Inoculation of Legumes




Дата канвертавання26.04.2016
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Inoculation of Legumes

Soil fertility depletion in smallholder farms is the fundamental bio-physical root cause of declining per capita food production in Africa, and soil fertility replenishment should be considered as an investment in natural resource capital (Sanchez et al. 1997). Nitrogen depletion results from crop removal, erosion and gaseous loss and must be offset by N inputs. Thus the nitrogen reserve of agricultural soils must be replenished regularly in order to maintain crop production. Replacement of soil nitrogen is accomplished by the addition of inorganic fertilizers and by biological nitrogen fixation (Giller et al. 2009).

Chemical fertilizer is the fuel that powered the Green Revolution’s forward thrust (Norman Borlaug). Nitrogen is an essential element for all living organisms. It is a key component of proteins.

Mineral nitrogen fertilizers are produced industrially by chemically fixing N2 gas in the air to produce ammonia. This process is energy expensive as it requires both high temperature and pressure, accounting for the high price of N fertilizer.





Biological Nitrogen Fixation (BNF)

Bacteria either free living or symbiotic are able to convert atmospheric N to biologically useful forms i.e. ammonia in a process called biological nitrogen fixation (BNF). However, only symbiotic bacteria in the root nodules of legumes are able to fix significant amounts of atmospheric N.

Legumes are the most important plants with a symbiotic relationship with rhizobium bacteria by forming root nodules. Symbiotic BNF requires that N supply in the soil be limiting which fits soil fertility status of SSA.

BNF in the rhizobium-legume symbiosis offers an inexpensive alternative for smallholder farmers to mineral fertilizers and its benefits can extend to subsequent crops as residual sources of organic N in crop residues, roots and nodules.




Prolifically nodulated soybean


The need to inoculate

In many soils, the nodule bacteria are not adequate in either number or quality. Under these conditions, it is necessary to inoculate the seed or the soil with highly effective rhizobia.

Root nodule bacteria are cultured in the laboratory and combine with a suitable carrier material, such as peat, compost or filter mud, to make an inoculant. The process of adding this inoculant to the seed is called inoculation.

Inoculation is often required when new legumes are introduced to an area. Host-specific rhizobia are frequently developed for new varieties of legumes.

Many soils are heavily infested with ineffective rhizobia capable of inducing nodulation without benefiting the legume host. Very large inoculant rates of competitive and effective strains may counteract these aggressive native rhizobia.

The production of rhizobial inoculants

Producing inoculants requires elite strains of rhizobia that are raised under sterile conditions and mixed with carrier material that protect the rhizobia and make it easier to apply





Types of inoculants

Legume inoculants are of two general types:



  1. those designed for application to seeds or

  2. directly to the soil.

Seed inoculants are the most common because they are easy to apply and are generally effective under most conditions.

Application of liquid formulation inoculants directly to the soil may be necessary to obtain effective nodulation when planting legume seeds in hot, dry or highly acidic soils or under adverse weather conditions, or when the seeds are treated with chemicals toxic to rhizobia.

High-quality peat-based inoculants are generally considered the most dependable. Shelf life varies with carrier, handling and storage temperature.

It is important that inoculant quality be monitored by an independent laboratory and conservative expiration dates be established to protect the interests of users.



Selecting quality inoculants

  1. Inoculant should contain only rhizobia capable of producing effective nodules Effective inoculants may consist of one or several elite strains.

  2. Inoculant should provide large numbers of viable rhizobia allowing for application of at least 10,000 bacteria per seed.

  3. Carrier medium must protect the rhizobia in the package and on the seed. It should be easy to apply and adhere well to the seed.

  4. Inoculant must be free of other bacteria which might be detrimental to rhizobia or to the young legume seedling. Some inoculants contain other beneficial root bacteria..

  5. Inoculant must be packaged to protect the rhizobia until it is used. The package should allow exchange of gases and retention of moisture.

  6. The package should provide clear instructions and list the legumes that it effectively nodulates and carry an expiry date beyond which the product cannot be considered dependable.

