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Plants growing on the soil subsist on the products of microbial activity

By Dr. S.M. ALAM
NIA, Tando Jam, Pakistan
Nov 05 - 11, 2001

Soils on earth differ from a heap of inert rock particles in many ways, but one of the more important is that they have a population of microorganisms living in them, which derives its energy by oxidizing organic residues left behind by the plants growing on the soil or by the animals feeding on these plants. In the final analysis, the plants growing on the soil subsist on the products of microbial activity, for the microorganisms are continually oxidizing the dead plant remains and leaving behind, in a form available to the plant, the nitrogenous and mineral compounds needed by the plants for their growth. On this concept, a fertile soil is one, which contains either an adequate supply of plant food in an available form, or a microbial population, which is releasing nutrients fast enough to maintain rapid plant growth; an infertile soil is one in which this does not happen, as for example, if the microorganisms are removing and locking up available plant nutrients from the soil. The soil microorganisms can be classified into major divisions, such as the bacteria, actinomycetes, fungi and algae the microflora, and the protozoa, worms and arthropods the microflora and fauna.

Microorganisms, like larger organisms, take in food and excrete by products, which are either products of respiration or components in the food supply. Microorganisms usually excrete the nitrogen of originally combined nitrogen, which is surplus to their requirements as ammonium ions under aerobic conditions, and they excrete urea, uric acid acids such as: citric, tartaric, formic, lactic, oxalic, dibasic acids, succinic acids. But, if the aeration is reduced, or anaerobic conditions set in, complex and usually foul smelling amines will also be produced such as the alipathic amines cadaverine and putrescine and the aromatic indols. All these products are harmful to the plant growth. Microorganisms can only use insoluble substances, such as cellulose and other polysaccharides and insoluble proteins as source of nutrients. But, due to production of enzymes, these substances convert into simpler compounds such as simple or amino acids and they are utilized by the microorganisms. The organisms composing the soil population:

Bacteria: Historically, bacteria have been for the most important group of soil organisms. They live mainly on the surface of the soil and humus particles. Their numbers in soil as have been reported to be 109 bacteria per gram of soil, which is reported from a soil containing sufficient and of farmyard manure. Fertile soils contain a greater proportion of bacteria than infertile. The sizes of the bacteria are very small. They are of the figures of small cocci short straight rods, short curved rods, long rods, rods with branching, thin flexible rods. These are of the sizes of 0.5u in diameter to 10u long. The bacteria's genus are: Arthrobacter, Corynebacter, Mycobacter, Pseudomonas, Actromobacter, Radiobacter Rhizobia. All these genus play an important role in the decomposition of organic matter in the soil, which helps in soil fertility. In the soil, the bacteria seem to be concentrated on clumps of humus particles rather than on mineral particles, but this could simply be because the humus farmed the nutrient substrate for these bacteria rather than these being any specific adsorption of humus by the bacterial cell wall.

Actinomycetes: The typical filamentous actinonycetes belong predominantly to the Streptomyces and Micromonospora groups. They form very find often much branched hyphae when growing, which break up into spores, either by the tip of the hyphae producing one or two spores. They use carbon and nitrogen compound such as cellulose, hemicelluloses, proteins and lignin. Most members are aerobic and some may have a limited ability to reduce nitrates. They also appear to be active under pastures and may be the dominant microorganisms in the surface layers of grass lands and thatch surfaces.

Fungi: The fungi form the second of the two great groups of soil microorganisms. Soil fungi can not be seen with either the naked eye or with a magnifying glass in normal arable soils. The soil fungi predominantly belong to the groups that form filaments or mycelia, though some species of Myxomycetes (slime fungi), yeasts and Chytridiales (Chytrids) are also found. Some of its genera are Mucor, Rhizopu, Zygorrhnchus, Trichoderma, Aspergillus, Penicillium; Cephalosporium and Fusarium. These are probably usually present as spores in soils to which fresh organic matter has not recently been added. The soil fungi are probably all hueterotrophic, but the species present have a wide variety of food requirements, ranging from those which can utilize simple carbohydrates, alcohols and organic acids, nitrates or ammonia as the source of nitrogen.

Nearly all soil fungi need to be supplied with either inorganic nitrogen salts or organic nitrogen compounds, though some yeasts, a Saccharomyces and a Rhodotorula, which are mainly subsoil inhabitants, can fix atmospheric nitrogen. The saprophytic fungi can be very efficient converters of food into microbial tissues, some can rynthesise 30 to 50 per cent of the carbon in the food into their cell substance. The filamentons fungi generally need arobic conditions all along their filaments. Penicillium nigricans, a fungus usually restricted to the upper 5 cm of the surface soil, is less tolerant of high CO2 concentration than Zygorrhynchus vuillemini, a species, which is usually more abundant below 10 cm. The fungi species plays very important role in the fertility of the soils.

