After successful completion of this Unit, you should be able to answer:

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Module V Groundnut Plant Nutrition
Virtual Academy for the Semi Arid Tropics
Lesson 2 Importance of Primary Plant Nutrients
Unit 2 Biological Nitrogen Fixation

• After successful completion of this Unit, you
  should be able to answer
• What is biological nitrogen fixation?
• What type of bacteria is involved in nitrogen
  fixation in groundnut?
• How do you know that Rhizobium infection has
  occurred in groundnut?

Course on Groundnut Production Practices
2

• After successful completion of this Unit, you
  should be able to answer
• Where actually the nitrogen fixation takes place
  in groundnut plants?
• How can you identify that the nitrogen fixation
  is low or poor in groundnut crop?
• What practice one should follow when nitrogen
  fixation is poor or low in a standing groundnut
  crop?

3

• After successful completion of this Unit, you
  should be able to answer
• What are the factors that affect the Rhizobium
  population in a soil?
• How can you ensure good nitrogen fixation before
  sowing of groundnut crop?
• What criteria need to be followed in selecting
  effective strain of Rhizobium?

4

• After successful completion of this Unit, you
  should be able to answer
• How much of carrier-based Rhizobium inoculum
  is required for one hectare of groundnut crop?
• What is the procedure for preparing the
  inoculum for seed inoculation?
• How do you practice seed inoculation in the
  field?
• What is the procedure for preparing the
  inoculum for liquid inoculation?

5

• After successful completion of this Unit, you
  should be able to answer
• How do you practice liquid inoculation in the
  field?
• What is the procedure for preparing the
  inoculum for granular inoculation?
• How do you practice granule inoculation in the
  field?
• How Rhizobium inoculation can be done using
  compost?

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Certain microorganisms present in the soil have
the ability to convert the abundant atmospheric
nitrogen into a form of nitrogen which can be
used by the legume plants. This process of
conversion of nitrogen in the atmosphere to
available nitrogen to plants is called nitrogen
fixation. Nitrogen fixation occurs in legume
plants in association with a group of bacteria
belonging to Rhizobium family.
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Groundnut being a legume crop utilizes the
nitrogen in the atmosphere by nitrogen fixation
in association with a group of bacteria belonging
to Rhizobium family. The Rhizobium bacteria
penetrate the roots of legumes and form root
nodules (Fig.). Nitrogen fixation takes place in
these nodules.
Nodules on groundnut roots
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Most of the groundnut cultivated fields in India
have large population of this Rhizobium and
ensure normal to abundant nodule formation in the
groundnut crop. However, sufficient nodulation
does not mean better nitrogen fixation. Some
of the nodules are found to be ineffective and do
not fix adequate nitrogen required by the
groundnut plants.
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The effective nodules are big and concentrate on
the tap root and the top lateral roots (left side
plant in the picture). Ineffective nodules are
small, and are distributed throughout the root
system (right side plant in the picture). The
effectiveness of nodulation and availability of
nitrogen to plants will result in dark green
foliage and better growth of plants (Observe in
the picture).
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To get a better look at the nodulation examine
the number of nodules on the roots. Fewer nodules
may indicate a problem. However, even a great
number of nodules does not mean that they are
actively fixing nitrogen. To determine this,
carefully slice open several nodules. Active
nodules are pink to dark red inside (Fig.). A
white color inside indicates that they nodule is
not yet active. Other colors may indicate that
little nitrogen is being fixed.
Ineffective white nodules on the left. Effective
ones on the right side.
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Nodulation Index Using a shovel, carefully dig
plants from three or four different locations in
the field at about six weeks after germination of
the crop. Count the number of nodules on the
roots, determine the average number of nodules
per plant and compare to the index on the left
side.
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Nodulation Index This early-season index is
only meant as a guideline of nodulation versus
the need for an application of nitrogen
fertilizer. If nodulation is found to be poor
or fair, consider your crop's nitrogen needs and
the possible application of nitrogen fertilizer
as top dressing.
Source Calvin Trostle, Extension Agronomist,
Texas AM University
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• After examining the nodules and nodulation in a
  groundnut crop indicates no or poor nitrogen
  fixation, it is necessary to assess the reasons
  for such a situation.
• The rhizobium population in a soil is affected
  by a number of factors.
• Acidity Rhizobia and/or their effectiveness may
  be reduced in soils with a pH below 5.5 or above
  8.0.

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• The rhizobium population in a soil is affected
  by a number of factors.
• Temperature Rhizobia populations can be reduced
  in hot, dry soils particularly at planting or
  may not be available to shallow-planted seed.
  Cool soil temperatures also slow down the
  movement of bacteria into the roots.
• Drought Moisture is needed for Rhizobia to
  survive. Prolonged drought, combined with high
  temperatures, can reduce bacteria levels.
• Flooding Flooding and the depletion of oxygen in
  the root zone will also kill the bacteria.

