It refers to the arrangement of soil particles. It is
one of the important property of soil, since it influences
aeration, permeability and water capacity.
" It is the vertical section of the soil through all its
horizons from the surface to the unaffected parent
materials''. Generally the profile consists of three mineral
horizons viz., A, B and C.
The surface soil or that layer of soil at the top which
is liable to leaching and from which some soil constituents
have been removed is known as horizon 'A' or the horizon of
eluviation. The intermediate layer in which the materials
leached from horizon ' A' have been re-deposited is known as
horizon ' B' or the horizon of illuviation. The parent
material from which the soil is formed is known as horizon '
C'.
The soil in each of these horizons is usually uniformly
developed and presents a more or less homogeneous character.
Each layer or horizon develops specific morphological
features such as the size and shape of particles, their
arrangement, colour, consistence etc. which distinguish from
one horizon to another.
Study of soil profile is important since it reveals the
characteristics and qualities of the soil.
Inorganic matter - Macro nutrients and Micro nutrients
Organic Matter
The plants and animals grown in weathered material and
the organic residues left behind decay with time and become
an integral part of the soil. The main source of soil
organic matter is plant tissue. Animals are subsidiary
source of soil organic matter.
The micro flora like bacteria, fungi, algae,
actinomycetes, and micro fauna like protozoa, nematodes,
macro fauna like earthworms, ants etc. play an important
role in formation of organic matter.
The organic matter influences the soil in respect to
colour, physical properties, supply of available nutrients
and adsorptive capacity.
Soil Organisms
Soil is the habitat for enormous number of living
organisms. Some of these organisms are visible to naked eye
where as others can be seen by microscope only.
Roots of higher plants are considered as soil macro
flora while bacteria, fungi, algae and actinomycetes are
considered as soil micro flora. Protozoa and nematodes are
the significant soil micro fauna where as the earthworms,
moles and ants constitutes soil macro fauna.
Soil Water
In order to function as a medium for plant growth, soil
must contain some water. The main functions of water in the
soil are as follows:
Promotes many physical and biological activities of
soil.
Acts as a solvent and carrier of nutrients.
As a nutrient itself.
Acts as an agent in photosynthesis process.
Maintains turgidity of plants.
Acts as an agent in weathering of rocks and minerals.
Soil Air
Oxygen is essential for all biological reactions
occurring in soil. Its requirement is met from the soil air.
The gaseous phase of soil acts as a path way for intake
of oxygen which is absorbed by soil micro organisms, plant
roots and for escape of carbondioxide produced by the
plants.
This two way process is called soil aeration. Soil
aeration become critical for the plant growth when water
content is high, because water replaces soil air.
Soil Inorganic Matter
The inorganic constituents of the soil comprises
carbonates, soluble salts, free oxides of iron, aluminium
and silica in addition to some amorphous silicates.
The inorganic constituents forms the bulk of the solid
phase of the soil. Soils having more than 20% of the organic
constituents are designated as organic soils.
Soils where inorganic constituents dominates they are
called mineral soils. The majority of the soils in India are
mineral soils.
Soil fertility deals with the nutrient status or ability
of soil to supply nutrients for plant growth under
favourable environmental conditions such as light,
temperature and physical conditions of soil.
Soil productivity is defined as the capability of the
soil for producing a specified quantity of plant produce per
unit area and the ability to produce sequence of crops under
a specified system of management.
Those soils with pH less than 6.5 and which respond to
liming may be considered as acid soils.
Reasons for Acidity
Humus decomposition results in release of large amounts
of acids. There by lowering the pH.
Rainfall : In areas with more than 100 cm rainfall
associated with high R.H ---- Ca, Mg is dissolved in water
and leached out due to this base saturation of soil
decreases.
Application of elemental sulphur under goes reactions
resulting in formation of H2So4.
Continuous application of acid forming fertilizers like
ammonium sulphates or ammonium chlorides results in
depletion of Ca by CEC ( cation exchange capacity)
phenomenon.
Parent Material : Generally rocks are considered as
acidic, which contain large amount of silica (Si o2) when
this combined with water, acidity increases.
Characteristics
PH is less than 6.5
This soils are open textured with high massive
Structure.
Low in Ca, Mg with negligible amount of soluble salts.
This soils appear as brown or reddish brown, sandy loams
or sands.
Injurity to Crops
Direct Affects
Plant root system does not grow normally due to toxic
hydrogen ions.
