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Organic Matter

Why Organic Matter is Important

The greatest factor contributing to the overall fertility of soil is the Decomposed Organic Matter.
Its values are manyfold, such as:

  1. Improves soil physical (crumb) structure and macroporosity (glues soil particles together)
  2. Absorbs and retains water
  3. Increases soil nutrient holding capacity
  4. Absorbs and retains heat
  5. Reduces loss of nutrients from soil by leaching
  6. Buffers pH fluctuation of acid soils

The rate of decomposition of organic matter and the mineralisation release of nutrients depends upon many fertility factors.Some of them nutritional, physical and microbiological.Many of these factors are inter-related and in turn the decomposed organic matter reversibly affects the original fertility factor.

The Organic Contents of the Soil

Organic matter, often called humus, is one of the most important constituents of soil, and as far as plant growth is concerned.For practical purposes it is necessary and important to distinguish between two main groups of soil substances which differ vitally from each other in biological and physiological properties.
The total amount of organic matter contained in a given soil consists of:

  1. Reserve organic matter, viz., that part which is still undecomposed or is in the process of decomposition and which, through continued chemical and biological influences, gradually changes into humus
  2. Humus, viz., that part which is fully decomposed to the colloidal state.
     

To know at all times in what proportion reserve organic matter and humus are contained in their soil is of vital importance for the practical farmer and gardener, since, without this knowledge, he can not properly gauge the condition of their soil, on which present and future productiveness depend.The reserve organic matter is determined by establishing the amounts of total organic matter as well as of humus and by subtracting the latter from the former.

Values of Humus

The qualities of humus which are of greatest interest and value for the practical farmer and gardener must be considered first.The subsequent discussion of the chemical constituents or organic matter will then be much easier to follow.

One of the most valuable qualities of humus is its ability to absorb and retain water, thus holding it in reserve and preventing it from being lost in percolation or through surface run-off.At the same time, humus acts as a sort of filter, retaining a large part of the nutrient salts dissolved in the soil solution, even after the water has disappeared, and releasing the nutrients again slowly when water is added.Its colloidal character gives it cohesive power, which means that it is able to hold soil particles together.

In this manner it serves to render light sandy soils more than retentive of water and nutrients, whereas in heavy clay soils it increases the pore spaces by tying the very small soil particles together into groups.Increased pore space means better aeration and improved drainage.This two way action in different soil types is as curious as it important, though it is readily understandable that the incorporation of raw fibrous organic matter will result in the loosening up of heavy soils.

The dark colour of humus also is of value to plant growth.Black absorbs and retains heat, whereas white reflects it and remains cool.The dark humus, therefore, serves to render heavy, cold soils warmer, and tends to equalise the often sudden and very undesirable fluctuations of temperature in light soils.

Values of Humus

The qualities of humus which are of greatest interest and value for the practical farmer and gardener must be considered first.The subsequent discussion of the chemical constituents or organic matter will then be much easier to follow.

One of the most valuable qualities of humus is its ability to absorb and retain water, thus holding it in reserve and preventing it from being lost in percolation or through surface run-off.At the same time, humus acts as a sort of filter, retaining a large part of the nutrient salts dissolved in the soil solution, even after the water has disappeared, and releasing the nutrients again slowly when water is added.Its colloidal character gives it cohesive power, which means that it is able to hold soil particles together.

In this manner it serves to render light sandy soils more than retentive of water and nutrients, whereas in heavy clay soils it increases the pore spaces by tying the very small soil particles together into groups.Increased pore space means better aeration and improved drainage.This two way action in different soil types is as curious as it important, though it is readily understandable that the incorporation of raw fibrous organic matter will result in the loosening up of heavy soils.

The dark colour of humus also is of value to plant growth.Black absorbs and retains heat, whereas white reflects it and remains cool.The dark humus, therefore, serves to render heavy, cold soils warmer, and tends to equalise the often sudden and very undesirable fluctuations of temperature in light soils.

Important Substances in Humus
  1. Carbohydrates, including cellulose, hemicellulose, starch, various types of sugars and pentosans
  2. Lignin substances which come from the woody skeleton of plant bodies and plant cells
  3. Proteins, rather than complex compounds.They decay resulting in the formation of amino acids which can be readily identified in the soil and which, in further decomposition, have for their end product ammonia or ammonium salts
  4. Fats and various types of waxes, oils and resins
     
Lignin as a Source of Humic Acid

Most of the above mentioned substances are rather rapidly decomposed by micro organisms into simpler compounds and eventually dissolve largely into the elements of which they consisted originally.The lignin substances, however, show considerable resistance to biological decomposition, and decomposing very slowly, they represent a rather stable component of soil organic matter.The most essential ingredient of all lignin substances is humic acid which, at the same time, represents the most vitally important component of humus.

Humic Acid

Humic acid is present in nearly all soils in varying amounts.In chemically pure form it represents a dark reddish-brown powder which is practically insoluble in water.