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• The rhizobium population in a soil is affected
  by a number of factors.
• Competition Other strains of bacteria and soil
  organisms competing for moisture and nutrients
  may reduce the amount of Rhizobia.
• Plant stress Any practice or conditions that
  puts stress on the plant can reduce the nutrients
  available to the bacteria thereby reducing
  formation of nodules.
• Nitrogen level Formation of nodules can be
  reduced with applied fertilizer nitrogen even as
  low as 30 kg N/ha in the soil.

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• The rhizobium population in a soil is affected
  by a number of factors.
• Seed treatment chemicals or pesticides Some of
  these products are toxic to the bacteria.
• Additional concerns Other soil conditions that
  reduce bacteria populations include compaction
  which reduces movement of the bacteria or erosion
  which removes bacteria from the soil or exposes
  them to adverse conditions.
• Organic matter Rhizobia survive better in soils
  with increased organic matter.

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To ensure effective nodules, nodulation and
nitrogen fixation, the crop has to be provided
with highly efficient Rhizobium in the vicinity
of its root system after correcting the soil
conditions which affect the survival of Rhizobia.
This can be achieved by artificially
inoculating pre selected effective and efficient
Rhizobium culture.
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• Inoculation Procedure
• Higher rate of Rhizobium inoculation is required
  to obtain sufficient nodulation to overcome the
  competition by the native Rhizobium.
• In general, 200 - 500 g of carrier based
  Rhizobium inoculum is recommended per hectare or
  inoculum containing 105 cells of Rhizobia / seed
  is the minimum requirement.
• The technology involves strain selection,
  preparation of inoculants and suitable method of
  application of Rhizobium.

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• Strain selection
• Rhizobium selected for inoculation must meet the
• following criteria for efficient nitrogen
  fixation
• The strain should be an effective nitrogen fixer
  
• It should be able to out compete the native
  Rhizobium population to form maximum number of
  effective nodules
• It should be able to survive on seed and soil for
  a longer period and should be able to multiply
  faster in the vicinity of the roots of host
  plants under a variety of environmental
  conditions.

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• Preparation and Application of Inoculum
• Seed Inoculation
• A solution of 5 jaggery or sugar is prepared
  using water. For treating 100 kg kernels of
  groundnut, 800 ml of solution is required.
• In Tamil Nadu the farmers use rice gruel as a
  sticker.
• Two hundred grams of carrier-based Rhizobium
  culture is added to the above solution to form a
  slurry.

Carrier-based Rhizobium culture
21

• Preparation and Application of Inoculum
• Seed Inoculation
• Seeds are evenly spread on a cement floor or on
  a polythene sheet.
• The slurry is evenly poured and gently smeared on
  the kernel surface.
• For uniform application of the inoculum small
  batches of seed may be treated.

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• Preparation and Application of Inoculum
• Seed Inoculation
• Seeds are evenly spread on a cement floor or on
  a polythene sheet.
• The slurry is evenly poured and gently smeared on
  the surface of the seeds.
• For uniform application of the inoculum small
  batches of seed may be treated.

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• Seed Inoculation
• The treated seeds are dried in the shade to avoid
  death of the Rhizobium.
• The seeds are then sown immediately.
• Most of the seed-treating fungicides used to
  prevent seed-borne diseases kill the inoculating
  Rhizobium.
• When both seed treatments are essential, the
  seeds may be treated with fungicide first and
  then with the Rhizobium culture.

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• Preparation and Application of Inoculum
• Liquid inoculation
• Liquid inoculum is prepared by mixing the
  Rhizobium culture in 50-100 liters water. The
  liquid culture is poured in seed furrows evenly
  where seeds are sown. The furrows need to be
  covered immediately.
• This method of Rhizobium application will be
  useful in areas where sowing is done behind the
  country plough.

Application of liquid inoculum
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• Preparation and Application of Inoculum
• Granular inoculation
• Granular inoculum is prepared by mixing 200 g of
  carrier-based inoculum with 1 to 1.5 liters of
  aqueous methyl cellulose. Then, this inoculum is
  mixed with 15 kg of washed river sand to evenly
  coat with the peat.
• The above inoculum mixture is air-dried, and then
  uniformly distributed in the seed furrows by
  hand. If seed cum fertilizer drill is used, the
  inoculum mixture is placed in fertilizer hopper.

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• Preparation and Application of Inoculum
• Rhizobium-inoculant mixed with compost
• 500 g of Rhizobium-inoculant is thoroughly mixed
  with 1000 kg of well-decomposed compost. This
  enriched compost is applied into the seed furrows
  and the crop is sown immediately.
• It is very important to use only well decomposed
  compost, as partially decomposed compost may
  bring down the population of inoculated
  Rhizobium.

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Module V Groundnut Plant Nutrition
Virtual Academy for the Semi Arid Tropics
Lesson 2 Importance of Primary Plant Nutrients
With this, the Unit 2 on Biological Nitrogen
Fixation in this Lesson concludes. The next
Unit in this Lesson is about phosphorus nutrition
in groundnut. The subsequent Units in this
Lesson deal with the other aspects of groundnut
nutrition. Select Unit 3 in Lesson 2 of Module
V from Course contents
Course on Groundnut Production Practices