What Is The Organic Material Formed In Soil From The Decayed Remains Of Plants And Animals Called?

Chapter 2. Organic affair decomposition and the soil food web

Soil organic thing

When plant residues are returned to the soil, diverse organic compounds undergo decomposition. Decomposition is a biological process that includes the concrete breakdown and biochemical transformation of complex organic molecules of dead material into simpler organic and inorganic molecules (Juma, 1998).

The continual improver of decaying found residues to the soil surface contributes to the biological activity and the carbon cycling process in the soil. Breakdown of soil organic affair and root growth and decay also contribute to these processes. Carbon cycling is the continuous transformation of organic and inorganic carbon compounds by plants and micro- and macro-organisms betwixt the soil, plants and the atmosphere (Figure 2)

Figure 2
Carbon cycle

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

Soil organisms, including micro-organisms, employ soil organic thing equally food. As they intermission downwards the organic thing, any excess nutrients (N, P and S) are released into the soil in forms that plants can use. This release procedure is called mineralization. The waste products produced past micro-organisms are likewise soil organic matter. This waste is less decaying than the original plant and brute material, but it tin exist used by 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 power of a soil to provide the crop with sufficient nutrients to harvest a healthy product. The organic matter content, specially the more than stable humus, increases the capacity to store h2o and store (sequester) C from the atmosphere.

The soil food web

The soil ecosystem (Box i) tin be divers as an interdependent life-support system composed of air, water, minerals, organic matter, and macro- and micro-organisms, all of which part together and interact closely.

The organisms and their interactions enhance many soil ecosystem functions and make up the soil food spider web. The energy needed for all food webs is generated by master producers: the plants, lichens, moss, photosynthetic leaner and algae that use sunlight to transform CO₂ from the temper into carbohydrates. Most other organisms depend on the principal producers for their free energy and nutrients; they are chosen consumers.

BOX one
Some functions of a salubrious soil ecosystem

Decompose organic matter towards humus.
Retain North and other nutrients.
Glue soil particles together for best construction.
Protect roots from diseases and parasites.
Make retained nutrients available to the plant.
Produce hormones that assistance plants abound.
Retain h2o.

Soil life plays a major office in many natural processes that make up one's mind food and water availability for agronomical productivity. The principal activities of all living organisms are growing and reproducing. By-products from growing roots and institute residues feed soil organisms. In plow, soil organisms support plant health as they decompose organic thing, cycle nutrients, enhance soil construction and command the populations of soil organisms, both benign 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 also includes found roots, insects, earthworms, and larger animals such equally moles, mice and rabbits that spend function of their life in the soil. The living portion represents about 5 pct of the total soil organic matter. Micro-organisms, earthworms and insects aid pause down ingather residues and manures by ingesting them and mixing them with the minerals in the soil, and in the process recycling energy and plant nutrients. Mucilaginous substances on the skin of earthworms and those produced past fungi and bacteria help bind particles together. Earthworm casts are also more strongly aggregated (bound together) than the surrounding soil as a result of the mixing of organic matter and soil mineral material, equally well as the abdominal mucus of the worm. Thus, the living part of the soil is responsible for keeping air and h2o bachelor, 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 flavour dependent). Therefore, the organisms are neither uniformly distributed through the soil nor uniformly nowadays all year. All the same, in some cases their biogenic structures remain. Each species and group exists where information technology tin find appropriate food supply, space, nutrients and moisture (Plate 2). Organisms occur wherever organic matter occurs (Ingham, 2000). Therefore, soil organisms are concentrated: around roots, in litter, on humus, on the surface of soil aggregates and in spaces betwixt aggregates. For this reason, they are most prevalent in forested areas and cropping systems that leave a lot of biomass on the surface.

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

T. MILLER

The activity of soil organisms follows seasonal as well every bit daily patterns. Not all organisms are active at the aforementioned time. Most are barely active or even dormant. Availability of food is an important cistron that influences the level of activity of soil organisms and thus is related to country use and management (Effigy 3). Practices that increase numbers and activity of soil organisms include: no tillage or minimal cultivation; and the maintenance of plant and annual residues that reduce disturbance of soil organisms and their habitat and provide a food supply.

Unlike groups of organisms tin be distinguished in the soil (Brussaard and Juma, 1995). Tabular array 1 classifies them by size. Tabular array 2 classifies them by role.

Decomposition process

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

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

Figure 3
Fluctuations in microbial biomass at unlike stages of crop development in conventional agronomics compared with systems with rest retentiveness and high organic matter input

Source: Balota, 1996

TABLE 1
Classification of soil organisms

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

Notation: Dirt particles are smaller than two µm.
Source: adapted from Swift, Heal and Anderson, 1979.

