Biophysical
Management of Soil
Soil Biology
The dead
plants contribute to the formation of soil organic matter, which in turn provides food,
energy and nutrients to microorganisms and also higher plants a process of cycling
of plant nutrients. Continuous decay of plant roots adds organic matter to soil, thereby
changing the soil properties, viz. Soil aggregation, cation exchange capacity, water and
nutrient retention capacity, etc. of soil. If the vegetation is removed, the soil
characters change completely. When the roots decay, the vacant space makes room for water
and air to move in.
Soil Microflora
Biological
nitrogen fixation plays an important role in the economy of crop production. The microbes
in this class of microflora are, besides bacteria, fungi, actinomycetes and algae. Of
these, bacteria are the most abundant in soil, next in order are actinomyeetes, followed
by fungi. Soil microorganisms are divided into two broad groups heterotrophs and
autotrophs.
Factors Moisture: - In
the presence of excess water, waterlogging, anaerobic condition occur the aerobes become
suppressed and inactive. In the absence of adequate moisture in soil, some of the microbes
die due to tissue dehydration and some of them change their form into resting stages of
spores or cysts.
Temperature: -
Temperature is the most important environmental factor influencing the biological
processes and the microbial activity. When the temperature is low, the number and activity
of microorganisms fall. Most of the soil organisms are mesophiles and grow well between 150C
and 450C. A temperature of 370C is considered to be optimum for most
mesophiles.
Aeration: - Microbes consume oxygen from soil air and give
out carbon dioxide. In the absence of such gaseous exchange, carbon dioxide accumulates in
soil air and becomes toxic to the microbes. Rate of oxygen intake and simultaneous
evolution of carbon dioxide are measures of microbial activity. Direct sunlight is
injurious to most of the microorganisms except algae.
Reaction: -
Bacteria prefer near neutral to slightly alkaline reaction between pH 6.5 and 8.0; fungi
grow in acidic reaction between pH 4.5 and 6.5; actinomyeetes prefer slightly alkaline
conditions.
Food: -
Well-aerated soil rich in organic matter is an essential prerequisite for maximum number
and activity of heterotrophic microorganisms. The microbial cells undergoing senescence
serve as a source of food for the organisms.
Soil Factor: -
A soil in good physical condition has good aeration and moisture supplying capacity, which
are so essential for optimum microbial activity.
Soil Bacteria Function: -
Through a number of transformations and biochemical reactions in soil and thereby directly
or indirectly help nutrition of biological fixation of nitrogen-symbiotic and
non-symbiotic; decomposition of carbohydrates and lignins; decomposition of proteins with
the liberation of ammonia or ammonification, nitrification and denitrification, the
transformation of carbon, nitrogen, phosphorous, sulphur, iron, manganese etc. All these
processes play a significant role in plant nutrition. The process of conversion of
molecular nitrogen into complex proteins through the agency of biological organisms is
known as biological nitrogen fixation. Symbiotic nitrogen fixing bacteria e.g. Rhizobium
and Nonsymbiotic nitrogen fixing bacteria e.g. Azotobacter, Clostridium
pasteurianum
Nitrogen Transforming Bacteria: - nitrogen is utilized by micro-organisms and
higher plants in inorganic form as nitrate or ammonium. The complex proteinaceous and
nitrogenous organic compounds are broken down to produce ammonia through a microbiological
process known as ammonification and the microbes responsible for this are ammonifiers or
ammonifying bacteria. In nitrification, first, nitrites are formed by nitrite forming
bacteria (Nitrosomonas) and then to nitrate by nitrifying bacteria or nitfifier (Nitrobacter).
The immobilized nitrogenous is dead bodies of the organisms is again converted by microbes
into inorganic forms, ammonium or nitrate, which can be utilized by plants and
micro-organisms.
Denitrifying Bacteria or Denitrifiers: - Denitrification is the process by which
nitrates are reduced to oxides of nitrogen and even too gaseous nitrogen. The bacteria,
which are responsible for this transformation, are known as denitrifying bacteria or
denitrifiers. e.g. Pseudomonas, Bacillus and Paracoccus. These microbes are
aerobic, but Denitrification mostly takes place under anaerobic condition.