Permeability of plant membranes are adversely affected
due to soil acidity.
Enzyme actions may be altered, since they are sensitive
to PH changes.
Indirect Affects
Deficiency of Ca, Mg occur by leaching.
Al, Mn, Fe available in toxic amounts.
All the micro nutrients except molybdenum are available.
So 'Mo' deficiency has been identified in leguminous crops.
Phosphorous gets immobilized and its availability is
reduced.
Actvity of Micro Organisms
Most of the activities of beneficial organisms like
Azatobacter and nodule forming bacteria of legumes are
adversely effected as acidity increases.
Potassium sulphate is a suitable source of 'K' for acid
soils. But MOP is better than K2So4 because Cl of MOP
replaces -OH ions, their by release of -OH ions tends to
increase the PH.
Alkali soils are formed due to concentration of
exchangeable sodium and high pH. Because of high alkalinity
resulting from sodium carbonate the surface soil is
discoloured to black; hence the term black alkali is used.
Reasons for Alkalinity
The excessive irrigation of uplands containing Na salts
results in the accumulation of salts in the valleys.
In arid and semi arid areas salt formed during
weathering are not fully leached.
In coastal areas if the soil contains carbonates the
ingression of sea water leads to the formation of alkali
soils due to formation of sodium carbonates.
Irrigated soils with poor drainage.
Characteristics
Injury to Crops
High exchangeable sodium decreases the availability of
calcium, magnesium to plants.
Dispersion of soil particles due to high exchangeable
'Na' leads to poor physical condition of soil, low
permeability to water and air, tends to be sticky when wet
and becomes hard on drying.
Toxicity due to excess hydroxyl and carbonate ions.
Growth of plant get affected mainly due to nutritional
imbalance.
Restricted root system and delay in flowering in
sensitive varieties.
Typical leaf burn in annuals and woody plants due to
excess of chloride and sodium.
The soils which owe characteristics that they can not be
economically used for the cultivation of crops without
adopting proper reclamation measures are known as problem
soils''.
Saline Soils
The saline soils contains toxic concentration of soluble
salts in the root zone. Soluble salts consists of chlorides
and sulphates of sodium, calcium, magnesium. Because of the
white encrustation formed due to salts, the saline soils are
also called white alkali soils.
Reasons For Salinity
In arid and semi arid areas salts formed during
weathering are not fully leached. During the periods of
higher rainfall the soluble salts are leached from the more
permeable high laying areas to low laying areas and where
ever the drainage is restricted, salts accumulate on the
soil surface, as water evaporates
The excessive irrigation of uplands containing salts
results in the accumulation of salts in the valleys.
In areas having salt layer at lower depths in the
profile, seasonal irrigation may favour the upward movement
of salts.
Salinity is also caused if the soils are irrigated with
saline water.
In coastal areas the ingress of sea water induces
salinity in the soil.
Characteristics
PH
Less than 8.3
Ec
More than 4.0 m.mhos/ cm
ESP( exchangeable sodium %)
Less than 15
Chemistry of soil solution
Dominated by sulphate and chloride ions
and low in exchangeable sodium
. Effect of electrolytes on soil particles
Flocculation due to excess soluble salts.
Main effect on plant
High osmotic pressure of soil
solution
Geographic distribution
Arid and semi arid regions.
Diagnosis under field condition
Presence of white crust
Presence of chloris barborata(weed)
Patchy growth of plants.
Injury to Crops
High osmotic pressure decreases the water availability
to plants hence retardation of growth rate.
As a result of retarded growth rate, leaves and stems of
affected plants are stunted.
Development of thicker layer of surface wax imparts
bluish green tinge on leaves
The salts are to be leached below the root zone and not
allowed to come up. However this practice is some what
difficult in deep and fine textured soils containing more
salts in the lower layers. Under this conditions a provision
of some kind of sub-surface drains becomes important.
The required area is to be made into smaller plots and
each plot should be bounded to hold irrigation water.
Separate irrigation and drainage channels are to be
provided for each plot.
Plots are to be flooded with good quality water upto 15
- 20 cms and puddled. Thus, soluble salts will be dissolved
in the water.
The excess water with dissolved salts is to be removed
into the drainage channels.
Flooding and drainage are to be repeated 5 or 6 times
till the soluble salts are leached from the soil to a safer
limit.
Green manure crops like Diancha can be grown upto
flowering stage and incorporated into the soil. Paddy straw
can also be used.
Super phosphate, Ammonium sulphate or Urea can be
applied in the last puddle. MOP and Ammonium chlorides
should not be used.
Scrape the salt layer on the surface of the soil with
spade.
Grow salt tolerant crops like sugar beet, tomato, beet
root, barley etc
Before sowing , the seeds are to be treated by soaking
the seeds in 0.1% salt solution for 2 to 3 hours.
Need : When land is brought under cropping, grain or
fruit and sometimes the entire plants are removed
(harvested) from the land. Hence, the soil losses a
considerable amount of its nutrients (up take by plants). If
cropping is continued over a period of time, without
nutrients being restored to the soil, its fertility will be
reduced and crop yields will decline.
Apart from removal by crops, nutrients may also be lost
from the soil through leaching and erosion. Even to maintain
soil productivity at the existing levels, it is necessary to
restore to the soil, the nutrients removed by crops as also
those lost through leaching and erosion.
Continued maintenance of a high level of soil fertility
is an indispensable for profitable land use and sustained
agricultural production. From time to time the inherent
fertility of soil has to be evaluated. There are different
methods for soil fertility evaluation as listed below:
Visual method of diagnosis
Plant analysis (Analysis of whole or part of plant
growing on the soil in question).
Biological tests in which higher plants or certain micro
organisms are used.
Soil tests.
Field experiments.
Advantages
Among the different methods soil testing is a better
method for the following reasons.
Soil testing, being a rapid method, is an advantage over
the biological methods which are relatively elaborate and
time consuming. It is also better than deficiency symptoms
and plant and tissue analysis, because the needs can be
determined before the crop is planted while in the other
methods the crop needs can be ascertained only after the
crop is grown, by which time it may be late to correct any
nutritional deficiencies that may be indicated.
The main purpose of soil testing is to evaluate the
fertility status of the soil. It provides a basis for
fertilizer, lime and gypsum recommendation. Laboratory test
is a means of making an inventory of the chemical conditions
of soil and determining treatments, if any, are needed. Soil
test information is then used along with an evaluation of
specific crop requirements, cropping history and physical
characteristics of the soil for determining the exact
amounts of different nutrients and soil amendments, if any,
needed for a certain crop or cropping sequence. With this
objective in view, a number of soil testing laboratories
have been established in the country by the State
Governments, Agricultural Universities and fertilizer
industry for making fertilizer recommendations to farmers on
the basis of the fertility status of their soils. This
service is generally rendered free of cost.
Methodology
Soil Sampling : Soil tests and their interpretation are
based on the soil samples sent in for analysis. It is,
therefore, important that soil sample should be properly
collected and be representative of the area to be tested.
Soil tests and their interpretation are as reliable as the
soil samples drawn.
Sampling Procedure
Each field should be sampled separately. When the areas
within the field distinctly differ in crop growth, in the
appearance of soil, in elevation or area known to have been
manured or cropped differently in the field should be
divided suitably and each area sampled separately.
Drawing samples from spots which do not represent the
field should be avoided. Such spots may be old bunds, marshy
spots, hedges, areas previously occupied by compost heaps,
etc. Sampling should not be done in a field within three
months of the application of lime, ash or fertilizer.
Proper sampling tool should be used. Samples can be
satisfactorily taken with a soil tube, an auger, a kassi
(spade) or khurpi. In a very friable soil, a large spoon can
also be used.
A composite sample may be taken from each area. After
scrapping the surface litter, a uniform core or a thin slice
of soil from the surface to plough depth (15 to 22 cm deep)
from 15 to 20 spots should be taken. In a hard soil, a small
pit of about 15 cm x 15 cm and of about 15 cm in depth be
made. Than a v-shaped slice from one of the slides be taken.
Where crops have been planted in lines, sampling may be
done between the lines.
Individual cores or slices should be collected in a
clean container. All lumps should be broken and mixed well
in the container or on a clean cloth. The size of the
composite sample should be reduced by successive quartering
to about half a kilogram.
The sample should be dried in shade for an hour or two
before putting it into a bag and dispatching it to the
nearest soil testing laboratory. Alkathene bags which are
available from soil testing laboratories or ordinary clean
cloth bags may be used.
Each sample should be identified by name or number to
correspond to the field name or number and also by the
cultivator's name.
The information sheet furnished by the soil testing
laboratory should be filled up completely. This is important
and will help the chemist to schedule a more accurate
fertilizer recommendation. The information sheet along with
the soil sample container should be sent to the soil testing
laboratory.
If standard information sheets are not available,
information may be given on the following points.
Legal or revenue description of the land (survey
number and name of the village)
Crops grown in the last two years:
Date of lost ploughing of the field:
Quantity of lime, gypsum and fertilizer used and when:
Date of legumes last grown on the field:
Whether green manuring practiced, if so, when:
Lay of the land, degree of erosion, drainage, crop
growth etc;
Crops to be grown in the next year:
In the soil testing laboratory, soil samples are
analyzed for the following five individual soil properties :
PH or soil reaction which indicates whether the soil is
acidic, alkaline or normal
Total soluble salts which indicates whether the soil is
saline or normal :
Organic carbon (as a measures of available nitrogen)
Available phosphorus
Available potash
Whenever facility exists samples are also analyzed for
micro nutrients, especially for Zinc.
Soil Testing
Based on analysis, soils are classified into three
categories i.e., low, medium and high, in respect of their
available content of each nutrient according to the ratings
of soil reaction.
Soil Test Interpretation and Fertilizer
Recommendations
From the results of analysis of soil samples sent by the
farmer and information sheet supplied by him, soil test
reports are prepared in the laboratories. Copies of these
reports are sent to the concerned farmer.
Soil test reports are usually in three main parts. First
part indicates results of analyses of the soil sample. Most
laboratories give actual analyses as well as the ratings.
Second part is fertilizer recommendations for the crop based
on soil analyses, history of the field like cropping
pattern, manures and fertilizers earlier applied, etc. This
part indicates quantities of nitrogen (N), Phosphate (
P205), Potash(K20), Zinc (Where facilities exist ) and also
of lime or gypsum to be applied per hectare. Most
laboratories also show in the report optimum quantities of
organic manures as per recommendations of the Agriculture
departments.
The third part of the report usually indicates time and
methods of fertilizer application and other practices
required to make the fertilizer use more efficient.
During the relatively short period that soil testing
service has been in operation in this country a large number
of soil samples have been analyzed in various laboratories.
Based on the results of these analyses , soil fertility maps
have been prepared indicating the nutrient status of
nitrogen, phosphorus, potassium and zinc in different parts
of the country. It must however, be noted that this is only
a broad classification , since it is based on limited soil
sample analysis.
Rating Chart for Soil Test Data
Nutrient
Low
Medium
High
Organic carbon ( as a measure of available
nitrogen)
Below o.5 %
0.5 - 7.5%
Above 0.75%
Available nitrogen( N)
Below 280Kg/ha
280-560kg/ha
Above 560Kg/ha
Available Phosphorus(P)
Below 10.0 Kg/ha
10.0-25Kg/ha
Above 25 Kg/ha.
Available potassium( K)
Below 110Kg/ha
110-280Kg/ha
Above280Kg/ha
Soil Types
pH
Acids
Below 6.0
Normal to saline
6.0 to 8.5
Tending to become alkaline
8.9 to 9.0
Alkaline
Above 9.0
Total Soluble Salts (Conductivity in milli
mhos/cm2)
Below 1ss
normal
1-2
critical for germination
2-4
critical for growth of the sensitive
crops
Above 4
Injurious to most crops
Infrastructure Related Information
The basic infra structure needed for soil testing are PH
meter, conductivity meter, spectrophotometer or calorimeter
for estimating phosphorous, flame photometer for potassium
and atomic absorption spectrophotometer for estimating micro
nutrients. Besides the instruments, glass - ware and
necessary chemicals are required.
In most of the soil testing laboratories PH, Ec, Organic
carbon (as an index of available nitrogen), available
phosphorus, available potassium are estimated. If necessary
micro nutrients like Fe, Cu, Mn, Zn and B are estimated, Ca,
Mg and S are also estimated if any deficiency symptoms are
observed on crops.
PH is estimated by a glass electrode PH meter in 1:2
soil water suspension.
Electrical conductivity is measured by a conductivity
meter in 1:2 soil water suspension.