Algae: The soil algae are microscopic chlorophyll-containing organisms and belong mainly to the Cyanophyceae (Myxophyceae) or blue-green algae, the Xanthophyceae or yellow-green algae, the Bacillariaceae or diatoms and the Chlorophyceae; or green algae. This soil forms typically comprise smaller and simpler species than the aquatic forms and consist either of species which only occur as small organisms or of dwarfed forms of species that can occur as large organisms. Many of the soil algae have their cell walls covered with a thick layer of a gummy substances while the cell walls of most diatoms are partially sikified.

The soil algae are found not only on the surface and just under the surface, where sunlight or diffused light may be able to penetrate, but also several centimeters below the surface where no light can penetrate. Algae develop most readily in damp soils exposed to the sun, hence usually have their maximum development in spraying and autumn when the soil is damp and the sun is not too hot and other vegetation sparse. They develop most freely on fertile soils well supplied with nutrients, and trend to be less numerous on light infertile acid soils. The green algae are the dominant group of algae in acid soils, but as the soils become more neutral the blue-green algae and the diatoms become equally important and on fertile soils the blue green algae may be the dominant group. Their members in soil vary from 2 x 105 to 3 x 105 per gram of soil. They add organic matter to soil help to bind the soil particles, improve the aeration of swamp soils and fix atmospheric nitrogen.

Some of the blue-green algae of the family Nostococeae, including members of genera Nostoc, Anabaens, Aulosira and Cylintospermum as well as a few belonging to the families Rivulariaceae Stigonemataceae and Seytonemataceae have been shown to possess the power of fixing nitrogen from the atmosphere, and thus have simpler food requirements than any other organisms, since they can obtain both their carbon and nitrogen from the air. They are important of rice crops in the field. One can summarize our present day knowledge of the importance of algae in the economy of the soil by saying that they have been shown to be of great importance in colonizing bare soil or soil devoid of organic matter, but that only the blue-green algae have proved important agriculturally, and then only in hot climates. It is possible that they are of prime importance in the cultivation of rice which is grown under water logged conditions by supplying the rice roots with free oxygen and by fixing atmospheric nitrogen, which they can use either directly or indirectly.

Protozoa: The soil protozoa are mostly rhizopods and flagellates. The rhizopods include the amoebae, of which Naegleria gruberi and Hartmanella hyalins are typical representatives. Protozoa can not yet counted directly in the soil.

Nematodes: The nematodes are non-segmented worms with thin spindle shaped bodies many of which when adult are between 0.5 and 1.5 mm in length and 10 to 30 u thick. There are several insect-killing nematodes that occur naturally in the soil and are parasitic on a wide variety of soil-inhabiting insects, including many major crop pests. Steinernema (= Neoplectana carpocapsae (Weiser) is one common species with considerable potential as a biological control agent because of its wide host range, rapid action, and environmental safety. Many species of caterpillars, root maggots, beetles, cutworms, weevils, cockroaches, ants, and borers are susceptible to this nematode. This nematode does not affect aphids, white-flies, scales, or sluges. Some other important nematode species that affect insects include S.feltiae (=Neoaplectana bibionis), S. (--N.) glaseri, and heterorhabditis bacteriophora (=heliothidis). These nematodes are harmless to people, animals, beneficial aboveground insects, and earthworms. (The scientific names of two nematodes are revised in 1992: What used to be called S. feltiae is now called S. carpocapsoe and what is now called S. feltiae used to be called N: bibionis. Be sure to clarify with suppliers which nematode is appropriate for your situation)!

Nematodes are minute animals that look like long, slender works under the microscope. They are about 0.5 mm (1/64 inch) long, transparent, and practically invisible to the naked eye. They seek out and reach insects underground or in protected plant parts where moisture is present. Moisture is essential for the movement and persistence of nematodes. The nematodes enter in insect and release a bacterium that kills the host within 24-48 hours. The bacteria serve as food for the nematodes, which complete their development in 1-3 weeks inside the dead insect. The next generation of nematodes leaves the insect in search of new hosts. Unlike plant parasitic nematodes, insect-killing nematodes do not damage to plants. Although, these nematodes naturally in soil, rarely are enough of them present to provide adequate control of most insect pests. They can be applied to crops in large quantities as a biological insecticide.