Tabular array 2
Essential functions performed by different members of soil organisms (biota)

Functions	Organisms involved
Maintenance of soil structure	Bioturbating invertebrates and plant roots, mycorrhizae and some other micro-organisms
Regulation of soil hydrological processes	Nigh bioturbating invertebrates and plant roots
Gas exchange and carbon sequestration (accumulation in soil)	Mostly micro-organisms and plant roots, some C protected in big compact biogenic invertebrate aggregates
Soil detoxification	Mostly micro-organisms
Nutrient cycling	Generally micro-organisms and plant roots, some soil- and litter-feeding invertebrates
Decomposition of organic matter	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, leaner and various other micro-organisms, collembola, earthworms, various predators
Sources of food and medicines	Plant roots, various insects (crickets, beetle larvae, ants, termites), earthworms, vertebrates, micro-organisms and their past-products
Symbiotic and asymbiotic relationships with plants and their roots	Rhizobia, mycorrhizae, actinomycetes, diazotrophic bacteria and various other rhizosphere micro-organisms, ants
Found growth control (positive and negative)	Direct effects: constitute roots, rhizobia, mycorrhizae, actinomycetes, pathogens, phytoparasitic nematodes, rhizophagous insects, plant-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. In that location are many dissimilar types of organic molecules in soil. Some are simple molecules that have been synthesized directly from plants or other living organisms. These relatively simple chemicals, such every bit 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, merely are more hard for soil organisms to suspension down.

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

Non-humic substances: significance and function

Not-humic organic molecules are released directly from cells of fresh residues, such as proteins, amino acids, sugars, and starches. This office of soil organic matter is the active, or hands decomposed, fraction. This agile fraction is influenced strongly by atmospheric condition conditions, moisture condition of the soil, growth phase of the vegetation, add-on of organic residues, and cultural practices, such equally tillage. It is the main food supply for various organisms in the soil.

Carbohydrates occur in the soil in three master 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 v-25 percent of the organic affair in most soils, but are hands broken downward by micro-organisms.

Polysaccharides (repeating units of sugar-blazon molecules connected in longer bondage) promote better soil construction through their ability to bind inorganic soil particles into stable aggregates. Research indicates that the heavier polysaccharide molecules may be more of import 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 activity.

The soil lipids form a very diverse group of materials, of which fats, waxes and resins make upwardly 2-6 pct of soil organic matter. The significance of lipids arises from the ability of some compounds to human action as growth hormones. Others may take a depressing effect on plant growth.

Soil N occurs mainly (> ninety pct) in organic forms as amino acids, nucleic acids and amino sugars. Small amounts exist in the form of amines, vitamins, pesticides and their deposition products, etc. The remainder is present every bit ammonium (NH₄ ^-) and is held past the clay minerals.

Compounds and function of humus

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

One of the most striking characteristics of humic substances is their ability to collaborate with metal ions, oxides, hydroxides, mineral and organic compounds, including toxic pollutants, to course water-soluble and water-insoluble complexes. Through the formation of these complexes, humic substances can dissolve, mobilize and send metals and organics in soils and waters, or accumulate in certain soil horizons. This influences nutrient availability, especially those nutrients present at microconcentrations only (Schnitzer, 1986). Accumulation of such complexes can contribute to a reduction of toxicity, e.k. of aluminium (Al) in acid soils (Tan and Binger, 1986), or the capture of pollutants - herbicides such as Atrazine or pesticides such as Tefluthrin - in the cavities of the humic substances (Vermeer, 1996).

Humic and fulvic substances raise plant growth directly through physiological and nutritional furnishings. Some of these substances part every bit natural institute hormones (auxines and gibberillins) and are capable of improving seed germination, root initiation, uptake of plant nutrients and tin serve as sources of Due north, P and S (Tan, 1994; Schnitzer, 1986). Indirectly, they may affect institute growth through modifications of physical, chemical and biological properties of the soil, for instance, enhanced soil water holding capacity and CEC, and improved tilth and aeration through skillful soil structure (Stevenson, 1994).

Nigh 35-55 percent of the not-living part of organic matter is humus. It is an important buffer, reducing fluctuations in soil acidity and nutrient availability. Compared with unproblematic organic molecules, humic substances are very circuitous and large, with high molecular weights. The characteristics of the well-decomposed role of the organic affair, the humus, are very dissimilar from those of elementary organic molecules. While much is known about their full general chemical limerick, the relative significance of the various types of humic materials to plant growth is yet to be established.

Humus consists of different humic substances:

Fulvic acids: the fraction of humus that is soluble in water nether all pH weather condition. Their color is ordinarily calorie-free yellow to yellow-chocolate-brown.
Humic acids: the fraction of humus that is soluble in water, except for weather more than acid than pH 2. Common colours are dark brown to black.
Humin: the fraction of humus that is non soluble in water at any pH and that cannot be extracted with a stiff base, such as sodium hydroxide (NaOH). Ordinarily black in color.

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

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

Fulvic acids are produced in the earlier stages of humus formation. The relative amounts of humic and fulvic acids in soils vary with soil blazon and management practices. The humus of forest soils is characterized by a high content of fulvic acids, while the humus of agricultural and grassland areas contains more humic acids.

BOX 2
Humic substances retain nutrients available on demand for plants

Functions of humus: