface layer of most soils can be accounted for in N-containing carbohydrates or amino sugars.

Nucleic Acids

Nucleic acids, which occur in the cells of all living organisms, consist of individual mononucleotide units (base-sugar-phosphate) joined by a phosphoricacid ester linkage through the sugar.Two types: ribonucleic acid (RNA) anddeoxyribonucleic acid (DNA).They have pentose sugar (ribose or deoxyribose),the purine: adenine, guanine and the pyrimidine: cytosine, thymine.RNA contains also the uracil. The N in purine and pyrimidine bases is usually considered to account forless than 1% of the total soil N. Small amounts of N are extrcted from soil in the form of glycerophosphatides, amines, vitamins, pesticide and pesticide degradation products.

Nitrogen transformation

A key feature of the internal cycle is the biological turnover of N betweenmineral and organic forms through the opposing processes of mineralizationand immobilization. The latter leads to incorporation of N into microbial tissues. Whereas much of this newly immobilized N is recycled through mineralization, some is converted to stable humus forms. The overall reaction leading to incorporation of inorganic forms of N intostable humus forms is depicted on the picture. Thus the decay of plant and animal residues by microorganisms results in theformation of mineral forms of N (NH₄⁺ and NO₃^-) and assimilation of part ofthe C into microbial tissue (reaction A). Part of the native humus undergoes a similar fate (reaction B). Subsequent turnover through mineralization-immobilization leads to incorporation of N into stable humus forms (reaction C). Stabilization of N may also occur through the reaction of partial decay products of lignin with nitrogenous constituents (raection D). Except under unusual circumstances, both mineralization and immobilizationalways function in soil, but in opposite direction.

Chemical reaction of ammonia and nitrite with organic matter

The fate of mineral forms of N in soil is determined to some extent bynonbiological reactions involving NH₄⁺, NH₃and NO₂^- as depicted in fig.
In addition to NH₄⁺ fixation by clay minerals (reaction A), NH₃ and NO₂^- react chemically with organic matter to form stable organic N complexes(reaction B and C). The chemical interaction of NO₂^- with organic matter may lead to the generation of N gases. Although both types of reactions can proceed over a wide pH range, fixation of NH₃⁺ is favored by a high pH (>7.0). In contrast, NO₂^- -organic matter interactions occur most readily under highly acidic conditions (pH of 5.0 to 5.5 or below).

Stability of soil organic nitrogen

1.      Proteinaceous constituents are stabilized through their reaction withother organic constituents, such as lignins, tanins, quinons.

2.      Biologically resistant complexes are formed in soil by chemical reactionsinvolving NH₃⁺ or NO₂^-with lignins or humic substances.

3.      Adsorption of organic N compounds by clay minerals (pariculary montmorillinitic types) protects the molecule from decomposition.

4.      Complexes formed between organic N compounds and polyvalent cations, such as Fe, are biologically stable.

5.      Some of the organic N occurs in small pores or voids and is physicallyinaccessible to microorganisms.

C/N ratio

For surface soils, and for the top layer of lake and marine sediments, the ratio generally falls within well-defined limits, usually from about10 to 12. In most soils, the C/N ratio decreases with increasing depth, often attaining values less than 5.0.Native humus would be expected to have a lower C/N ratio than most undecayedplant residues for following reasons. The decay of organic residues by soilorganisms leads to incorporation of part of the C into microbial tissue with the remainder being liberated as CO₂. As a general rule, about one-third of the applied C in fresh residues will remain in the soil after the first few months of decomposition. The decay process is accompanied by conversion of organic form of N to NH₃ and NO₃^- and soil microorganisms utilize partof this N for synthesis of new cells. The gradual transformation of plantraw material into stable organic matter (humus) leads to the establishmentof reasonably consistent relationship between C and N. Other factors which may be involved in narrowing of the C/N ratio include chemical fixation of NH₃ or amines by ligninlike substances.
The C/N ratio of virigin soils formed under grass vegetation is normally lower than for soils formed under forest vegetation, and for the latter,the C/N ratio of the humus layers is usually higher than for the mineral soil proper. Also the C/N ratio of a well-decomposed muck soil is lower than for a fibrous peat.
As a general rule it can be said that conditions which encourage decompositionof organic matter result in narrowing of the C/N ratio. The ratio nearly alwaysnarrows sharply with depth in the profile; for certain subsurface soils C/Nratios lower than 5 are not uncommon.

10. Organic forms of soil phosphorus

Organic phosphorus

Phosphorus rank in importance with N and K as major plant nutrients. Phosphorus compounds in soil can be placed into the following three classes:

1.      organic compound of the soil humus,

2.      inorganic compounds in which the P is combined with Ca, Mg, Fe, Al and with clay minerals,

3.      organic and inorganic P compounds associated with the cells of living matter. Microorganisms are involved in transformations of phosphorus between organic and mineral forms.