Available nitrogen is estimated by Subbaiah and Asija
(1956) method (distillation of soil with alkaline potassium
permanganate solution). But in most of the laboratories
organic carbon is taken as an index of available nitrogen
content of soil assuming C:N ratio is 10. Organic carbon is
determined by chromic acid oxidation by rapid titration
(Walkley and Black ( 1934) rapid titration).
Phosphorous is determined by Olsen's (1954) using 0.5 M
sodium bicarbonate as extractant and phosphorus is analyzed
calorimetrically.
Neutral normal ammonium acetate solution is the most
widely used extractant for available potassium which is
analyzed by flame photometer.
Micro nutrients are extracted by DTPA and determined by
atomic absorption spectro photo meter.
There are 8 major group of soils in India which are
furnished below
Red Soils
Red colour is due to various oxides of iron. They are
poor in N, P, K and with pH varying 7 to 7.5. These soils
are light textured with porous structure. Lime is absent
with low soluble salts.
Red soils occurs extensively in Andhra Pradesh , Assam,
Bihar, Goa, Parts of kerala, Maharastra, Karnataka,
Tamilnadu and West Bengal. Most of the red soils have been
classified in the order ' Alfisols'.
Lateritic Soils
Seen in high rainfall areas, under high rainfall
conditions silica is released and leached down wards and the
upper horizons of soils become rich in oxides of iron and
alluminium. The texture is light with free drainage
structure.
Clay is predominant and lime is deficient. pH 5 to 6
with low in base exchange capacity, contained more humus and
are well drained. They are distributed in summits of hills
of Daccan karnataka, Kerala, Madhyapradesh, Ghat regions of
Orissa, Andhra pradesh, Maharastra and also in West Bengal,
Tamilnadu and Assam.
Most of the laterite soils have bee classified in the
order ' ultisols' and a few under ' oxisols'.
Alluvial Soils
These are the most important soils from the agriculture
point of view. The soils are sandy loam to clay loam with
light grey colour to dark colour, structure is loose and
more fertile. But the soils are low in NPK and humus.
They are well supplied with lime; base exchange capacity
is low, pH ranges from 7 to 8. These soils are distributed
in Indo-Gangetic plains, Brahmaputra valley and all most all
states of North and South. Most of the alluvial soils have
been classified in the orders ' Entisols', ' Inceptisols'
and ' Alfisols'.
Black Soils
This is well known group of soils characterised by dark
grey to black colour with high clay content.
They are neutral to slightly alkaline in reaction. Deep
cracks develop during summer, the depth of the soil varies
from less than a meter to several meters. Poor free drainage
results in the soils, base exchange is high with high pH and
rich in lime and potash. Major black soils are found in
Maharastra, Madhyapradesh, Gujarat and Tamilnadu.
Cotton is most favourable crop to be grown in these
soils. These soils are classified in the order 'Entisols', '
Inceptisols' and ' vertisols'.
Forest Soils
This group of soils occur in Himalayas. Soils are dark
brown with more sub-soil humus content. They are more
acidic.
Desert Soils
These soils are mostly sandy to loamy fine sand with
brown to yellow brown colour, contains large amounts of
soluble salts and lime with pH ranging 8.0 to 8.5. Nitrogen
content is very low.
The presence of Phosphate and Nitrate make the desert
soils fertile and productive under water supply. They are
distributed in Haryana, Punjab, Rajasthan. They are
classified in the order ' Aridisols' and ' Entisols'.
Peaty and Marshy Soils
These soils occur in humid regions with accumulation of
high organic matter. During monsoons the soils get submerged
in water and the water receipts after the monsoon during
which period rice is cultivated. Soils are black clay and
highly acidic with pH of 3.5. Free alluminium and ferrous
sulphate are present.
The depressions formed by dried rivers and lakes in
alluvial and coastal areas some times give rise to water
logged soils and such soils are blue in colour due to the
presence of ferrous iron.
Peaty soils are found more in Kerala and marshy soils
are found more in coastal tracks of Orissa, West Bengal and
South - East coast of Tamilnadu.
Saline - Sodic Soils
Saline soils contain excess of natural soluble salts
dominated by chlorides and sulphates which affects plant
growth. Sodic or alkali soils contain high exchangeable
sodium salts.
Both kinds of salt effected soils occur in different
parts of India like Uttarpradesh, Haryana, Punjab,
Maharastra, Tamilnadu, Gujarat, Rajastan and Andhra pradesh.
These soils are classified under ' Aridisols', ' Entisols'
and ' Vertisols'