Soil Fungi
Fungi are
heterotrophic plants larger than the bacteria. Those that live on the dead tissues of
organic substances are saprophytic. Fungi may be regarded as the scavengers who will
decompose in soil almost anything of organic nature that bacteria cannot tackle and many
of them serve as food for the bacteria.
Cellulose
and Hemicellulose Decomposing Fungi
In acid
soils, the fungi are the main decomposers of cellulose as under acidic conditions,
bacteria and actinomycetes become inactive. In acid soils, Penicillium and Trichoderma
take part in cellulose decomposition, whereas in other soils the fungi species are Aspergillus,
Fusarium, etc.
Certain
species of fungi, Alternaria, Aspergillus, etc. produce substances similar to humic
substances in soil and may be important in the synthesis of soil humus. The black or dark
brown colour of soil humus may be due to their presence.
Soil Actinomycetes
Actinomycetes
have characteristics, which are transitional between bacteria and fungi and are sometimes
called fungi-like bacteria. Actinomycetes are more abundant in dry soil than in wet soils
and more in grassland and pasture soils than in the cultivated soils. They are responsible
for the decomposition of the more resistant organic matter of soil and produce a number of
dark black to brown pigments, probably contributing to the dark colour of soil humus.
Soil Algae
Soil algae
are microscopic, chlorophyll containing organisms, being the simplest chlorophyllous
plants. Moisture and adequate sunlight are the most significant environmental conditions
influencing the algal population. One of the important roles of certain strains of
blue-green algae is the fixation of nitrogen from air.
Soil Macrofauna Earthworms: -
They transform the food so as to be more beneficial for the higher plants. These animals
are more abundant in moist soils having high organic matter and undecomposed plant
residues. They are more common in fine textured soils than in the coarse sandy soils.
Moles: - Molehills,
frequently observed in some fields, are composed of subsoil deposited by moles. They need
a good reserve of calcium in soil.
Ants: - More
active in humifying insects than plants.
Soil Microfauna
Form only
a small part of the soil population. They are more abundant in surface soil. Feeds on
decaying organic matter are called saprohytic; those on earthworms, other nematodes, etc.
are predatory, and those on roots of higher plants are parasitic. As they feed on the
bacteria and actinomycetes, they probably help to maintain a favourable balance of the
microflora in soil. Nematodes are the abundant soil microfauna in soil. They cause loss of
vigour of the root system and make plants growing in nematode infested soil liable to
diseases.
The
viruses in soils are known to parasitize bacteria and are specifically known as
bacteriophages. Clay and organic matter in soil adsorb bacteriophages and thus cause their
retention and spread in soil.
Beneficial Role of Soil Organisms
The soil
organisms play a significant role in the life cycles of plants and animals through a
number of processes such as decomposition, synthesis and transformation. The most
important reactions which the micro-organisms carry out and have significant bearing on
soil properties and plant growth are decomposition of organic matter and synthesis of
humic substances in soil, biological fixation of nitrogen, microbial transformation of
nutrients and nutrient recycling in soil. From the animal and plant bodies, through
processes of decomposition, nitrogen enters into the bodies of soil organisms and organic
substances (humus) in soil and is recycled through plants and animals. Carbon and nitrogen
cycles are the two major biological processes, which take part in the decomposition of
added organic matter to soil and formation of humus substances in soil.
Synthesis of Humus Substances in Soil
The carbon
and nitrogen cycles show the probable pathways of breakdown of added organic substances in
soil. They ultimately lead to the formation of microbial cells in soil and soil organic
matter. The main partners are plants, microorganisms, and soil. The organic residues,
which undergo breakdown and transformation, are complex in nature, containing
carbohydrates, proteins and other nitrogenous compound lignins, fats, etc. in course of
the microbial reactions, the soil is enriched with the dead tissues of organisms forming
part of its organic matter. Humus is a complex mixture of amorphous and colloidal organic
substances. The whole process of decomposition of organic matter, mainly of plant origins,
is due to the microbes.
Phosphorus: - microbial grown a requires available and
utilizable forms of phosphorus. Immobilization of available soil phosphate may occur when
large amounts of available carbon and nitrogen are present in the decomposing material.
Thus addition of straw and similar materials causes biological phosphorus depletion and
immobilization.
Sulphur: - Sulphur ion in soil is the major source of
sulphur for plants, which can also utilize a small amount of sulphur dioxide from air.
When plant and animal residues are incorporated in soil, organically bound sulphur is
mineralized by micro-organisms into sulphate ions. Part of the mineralized sulphur is
immobilized by the microflora and part is available to the plants for their nutrition. The
autotrophic bacteria, chiefly members of the genus Thiobacillus, are capable of oxidizing
inorganic sulphur compounds. Hydrogen sulphide, it should be noted, is toxic to plants
even in small concentration.
Other Elements: -Other plant nutrient elements that undergo
microbial transformations and influence their availability for plant growth are potassium,
calcium, magnesium, iron, manganese, copper, zinc, molybdenum, boron, etc. certain
bacteria and fungi are capable of decomposing alumina silicate minerals in soil, thus
releasing a portion of the potassium contained therein. The potassium thus liberated to
satisfy the demand of the microbial nutrition becomes ultimately available to plants due
to the release of soluble potassium ion during decomposition of microbial cells. Iron
transformation in soil depends on the activity of the microorganisms which, in turn,
depend upon the compounds of iron. The iron bacteria may be performing the function of
oxidation and reduction, and iron precipitation. Bacteria are more dominant in these
transformations. Deficiency of manganese is closely related to the number of manganese
oxidizing bacteria in soil.
Biofertilizers
Biofertilizers
are the cultures of micro-organisms used for inoculating seed or soil or both under ideal
conditions to increase the availability of plant nutrients. Their purpose is to supplement
chemical fertilizers and not to replace them. Some of the microorganisms have the
beneficial role of biological nitrogen fixation to supply nitrogen to crops, solubilizing
insoluble phosphates to soluble forms to make them available to crops, synthesizing
biomass for manuring crops, particularly rice, and hasten the process of decomposition of
cellulose in composts and farmyard manures through cellulolytic organisms.
Soil Aggregation
Some
organisms may play a beneficial role indirectly be creating better soil physical
condition, e.g. by improving soil aggregation. Soil microorganisms cause soil aggregation
probably by the gum or polysaccharides produced by them. Azotobacter, Beijerinckia
and Rhizobium are examples of gum producing bacteria.
Effects of cultural practices on soil organisms
Cultural
practices, viz. Cultivation, crop rotation, application of manures and fertilizers, liming
and gypsum application, application of pesticides for crop production have their effect on
the soil organisms. Ploughing and tilling operations facilitate air movement in soil and
expose soil surface to sunshine, and thereby increase biological activity, particularly of
bacteria. Cultivation of a single crop causes accumulation of a particular group of
microbes, which dominate over the others. Crop rotation
disturbs the unfavourable population balance. Crop rotation with a legume is the common
practice. Irrigation of soil brings about a
significant proliferation of soil microbes.
Soil Amendments: - Liming of
acid soil increases activity of bacteria and actinomycetes and lowers the fungal
population. Gypsum application of sodic soil is
favourable for bacterial activity. Application of fertilizers and manures increases crop
production supply food and nutrition not only to the crops but also to the microbial
population in the soil. Phosphatic fertilizers applied to berseem (Trifolium
alexandrimum) increase significantly the activity of nitrogen fixing bacteria. Organic
manures provide a readily available source of carbon for the heterotrophs. Repeated heavy
fertilization with nitrogenous fertilizer alone promotes rapid growth of fungi. Oilcakes,
viz. Neem, karanj and groundnut encourage the growth of nematode-trapping fungi.
Application of chemicals (pesticides) to control damage to crops by insects, fungi,
nematodes and weeds is imperative. The pesticide molecules may interact with soil
constituents and get adsorbed on the soil particles or they may move with water to find
their way into ground water, rivers and stems. The pesticides are systemic in nature, they
persist in soil and may be taken up by plants and ultimately by animals. Some pesticides
are biodegradable by soil organisms, other are liable to leave toxic residues, which are
likely to be hazardous. The usual dosage is not high enough to cause any profound change
in the normal microbial activity of soil.
Physical Properties
Soil is a
heterogeneous mixture of silicate particles, humus, and a variety of insoluble salts and
oxides of metals called the solid phase, a liquid phase and a gaseous phase. Depending on
the relative proportions of the various size groups we can define soil texture. Depending
on the size and shape of the aggregates we can define soil structure.
Many of
the important soil properties are related to texture. Clayey soils show high water holding
capacity, high plasticity, stickiness and swelling whereas sandy soils are conspicuous by
the absence of these properties. The most important way in which soil texture affects
plant growth is water and with it the nutrient supply. The available water capacity of
soil is related to soil texture.
This kind
of arrangement and organization of secondary and primary particles under natural condition
brings about what has been termed as soil structure. Under natural soil conditions, the
primary particles (clay, silt and sand) are mostly bonded together by cementing agents
into secondary aggregates of varying sizes.
Bulk and
Particle Density and Pore Space
Bulk
density of a soil is defined as the mass per unit volume of soil consisting of solid and
gas phases. Particle density of a soil is the mass per unit volume occupied by the soil
particles alone. Bulk density of soil is influenced by soil texture, organic matter
content and cultivation practices. The constituents of soil organic matter contribute
significantly to aggregation of soil particles. The humus portion in the aggregation is
susceptible to biodegradation. To maintain soil aggregation status of cultivates soils,
renewed addition of organic matter is essential.
Management Practices
The
management practices for crop production have a profound effect on the formation and
stability of soil aggregates
Cultivation: -
Continuous cultivation of arable land year after year, without incorporation of organic
matter, deteriorates soil structure, lowering the level of soil structure, lowering the
level of soil organic matter. Addition of organic matter helps in improving the structural
status of soil. The tillage operation affects the size distribution of peds, density and
packing of soil particles, amount of organic matter and moisture content of soil. Tillage
is to loosen the surface soil, to facilitate water infiltration and aeration. Friable
conditions of soil are the optimum soil condition for tillage operation to produce
aggregates of suitable size.
Crops and Cropping Practices: - Crops affect soil structure through their
vegetative canopy above the ground and their roots below the ground. Grasses are conducive
to well structured soil. The vegetative canopy protects the soil from the beating action
of raindrops and destruction of the structure of the surface soil and prevents crusting,
improve soil structure. The role of legumes in building up soil fertility is well known
and legumes have, therefore, their place in sound crop rotation practices. The beneficial
effect is usually attributed to nitrogen added to the soil by legumes. The structure is
built up with the advancement of stages of crop growth. Legumes treated with phosphatic
fertilizers significantly improve soil structure.
Organic
matter level and structural improvement of soil can be built up, to a varying degree, and
maintained by continuous judicious application of manures. The degree of improvement
depends upon the quantity and length of application of manure, the climatic conditions,
and nature of soil. Phosphatic fertilizers used in conjunction with nitrogenous
fertilizers improve and maintain soil structure.
Management of Soil Structure
The
objective of soil structure management is the improvement and maintenance of soil
structure. Inclusion of a suitable legume in a crop rotation is the most effective way of
improving and maintaining soil structure. Coupled with phosphatic fertilizers, legumes
improve soil structure still further. Use of balanced fertilizers for raising crops is the
other means. Judicious application of adequate quantities of well-decomposed manures, as
frequently as possible, improves soil structure. In coarse-textured soils, use of organic
manures is the only way of improving structure. Application of pond sediments or clay soil
brought from another locality is helpful in this respect. In the case of very
fine-textured soils, organic manure is no doubt helpful, but the quantity required well be
high. Crop rotation and use of phosphatic fertilizers are better methods for such soils.
