How Do Plant And Animal Remains Become Part Of Soil And Help To Make Soil Healthy

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Chapter two. Organic matter decomposition and the soil food web

Soil organic matter

When found residues are returned to the soil, various organic compounds undergo decomposition. Decomposition is a biological process that includes the physical breakup and biochemical transformation of circuitous organic molecules of dead material into simpler organic and inorganic molecules (Juma, 1998).

The continual addition of decomposable found residues to the soil surface contributes to the biological action and the carbon cycling process in the soil. Breakdown of soil organic affair and root growth and disuse as well contribute to these processes. Carbon cycling is the continuous transformation of organic and inorganic carbon compounds by plants and micro- and macro-organisms between the soil, plants and the atmosphere (Figure 2)

FIGURE 2
Carbon cycle

Decomposition of organic thing is largely a biological process that occurs naturally. Its speed is determined by three major factors: soil organisms, the concrete environs and the quality of the organic affair (Brussaard, 1994). In the decomposition process, different products are released: carbon dioxide (CO₂), energy, water, establish nutrients and resynthesized organic carbon compounds. Successive decomposition of dead material and modified organic matter results in the formation of a more circuitous organic matter called humus (Juma, 1998). This process is chosen humification. Humus affects soil properties. Equally information technology slowly decomposes, it colours the soil darker; increases soil aggregation and aggregate stability; increases the CEC (the power to concenter and retain nutrients); and contributes N, P and other nutrients.

Soil organisms, including micro-organisms, use soil organic affair as food. As they pause downward the organic thing, whatsoever excess nutrients (N, P and Southward) are released into the soil in forms that plants can apply. This release process is called mineralization. The waste products produced by micro-organisms are likewise soil organic thing. This waste cloth is less decomposable than the original constitute and animal textile, but it tin can be used past a large number of organisms. By breaking down carbon structures and rebuilding new ones or storing the C into their own biomass, soil biota plays the most important role in nutrient cycling processes and, thus, in the ability of a soil to provide the crop with sufficient nutrients to harvest a healthy production. The organic thing content, especially the more stable humus, increases the capacity to store h2o and shop (sequester) C from the atmosphere.

The soil nutrient web

The soil ecosystem (Box 1) can be defined as an interdependent life-support arrangement composed of air, h2o, minerals, organic matter, and macro- and micro-organisms, all of which function together and interact closely.

The organisms and their interactions enhance many soil ecosystem functions and brand up the soil nutrient spider web. The energy needed for all food webs is generated by primary producers: the plants, lichens, moss, photosynthetic bacteria and algae that utilise sunlight to transform CO₂ from the atmosphere into carbohydrates. Most other organisms depend on the master producers for their energy and nutrients; they are called consumers.

BOX ane
Some functions of a healthy soil ecosystem

• Decompose organic thing towards humus.

• Retain Northward and other nutrients.

• Glue soil particles together for all-time structure.

• Protect roots from diseases and parasites.

• Make retained nutrients available to the establish.

• Produce hormones that help plants grow.

• Retain water.

Soil life plays a major role in many natural processes that determine food and water availability for agricultural productivity. The chief activities of all living organisms are growing and reproducing. Past-products from growing roots and plant residues feed soil organisms. In turn, soil organisms back up plant health as they decompose organic thing, wheel nutrients, enhance soil structure and control the populations of soil organisms, both beneficial and harmful (pests and pathogens) in terms of crop productivity.

The living role of soil organic matter includes a wide variety of micro-organisms such as bacteria, viruses, fungi, protozoa and algae. It likewise includes establish roots, insects, earthworms, and larger animals such as moles, mice and rabbits that spend role of their life in the soil. The living portion represents most 5 per centum of the total soil organic matter. Micro-organisms, earthworms and insects help break down ingather residues and manures past ingesting them and mixing them with the minerals in the soil, and in the procedure recycling energy and plant nutrients. Gluey substances on the skin of earthworms and those produced by fungi and bacteria aid bind particles together. Earthworm casts are likewise more strongly aggregated (leap together) than the surrounding soil as a outcome of the mixing of organic matter and soil mineral material, equally well as the intestinal fungus of the worm. Thus, the living part of the soil is responsible for keeping air and water available, providing plant nutrients, breaking down pollutants and maintaining the soil structure.

The composition of soil organisms depends on the food source (which in turn is season dependent). Therefore, the organisms are neither uniformly distributed through the soil nor uniformly present all year. However, in some cases their biogenic structures remain. Each species and group exists where it can observe appropriate food supply, space, nutrients and wet (Plate 2). Organisms occur wherever organic thing occurs (Ingham, 2000). Therefore, soil organisms are concentrated: around roots, in litter, on humus, on the surface of soil aggregates and in spaces between aggregates. For this reason, they are most prevalent in forested areas and cropping systems that go out a lot of biomass on the surface.

Plate ii
Termites create their own living weather well-nigh their
preferred food sources. Within the colony life is highly organized.