Inoculant labeling and storage requirements

The information required on the legume inoculant package should include:



  1. Name of the crops for which the inoculant is intended

  2. Scientific name of the Rhizobium species

  3. Number of live rhizobia per gram

  4. Expiration date beyond which the product cannot be used

  5. Lot number for quality control feedback

  6. Instructions for use

  7. Net weight of inoculant

  8. Trade name, manufacturer and address

  9. Necessary storage conditions


BIOFIX complies closely with industry standards of product labeling


Inoculant storage and handling

Legume inoculants are perishable and quickly lose their effectiveness when exposed to a temperature of 40o C or more. Inoculants retain their effectiveness for six months or longer when stored at a temperature around 20o C. This period can be extended if refrigerated near 4o C but freezing inoculants damages the product.


Transferring rhizobia in soil is inefficient compared to the use of inoculants

  1. Inoculants contain superior strains of rhizobia, soil does not

  2. Inoculants contain only rhizobia, soils may also be infested with pests and disease organisms

  3. Inoculants are light and easily transported, soils are bulky and transport expensive.





Inoculating legume seeds using the slurry method

Preparing the slurry. For soybean seed, a slurry consisting of 1 part of inoculant and 3 parts sticker is recommended. For larger seeds, the ratio of sticker decreases (e.g. 1:2 for bean). Inoculant and adhesives are mixed together before adding to legume seeds.


Slurry inoculation




Legume Inoculants and their Use, 1984. FAO, Rome.
Inoculating seeds using the two-step method

  • Place 5 kg of soybean seeds into a plastic bag.

  • Add 100 ml of gum arabic sticker

  • Inflate the bag and twist it shut

  • Shake the bag vigorously for about one minute

  • Open the bag and add 50 g of inoculant

  • Shake again, but more gently for one minute.

  • Immediately after coating, spread the seeds on paper and allow them to dry in a shady place




A different amount of sticker is required for various legume seed depending upon their size (surface area). More adhesive is required for smaller seed. More adhesive is also required for the slurry method.


legume

seed

--- slurry method ---

-- two-step method --

seed

weight

sticker

inoculant

sticker

inoculant




g/seed

ml/kg seed

g/kg seed

ml/kg seed

g/kg seed

soybean

0.14

30

10

20

10

bush bean

0.42

22

10

19

10

climbing bean

0.45

20

10

18

10

groundnut

0.50

18

10

16

10

cowpea

0.14

30

10

20

10


Pelleting limestone after the two step method

Seeds may be pelleted with finely ground limestone or rock phosphate using either the slurry or two-step method. In both cases, the amount of inoculant remains the same (10 g per kg seed) but the amount of adhesive increases by 33%. The following describes seed pelleting with limestone using the two-step method.



  • Inoculate 5 kg soybeans using the two-step method but use twice (200 ml) the adhesive

  • Add one kg of limestone and again shake gently

  • Inspect that seeds are uniformly coated

  • Spread pelleted seeds on paper or canvas

  • Dry under shade

  • Plant dried, pelleted seed as soon as possible

Pelleting seed with minerals requires that more adhesive be applied. More minerals may be applied to smaller seeds. Acidic minerals, such as super phosphate or sulfur must not be applied as pellets as these injure rhizobia.

legume

seed

---------- two-step pelleting ----------

seed

weight

sticker

inoculant

coating




g/seed

ml/kg seed

g/kg seed

g/kg seed

soybean

0.14

40

10

200

bush bean

0.42

33

10

160

climbing bean

0.45

30

10

150

groundnut

0.50

25

10

100

cowpea

0.14

40

10

200


Sound use of legume inoculants

  • Use the correct inoculant for each legume. Check the label for the legume species you are planting.

  • Protect inoculant from sun and heat to keep it alive. The ideal storage temperature is between 4o and 26o C.

  • Store inoculant in tightly closed bags.

  • Use a sticker when inoculating seeds.

  • Use the recommended amount of inoculant. Use no less than 5 g per kg of seeds.

  • Inoculate seeds just before planting.

  • Apply soil inoculant when the soil is moist or just before irrigation.

  • Cover the furrows after planting inoculated seeds.

Some common mistakes in inoculant handling and use

  • Exposing inoculants to temperatures above 30 C

  • Using inoculants after their expiration date or after they have been exposed to high temperatures.

  • Letting inoculants dry out.

  • Mixing fertilizer with inoculated seeds.

  • Broadcasting inoculants onto dry soil.

  • Appling additional inoculant to the surface when the soil is dry.

  • Planting commercially prepared, pre-inoculated seeds.



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