From 15 to 80% of the phosphorus in soils occurs in organic forms, the exact amount being dependent upon the nature of the soil and its composition. The higher percentages are typical of peats and uncultivated forest soils. From the standpoint of plant nutrion, phosphorus is adsorbed by plants largely as the negatively charged primary and secondary orthophosphate ions (H₂PO₄^- and HPO₄^2-) which are present in the soil solution. Small quantities of soluble organic P compounds are also present in water extracts of soil.

The phosphorus cycle

In a broad sense, the phosphorus cycle in soil involves the uptake of phosphorus by plants and its return ti the soil in plant and animal residues.

As can be seen from picture three general types of compounds make up the bulk of the organic phosphorus in plants, namely: phytin, phospholipids, and nucleic acids. Approximate recoveries of organic phosphorus in these forms are as follows:

When crop residues are returned to the soil, net immobilization of P will occur when the C/organic- P ratio is 300 or more; net mineralization will result when the ratio is 200 or less.

11. Types of humus in soils

Types of humus in soils

Humus occurs in soils in many types, differentates in regard to morphology and fractional composition. A type of humus is it a morphological form of naturals accumulation of humic substances in profile or on the surface of soil, conditioned by general direction of soil-forming process and humification of organic matter. A types of humus in terrestrial enviroment are following:

1.      mor

2.      moder

3.      mull

Mor is a type of humus, which occur largely in coniferous forest soils and the moorlands soils. This humus arise under conditions of low-biological activity in soil. The mineralization of organic matter proceed slowly and create layers, which maintain a structure of vegatable material.Acidophilic fungi and low activeinvertebrates participates in transformations of plant residues. Under these circumstances forms a litter of large thickness. C/N ratio of mor humus is always more than 20, or even 30-40, whereas pH is acid.

Moder is a transitional form of humus between mull and moder, characteristic for sod-podzolic soils, loesses and mountain grassland soils. The organic horizons with moder humus consist of low-thicknessed litter (2-3 cm), which gradually, without bounds, pass on to humus-accumulative horizons. Moder is a type of medium humified humus. Acidophilic fungi and arthropodan participates in transformations of plant residues. C/N ratio equal 15-25. Produced mineral-organic complexes are labile and weakly bounded with mineral portion of soil.

Mull is a type of humus characteristic for chestnut soils, phaeozems, rendzinas and others soils. This type of humus arise under grass vegetation. Mull is a well humified organic matter, which is produce in very biologically active habitat. This type of humus is characterized by neutral pH, C/N ratio nearing to 10 and ability to creation stable mineral-organic complexes. Mull is a type of humus which occurs in soils under cultivation.

According to Kononova, the types of humus are divide as follows:

First type of humus is characteristic for podzolic soils, grey brown soils and lateric soils under forest communities. In this humus predominate humic acids, thus humic acid/fulvic acid ratio is below 1. Humic acid indicate small extent of aromatic rings condensation and they are approximate to fulvic acids. Considerable hydrophilic properties of humic acids favor to creation of chelates with polyvalent cations and ability to displacement deep into profile of soil. Considerable mobility of this humus favor process of podsolization.

Second type of humus is characteristic for phaeozems, rendzinas, black earths and brown soils. Humic acid/fulvic acid ratio is upper than 1, Extent of aromatic rings condensation is high in humic acids, which cause their hydrophobic properties and inability to creation of chelates. Humic acids are strongly connected with mineral portion of soil in this type of humus.

Third type of humus is characteristic for semidesert soils. In this humus predominate fulvic acids fraction, whereas arise of humic acids is limited. Beyond this, humic acids are largely bounded with mineral portion of soil.

Division and order

Type,genera and kind

Humus content %

Calcisols:

Calcarious Rendzinas

3.4

2.1 - 6.3

4.4

Jurasic Rendzinas

1.5 - 7.0

Phaeozems:

Haplic Phaeozems

2.8

1.8 - 4.0

Cambisols:

Brown soils formed from sands

1.5

0.9 - 2.2

Brown soils formed from light and medium loams

1.8

1.1 - 3.0

Brown soils formed from heavy loam

2.5

1.6 - 3.7

Brown soils formed from silt formations

1.7

1.3 - 1.9

Brown soils formed from loess and loesslike materials

1.9

1.4 - 2.6

Luvisols:

Grey brown soils formed from silt formations

1.9

1.4 - 2.4

Grey brown soils formed from loess and loesslike materials

1.8

1.0 - 2.5

Grey brown soils formed from light loam

1.6

1.0 - 2.6

Podzols:

Podzolic soil formed from sands

1.5

1.1 - 2.0

Gleysols:

Boggy soils formed from silts

1.6

1.2 - 2.1

Gleysols:

Black earth formed from sands

2.8

1.2 - 4.1

Black earth formed from light and medium loams

2.6

1.2 - 5.7

Black earth formed from heavy loams and clays

4.9

2.5 - 5.6

Fluvisols:

Alluvial soils formed from sands

2.9

1.5 - 5.2

Alluvial soils formed from silts

3.5

1.7 - 5.8

Alluvial soils formed from clays

4.2

2.4 - 6.8

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