Solubility
  • An important property of humic acid is that, if brought into contact with certain bases, if it forms salts.
  • Most of these salts (such as those formed with calcium, magnesium, iron, aluminium and manganese) are likewise nearly water insoluble.
  • Some of these salts (such as the humic acid salts formed with potassium, ammonia, and sodium) are readily soluble in water.
  • This solubility in certain alkaline solutions, produced in the soil under certain conditions, is highly significant.
     
Colloidal Character of Humic Acid and its Salts

Of equal importance is the colloidal character of humic acid and its salts.Like other amorphous substances, such as glue and gelatine, humic acid and its salts are able to absorb and to hold considerable amounts of water.This process is accompanied by swelling to several times the original size and by the formation of a jelly-like matter.In the soil, such action results in the preservation of moisture and, at the same time, becomes of considerable value in making more permanent the soil crumbs, formed through cohesion of the soil particles.

Organic Matter – So What?

The decomposed organic matter influences physical and chemical properties of soils far out proportion to the small quantities present.It commonly accounts for at least half the cation exchange capacity of surface soils and is responsible perhaps more than any other single factor for the stability of soil aggregates.Furthermore, it supplies energy and bodybuilding constituents for most of the micro-organisms.

If it is good – Where Can I get It?

Sources of Soil Organic Matter

The original source of the soil organic matter is plant tissue.Under natural conditions, the tops and roots of trees, shrubs, grasses, and other native plants annually supply large quantities of organic residues.A good portion of cropped plants are commonly removed from cropped soil, but one-tenth to one-third of the tops and all of the roots are left in the soil.

My Neighbours or mine – which has more goodies in it?

Composition of Plant Residues

The carbohydrates, which range in complexity from simple sugars to the celluloses.The fats and oils are glycerides of fatty acids such as butyric, stearic, and oleic.These are associated with resins of many kinds and are somewhat more complex than most of the carbohydrates.
Lignin occurs in older plant tissue such as stems and other woody tissues.They are complex compounds, some of which may have “ring” structures.The major components of lignin’s are carbon, hydrogen, and oxygen.They are very resistant to decomposition.
Of the various groups the crude proteins are among the more complicated.They contain not only carbon, hydrogen, and oxygen, but also nitrogen and smaller amounts of elements as sulphur, iron and phosphorous.As a consequence, they are compounds of great significance as carriers of essential elements.Their reactions in soils are means by which these nutrients are first observed and eventually made available for plant uptake.

Acid-Mat – I want to decompose it, what do I do?

Decomposition of Organic Compounds

Organic compounds vary greatly in their rate of decomposition.They may be listed in terms of ease of decomposition as follows:

  1. Sugars, starches and simple proteins
  2. Crude proteins
  3. Hemicelluloses
  4. Cellulose
  5. Lignin’s fats, waxes, etc.
     

It should be remembered that all of these compounds usually begin with to decompose simultaneously when fresh plant tissue is added to a soil.The rate at which decomposition occurs, however, decreases as we move from the top to the bottom of the list.Thus, sugars and water-soluble proteins are examples of readily available energy sources for soil organisms.Lignins are a very resistant source of food, although they eventually supply much of the total energy.

I Stole Some Acid Mat – What Happens To It Now?

When organic tissue is added to the soil, three general reactions take place:

  1. The bulk of the material undergoes enzymatic oxidation with carbon dioxide, water, energy and heat as the major products.
  2. The essential elements, nitrogen, phosphorous and sulphur are released and/or immobilized by a series of specific reactions relatively unique for each element.
  3. Compounds resistant to microbial action are formed either through modification of compounds in the original plant tissue or by microbial synthesis.
     

Each kind of reaction has great practical significance.

Decomposition-an Oxidation Process

In spite of the difference in composition of the various organic compounds, the similarity of the ultimate end products of decay is quite striking, especially if aerobic organisms are involved.Under such conditions the major portion of all these compounds undergoes essentially a “burning” or oxidation process.

Breakdown of Proteins

The plant proteins are related compounds yield other very important products upon decomposition they break down into amides and amino acids of various kinds, the kind of rate breakdown depending on conditions.Once these compounds are formed, they may be hydrolysed readily to carbon dioxide, ammonium compounds, and other products.The ammonium compounds may be changed to nitrates, the form in which the higher plants take up much of their nitrogen.

Example of Organic Delay

The process of organic delay in time sequence is illustrated in Figure 1.First assume a situation where no readily decomposable materials are present in a soil.The microbial numbers and activity are low.Next, under favourable conditions, introduce and abundance of fresh, decomposable tissue.A marked change occurs immediately as the number of soil micro-organisms suddenly increase manyfold.Soon microbial activity is at its peak, at which point energy is being liberated rapidly and carbon dioxide is being formed in large quantities.General-purpose decay bacteria, fungi, and actinomycetes are soon fully active and are decomposing and synthesising at the same time.