Structural management of acid soils involves liming followed by organic manuring. Highly
alkali soil application of gypsum or other amendments in combination with green manuring
or manures or incorporation of crop residues has been successful in improving structure
and helps in reclamation of such soils.
Water Infiltration into Soil
Water
infiltration is the process of water entry into soil through the surface and the direction
of entry may be either downward or lateral or both. Water infiltration characteristics of
soil are of practical significance in soil and water conservation, irrigation and
watershed management. Factors influencing infiltration- texture, structure and initial
moisture content of soil control infiltration rate. Coarse textured and well aggregated
soils have usually high infiltration rate. Dry soil condition is conducive to rapid
infiltration. Vegetative covers help infiltration while the presence of a somewhat
impervious subsoil layer reduces it. The volume of water that enters into soil and the
depth to which water moves and wets the soil below the surface are of utmost importance in
the soil water plant relationship as this is one of the main sources of water supply to
plants.
Importance
of Soil Water Studies
Infiltration
studies are important for soil and water conservation. The design of furrows or basins for
ensuring even distribution of irrigation water will, therefore, depend upon the
infiltration rate. Plants meet their water requirement from water stored in soil. Microbes
and plants require, besides water, an adequate level of oxygen in soil for their growth
and activity. Low porosity and hydraulic conductivity of soil cause inadequate aeration.
Proper drainage ensures adequate aeration and removal of salts and other toxic substances
from the root zone. Recharge of groundwater is often necessary where excess of rainfall is
lost as surface runoff, due to low infiltration capacity of the soil. The soil air and
soil temperatures are closely related to water content of soil.
Soil Aeration and Plant Growth
Oxygen is
required by microbes and plants for respiration. Oxygen taken up and carbon dioxide
evolved are stoichiometric. Root elongation is particularly sensitive to aeration
conditions. Oxygen deficiency disturbs metabolic processes in plants, resulting in the
accumulation of toxic substances in plants and low uptake of nutrients. Certain plants
such as rice are adapted to grow under submerged conditions. These plants have large
internal air spaces, which facilitate oxygen transport to the roots.
Soil Temperature and Management
Soil
micro-organisms show maximum growth and activity at optimum soil temperature range. Soil
temperature has a profound influence on seed germination, root and shoot growth, and
nutrient uptake and crop growth. Seeds do not germinate below or above a certain range of
temperature. Root elongation is very much dependent on soil temperature. Each crop plant
has a specific optimum range of soil temperature for its rapid growth and maximum yield.
Soil
temperature under field conditions can be altered by suitable cultural practices such as
mulching, irrigation, drainage, and tillage. Tillage loosens the surface soil, increases
its porosity and decreases its thermal conductivity. Soil compaction has the reverse
effect on porosity and soil temperature. Mulching the surface soil with plastic cover or
crop residues may increase or decrease surface soil temperature, depending upon
environmental conditions. Mulches conserve soil moisture also. In tropical areas,
irrigation generally causes rapid and substantial reduction in maximum temperature in
summer and increase in minimum soil temperature in winter. In the cold season, the higher
temperature of irrigation water relative to soil, and high heat capacity of irrigation
water check lowering of minimum soil temperature.
Soil Compaction
Soil
compaction is the process of increasing dry bulk density of soil, reducing the pore space
by expulsion of air through applied pressure on a soil body. Initially, when water content
is low, the soil is stiff and difficult to compress, low density is the result. Compaction
of coarse-extured soil, sometimes desirable, for better seed germination, reducing
hydraulic conductivity of soil, and enhancing moisture conservation. Continued compaction
will practically remove all air. As the water content of soil increases, the water acts as
a lubricant, and the soil becomes workable with the expulsion of soil air. Compacting
creating problems for seed germination water transmission and aeration.
Soil Crusting
Crusting
of soil is a form of soil compaction. The crusts present a serious barrier for seedling
emergence, high exchangeable sodium percentage, poor structure, low organic matter content
of soil and also puddling during tillage operations are some of the factors responsible
for crust formation. Lowering the exchangeable sodium percentage and incorporation of
organic matter prevent crust formation. |
Ag.
Technologies
(Soil Magt.)
|