Earthworms: The important of earthworms was first stressed by Charles Darwin in lies book "The Formation of Vegetable Mould through the Action of Worms". But, in spite of this early start, and of their obvious importance in soils, for a long time little work was due on they ecology. Earthworms are very important in the development of soil fertility. They vary in size from large Lumbricus terrestris, which may have a length exceeding 25 cm fled weighing between 2 and 7 g to small pieces with lengths about 2.5 cm and weighing about 50 mg. The principal food of earthworms is dead or decaying plant remains including both leaf litter and dead roots. Animal dung is an attractive food for many species of earthworms, and large populations are frequently found under cow parts in pasture. Additions of farmyard manure to soils with increase the size of the population. Earthworms excrete sufficient amount of nitrogen and calcium along with soils from their bellies through digestive system.

Earthworms can only thrive in soils under certain specific conditions. They are intolerant to drought and frost and hence the dry sandy soils and thin soils overlying rock are not usually favourable environments for them. They need a reasonably aerated soils, hence heavy clays or undrained soils are also unfavourable as are pastures whose surface is pudded by over grazing in wet weather. They are numerous in loams and less in sands, gravels and days. Many can survive up to a year in water if it is reasonably aerated. Most earthworms, including all the larger species, need a continuous supply of calcium and if they are feeding on a calcium-rich material will excrete calcium surplus to their requirement as calcite from special plants in their digestive tract.

Earthworms are, therefore, absent on soils low in calcium. They are absent from acid soils. the numbers and weights of earthworms record in some soils are 21/2 to 31/2 million per hectare, weighing 1700 to 2000 kg, respectively. In some British soils their number recorded are 11/4 million hectares and weighing up to 650-1100 kg. Earthworms, where they flourish, as in virgin soils and pastures are the principal agents in mixing the dead surface litter body of the soil. Earthworms are extensive channelers and burrowers, improve aeration and drainage of the surface soil. They create about 15 million channels per hectare. They overall play important role in improving the fertility of the soils.

Arthropods: The soil inhabiting acarine mites vary from about 0.1 to 1 mm in size and are present in very considerable numbers belonging to a large number of genera and species. They are 0.5 to 2 mm in size and are minute wingless insects. They feed on decaying plant tissue and the microorganisms decomposing it on dead insects and presumably on the remains of other members of the soil fauna. The species present in soil vary with depth at which they live. Their members in soils receiving regular dressings of farmyard manure are higher than those receiving fertilizers.

Larvae of beetles and dipterous flies: The Larvae of beetles and dipterous flies live in the soil and can be important burrowers and channelers in it. They occur in considerable numbers in forest soils.

Ants and Termites: Ants and termites have many features in common. They are social insects living in nests present in the soil. Some genera fill the soil with passages and chambers for their brood without making any mound, while others make mounds either out of surface or of subsoil without, however, mixing in any humic matter in the way the earthworms do. This channeling and burrowing in the soil can have a very appreciable effect on the aeration and eat of soils and they could be as efficient as earthworms in improving plant growth in this way. They are very active insects in the soil body. Ants and termites have different works in the soils. Termites are the dominant animals in many tropical soils, in the sense that they probably consume more organic matter per hectare per year than any other group. The dry-wood termits live in the trees. They also play important role in soil development They develop mound on the soils.

Myriapods and Isopods: Both millepedes (Diplopoda) and Centipedes (Chilopoda) are present in soils, often in considerable numbers. In general the food of the millipedes is decaying plant litter, and centipedes are depend on the inmmature forms of the large soil fauna. In agriculture, millepedes are found in some pastures and may occur in considerable numbers in the arable soils well supplied with farmyard manure. The wooklice-isopods are predominantly saprophagous or phytophagous play important role in decay of organic matter and in altering the soil structure and pared ways for humification of plant residues in the soil. They also serve as channelers and burowers inside the soils.

Gasteropods: Slugs and mails are the two soil representatives of the gasteropods. Most are surface-feeders, usually active in damp conditions, hence during the day either burrow into the soil or into a dense, shady spot, such as under stones or leaves or in thick grass tussocks. Some species of smials; contain high concentrations of the enzyme cellulose in their gut. They are scarengers and attack on old plant tissues. They carry 11/2 million slugs per hectare in wheat field and weighing about 400 kg.

The soil-inhabiting mammals: Animals of the mouse family-mice, voles and shrews and moles are present in appreciable numbers in some undisturbed soils such as forest and prairie soils and to a less extent in some pastures. They play important roles in channeling the soil and for better growth.

Conclusion: The various groups of soil organisms do not live independently of each other, but form an interlocked system more or less in equilibrium with the environment. Their activity in soil depend on moisture content, temperature, soil enzymes, dissolution of soil minerals, breakdown of toxic chemicals. All have play a tremendous role in the development of soil fertility. Their actions involve in the formation of structural systems of the. soils which help in the increase of agricultural productivity.