T. MILLER

The activity of soil organisms follows seasonal every bit well every bit daily patterns. Not all organisms are active at the same time. Most are barely active or even fallow. Availability of food is an of import factor that influences the level of activeness of soil organisms and thus is related to land use and direction (Effigy 3). Practices that increase numbers and action of soil organisms include: no tillage or minimal tillage; and the maintenance of plant and annual residues that reduce disturbance of soil organisms and their habitat and provide a food supply.

Different groups of organisms tin be distinguished in the soil (Brussaard and Juma, 1995). Table i classifies them past size. Table ii classifies them past function.

Decomposition procedure

Fresh residues consist of recently deceased micro-organisms, insects and earthworms, old plant roots, ingather residues, and recently added manures.

Crop residues contain mainly complex carbon compounds originating from cell walls (cellulose, hemicellulose, etc.). Chains of carbon, with each carbon atom linked to other carbons, form the "backbone" of organic molecules. These carbon chains, with varying amounts of fastened oxygen, H, N, P and Southward, are the footing for both simple sugars and amino acids and more complicated molecules of long carbon chains or rings. Depending on their chemic structure, decomposition is rapid (sugars, starches and proteins), slow (cellulose, fats, waxes and resins) or very slow (lignin).

Figure 3
Fluctuations in microbial biomass at dissimilar stages of crop development in conventional agriculture compared with systems with residue retention and high organic thing input

Source: Balota, 1996

TABLE one
Classification of soil organisms

Micro-organisms	Microflora	<5 µm	Leaner Fungi
	Microfauna	<100 µm	Protozoa Nematodes
Macro-organisms	Meso-organisms	100 µm - 2 mm	Springtails Mites
	Macro-organisms	2 - twenty mm	Earthworms Millipedes Woodlice Snails and slugs
Plants	Algae	10 µm
	Roots	> 10 µm

Annotation: Clay particles are smaller than 2 µm.
Source: adapted from Swift, Heal and Anderson, 1979.

TABLE 2
Essential functions performed past different members of soil organisms (biota)

Functions	Organisms involved
Maintenance of soil structure	Bioturbating invertebrates and plant roots, mycorrhizae and another micro-organisms
Regulation of soil hydrological processes	Most bioturbating invertebrates and plant roots
Gas commutation and carbon sequestration (aggregating in soil)	By and large micro-organisms and found roots, some C protected in large meaty biogenic invertebrate aggregates
Soil detoxification	Generally micro-organisms
Food cycling	Mostly micro-organisms and plant roots, some soil- and litter-feeding invertebrates
Decomposition of organic affair	Various saprophytic and litter-feeding invertebrates (detritivores), fungi, bacteria, actinomycetes and other micro-organisms
Suppression of pests, parasites and diseases	Plants, mycorrhizae and other fungi, nematodes, bacteria and diverse other micro-organisms, collembola, earthworms, diverse predators
Sources of nutrient and medicines	Found roots, various insects (crickets, protrude larvae, ants, termites), earthworms, vertebrates, micro-organisms and their by-products
Symbiotic and asymbiotic relationships with plants and their roots	Rhizobia, mycorrhizae, actinomycetes, diazotrophic bacteria and various other rhizosphere micro-organisms, ants
Plant growth control (positive and negative)	Direct effects: institute roots, rhizobia, mycorrhizae, actinomycetes, pathogens, phytoparasitic nematodes, rhizophagous insects, constitute-growth promoting rhizosphere micro-organisms, biocontrol agents Indirect effects: most soil biota

During the decomposition process, microorganisms convert the carbon structures of fresh residues into transformed carbon products in the soil. There are many different types of organic molecules in soil. Some are simple molecules that accept been synthesized directly from plants or other living organisms. These relatively simple chemicals, such as sugars, amino acids, and cellulose are readily consumed by many organisms. For this reason, they practise not remain in the soil for a long time. Other chemicals such as resins and waxes also come directly from plants, simply are more difficult for soil organisms to break down.

Humus is the result of successive steps in the decomposition of organic matter. Because of the complex construction of humic substances, humus cannot be used by many micro-organisms as an energy source and remains in the soil for a relatively long time.

Non-humic substances: significance and function

Non-humic organic molecules are released directly from cells of fresh residues, such equally proteins, amino acids, sugars, and starches. This part of soil organic affair is the active, or hands decomposed, fraction. This agile fraction is influenced strongly by atmospheric condition atmospheric condition, moisture status of the soil, growth stage of the vegetation, addition of organic residues, and cultural practices, such as cultivation. It is the principal food supply for diverse organisms in the soil.

Carbohydrates occur in the soil in three main forms: free sugars in the soil solution, cellulose and hemicellulose; complex polysaccharides; and polymeric molecules of various sizes and shapes that are attached strongly to clay colloids and humic substances (Stevenson, 1994). The simple sugars, cellulose and hemicellulose, may constitute 5-25 percent of the organic matter in most soils, but are hands cleaved downwards past micro-organisms.