The organic matter at this stage contains a great variety of substances:intermediate products of all kinds, ranging from the more stable compounds, such as modified lignin’s, to microbial cells, both living and dead.The microbial tissue may even at times account for as much as half of the organic fraction of a soil.

Dead microbial cells soon decay, and the compounds present are devoured by living microbes, with the profuse evolution of carbon dioxide.As the readily available energy is used up and food supplies diminish, microbial activity gradually lessens and the general-purpose soil organisms again sink back into comparative quiescence.This is associated with a release of simple products such as nitrates and sulphates.The organic matter now remaining is a dark, incoherent, and heterogeneous colloidal mass usually referred to as humus.The decomposition of both plant residues and soil organic matter is nothing more than a process of enzymatic digestion.It is just truly a digestion as though the plant materials entered the stomach of a domestic animal.The products of these enzymatic activities, although numerous and tremendously varied, may be listed for convenience of discussion under three headings:

  1. Energy appropriated by the micro-organisms or liberated as heat
  2. Simple end products
  3. Humus
     
Energy of Soil Organic Matter

The micro-organisms of the soil must not only have substance for their tissue synthesis but energy as well.For most of the micro-organisms, both of these are obtained from the soil organic matter.All manner of compounds are utilised as energy sources, some freely, other slowly and indifferently.

Simple Decomposition Products

As the enzymatic changes of the soil organic matter proceed, simple products begin to manifest themselves.Some of these, especially carbon dioxide and water, appear immediately.Others, such as nitrate nitrogen, accumulate only after the peak of the vigorous decomposition has passed and the general-purpose decay organisms have diminished in numbers.

The Carbon Cycle

Carbon is a common constituent of all organic matter and is involved in essentially all life processes.Consequently, the transformations of these elements, termed the carbon cycle are in reality a biocycle that makes possible the continuity of life on earth.These changes are shown graphically in Figure 2.Note that humus and carbon dioxide are relatively stable components of this cycle.

Release of Carbon Dioxide

Through the process of photosynthesis, carbon dioxide is assimilated by higher plants and converted into numerous organic compounds, classes of which were described earlier.As these organic compounds reach the soil in plant residues, they are digested and carbon dioxide is given off.Microbial activity is the main source of carbon dioxide, although appreciable amounts come from the respiration of rapidly growing plant roots and some is bought down in rainwater.Under optimum conditions more than 100Kg/Ha of carbon dioxide may be evolved per day, 25-30Kg being more common.Much of the carbon dioxide of the soil ultimately escapes to the atmosphere, where it may again be used by plants, thus completing the cycle.

Simple Products Carrying Nitrogen

Ammonium salts are the first inorganic nitrogen compounds produced by microbial digestion, but other inorganic or mineral forms follow.Hence the process is called mineralisation.Proteins split up into amino acids and similar nitrogenous materials that readily yield ammonium compounds by enzymatic hydrolysis.These transformations are brought about by a large number of general purpose heterotrophic organisms – bacteria, fungi and actinomycetes.The ammonium ion is readily available to micro-organisms and most higher plants.

Nitrification

If conditions are now favourable, ammonium ions are subject to ready oxidation, principally by two special-purpose organisms, the nitrate and the nitrate bacteria.

Simple Products Carrying Sulphur

Many organic compounds, including proteins, carry sulphur, which in turn appears in simple forms as decay progresses.General heterotrophic types of organisms apparently simplify the complex organic compounds.The sulphur of these simplified by-products is then subjected to oxidation by special autotrophic bacteria.

The organisms involved obtain energy by the transfer and leave the sulphur as sulphate, the form in which it is taken up by plants.Note that this process also increases soil acidity.

Mineralisation of Organic Phosphorous

A large proportion of the soil phosphorous is carried in organic combinations.Upon attack by micro-organisms the organic phosphorous compounds are mineralised; that is, they are changed to inorganic combinations.The particular forms present depend to a considerable degree upon soil pH.As the pH goes up from 5.5 to 7.5, the available phosphorous changes from dihydrogen to monohydrogen phosphate.Both of these forms are available to higher plants.Since the small amount of phosphorous held in complex mineral combinations in soils usually is very slowly available, the organic sources mentioned in the above become especially important.

It must not be assumed, however, that the maintenance of soil organic matter at normal or even high levels will solve the phosphorous problem.Most field soils require phosphatic fertilizers for best plant growth.Yet strangely enough, the economic use of phosphorous depends to a considerable degree upon the organic transformation previously described.Since micro-organisms utilise phosphorous freely, some of that added in fertilizers quickly become part of the soil organic matter.Thus, this phosphorous is held in an organic condition and is later mineralised by microbial activity.

TNN Supplies a complete range of liquid humus and humic acids and crop digesters to rapidly break down organic matter and speed up the process of producing humus.