Polysaccharides (repeating units of sugar-type molecules connected in longer chains) promote better soil construction through their ability to bind inorganic soil particles into stable aggregates. Inquiry indicates that the heavier polysaccharide molecules may be more important in promoting aggregate stability and water infiltration than the lighter molecules (Elliot and Lynch, 1984). Some sugars may stimulate seed germination and root elongation. Other soil properties affected by polysaccharides include CEC, anion retention and biological activeness.

The soil lipids form a very diverse group of materials, of which fats, waxes and resins make upwards 2-6 percent of soil organic thing. The significance of lipids arises from the ability of some compounds to act as growth hormones. Others may accept a depressing effect on plant growth.

Soil N occurs mainly (> 90 per centum) in organic forms equally amino acids, nucleic acids and amino sugars. Small amounts exist in the grade of amines, vitamins, pesticides and their degradation products, etc. The remainder is nowadays as ammonium (NH₄ ^-) and is held by the clay minerals.

Compounds and part of humus

Humus or humified organic matter is the remaining role of organic matter that has been used and transformed by many different soil organisms. Information technology is a relatively stable component formed past humic substances, including humic acids, fulvic acids, hymatomelanic acids and humins (Tan, 1994). It is probably the nearly widely distributed organic carbon-containing textile in terrestrial and aquatic environments. Humus cannot be decomposed readily because of its intimate interactions with soil mineral phases and is chemically as well circuitous to be used past about organisms. It has many functions (Box 2).

Ane of the most hit characteristics of humic substances is their ability to interact with metal ions, oxides, hydroxides, mineral and organic compounds, including toxic pollutants, to form water-soluble and water-insoluble complexes. Through the formation of these complexes, humic substances can dissolve, mobilize and transport metals and organics in soils and waters, or accumulate in certain soil horizons. This influences food availability, especially those nutrients present at microconcentrations simply (Schnitzer, 1986). Accumulation of such complexes can contribute to a reduction of toxicity, due east.m. of aluminium (Al) in acid soils (Tan and Binger, 1986), or the capture of pollutants - herbicides such as Atrazine or pesticides such every bit Tefluthrin - in the cavities of the humic substances (Vermeer, 1996).

Humic and fulvic substances heighten plant growth direct through physiological and nutritional effects. Some of these substances function equally natural plant hormones (auxines and gibberillins) and are capable of improving seed germination, root initiation, uptake of plant nutrients and tin serve as sources of N, P and S (Tan, 1994; Schnitzer, 1986). Indirectly, they may bear on found growth through modifications of physical, chemical and biological properties of the soil, for case, enhanced soil water holding capacity and CEC, and improved tilth and aeration through practiced soil construction (Stevenson, 1994).

About 35-55 percent of the not-living part of organic matter is humus. Information technology is an important buffer, reducing fluctuations in soil acidity and nutrient availability. Compared with simple organic molecules, humic substances are very complex and big, with high molecular weights. The characteristics of the well-decomposed part of the organic thing, the humus, are very dissimilar from those of simple organic molecules. While much is known about their general chemic composition, the relative significance of the various types of humic materials to plant growth is nevertheless to exist established.

Humus consists of unlike humic substances:

• Fulvic acids: the fraction of humus that is soluble in water under all pH atmospheric condition. Their color is usually light yellow to yellowish-brown.

• Humic acids: the fraction of humus that is soluble in water, except for conditions more acrid than pH 2. Common colours are nighttime brown to black.

• Humin: the fraction of humus that is not soluble in water at any pH and that cannot be extracted with a strong base, such as sodium hydroxide (NaOH). Commonly black in colour.

The term acrid is used to describe humic materials because humus behaves like weak acids.

Fulvic and humic acids are circuitous mixtures of big molecules. Humic acids are larger than fulvic acids. Research suggests that the different substances are differentiated from each other on the basis of their water solubility.

Fulvic acids are produced in the before stages of humus formation. The relative amounts of humic and fulvic acids in soils vary with soil type and direction practices. The humus of wood soils is characterized past a loftier content of fulvic acids, while the humus of agronomical and grassland areas contains more humic acids.

BOX 2
Humic substances retain nutrients available on demand for plants

Functions of humus:

• improved fertilizer efficiency;

• longlife N - for instance, urea performs sixty-80 days longer;

• improved nutrient uptake, particularly of P and Ca;

• stimulation of beneficial soil life;

• provides magnified nutrition for reduced disease, insect and frost affect;

• salinity management - humates "buffer" plants from backlog sodium;

• organic humates are a catalyst for increasing soil C levels.

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Source: https://www.fao.org/3/a0100e/a0100e05.htm#:~:text=Micro%2Dorganisms%2C%20earthworms%20and%20insects,bacteria%20help%20bind%20particles%20together.

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