Seed inoculation with rhizobia


Reduced legume production is associated with declining soil fertility and low availability of soil nitrogen (N) to plants. Nitrogen is essential for plant growth and is the most limited nutrient in the African soils. This needed nitrogen may be applied as mineral fertilizers, as organic residues, or in the case of many legumes through securing abundant antmospheric N through Biological Nitrogen Fixation (BNF). Legume BNF occurs in the root nodules of many legumes, including common bean (Phaseolus vulgaris), when Rhizobium bacteria use plant sugars to transform atmospheric nitrogen (N≡N) into forms useful to plants (e.g. amino acids). This process is managed by supplying beans with elite strains of rhizobia through seed inoculation. These elite strains are available from several laboratories in Africa, and combined into either solid or liquid formulation inoculants by private companies, parastatal service providers or microbiology laboratories. Solid inoculants are most common and are added to seeds before planting, while liquid formulation inoculants are best suited to more mechanized planting operations. In some soils, large numbers of less effective native rhizobial may reduce the benefits of inoculation. More information on inoculation of bean is available from Dr. Paul Woomer of the TAAT Clearinghouse by email

About the Solution

Inoculation may be defined as the process of adding effective bacteria to the host plant seed before planting. The purpose of inoculation is to make sure that there is enough of the correct type of rhizobia present so that a successful legume-rhizobia symbiosis is established. Rhizobia bacteria live in nodules located on the roots of legume plants. As legumes germinate, the Bacteria “infect” the developing roots and the host plant responds by developing a nodule. The legume Host plant provides nutrients to the rhizobia bacteria, and in exchange, the plant utilizes the N produced by the rhizobia bacteria to make protein and other compounds needed for growth and development. Healthy nodules are easily visible on the root surface and will appear red in color when cut open, indicating that the nodule is actively fixing N. Rhizobia bacteria are specific with regards to which Legumes they will effectively nodulate and fix nitrogen. This forms the basis of seed inoculation technology involving the precise matching of rhizobia strains with the legume species such as common bean to enhance maximization of symbiotic nitrogen fixation (SNF). Some rhizobia strains can also infect species of legumes other than those listed, but will not produce a nodule capable of fixing N. Because we cannot always be certain that the appropriate rhizobia strains exist in sufficient populations in soil to ensure nodulation, inoculation at planting is generally recommended for legumes.

The manufacture of high quality rhizobial inoculants has enabled the utilization of seed inoculation in common bean production in several African countries. Many improvements to inoculants and the advent of inoculants’ quality control systems in several African countries have ensured that quality is optimised and maintained. Legume seeds inoculation represents an efficient and sustainable source of plant available N for its growth and subsequent crop. Studies have shown yield increases of subsequent crops planted after harvesting of legumes are often equivalent to those expected from application of 30 to 80 kg of fertilizer-N/ ha. Inoculation with elite rhizobia has resulted in increase legume production tackling nutritional and food security particularly in Eastern and Southern Africa. It’s also cost effective and environmentally friendly as compared to the use of inorganic nitrogenous fertilizers consequently reducing the cost of production and increasing grain yields. Seed inoculation is recommended in low fertile soils particularly low soil nitrogen. However, to effectively harness the benefits of seed inoculation in these poor soils, a supplemental basal fertilizer containing nutrients such as phosphorus and micronutrients, with liming effect, should be applied. Rhizobia are sensitive to soil pH outside the range of <5 or >8, high soil N and low Phosphorus, exposure to light, extreme temperatures, drought and excessive moisture

Highly effective nitrogen-fixing rhizobial strains specific for a legume such as common bean are used in the production of commercial rhizobial inoculants. These strains are selected and identified through a step-wise approach involving laboratory, greenhouse and field-based experiments and later their evaluation in a wide range of agro-ecology. These elite strains are highly infective in relation to nodule formation and highly adapted to prevailing environmental conditions. The strains are stored and maintained in selected laboratories such as MIRCEN spread all over Africa. Inoculants are manufactured through artificially culturing (multiplying) the rhizobia in a set of controlled conditions (Laboratory) to produce a culture. The rhizobia culture is then introduced into a sterile carrier material, normally carbon-rich substrate like filter mud, for proliferation and maintenance of bacterial populations before being used for seed inoculation. They are marketed in either liquid or solid form.

Proper handling and storing procedures are key to maximize the effectiveness of rhizobia-containing inoculant products. Rhizobial cells are sensitive to UV light so the whole procedure of seed inoculation is done under a shade. Seed inoculation with solid-based rhizobia inoculants start with preparation of sticker (e.g Gum Arabic), normally accompanying the inoculants, by mixing it with luke-warm water to form a sticky solution. The sticker helps ensure that the majority of the seed surface is well coated with rhizobia. The seeds are placed in a clean basin; then the sticker solution is poured in and mixed thoroughly with the seeds to ensure that all the seeds have a sticky covering. The rhizobia inoculant is then added and the mixture mixed gently ensuring all the legumes seeds are covered with the inoculants. The inoculated seeds are then allowed to stand for a few minutes to allow the rhizobial inoculants adhere onto the seeds. Once dry, the legume seeds can be sown following the guidelines provided for specific crop and region. Recently, liquid-based inoculants are being introduced in the African market and are mostly preferred for their ease of application among the smallholder farmers and commercial farms where mechanization is employed to sow legume seeds. Generally, the liquid inoculants have adhesives incorporated in their formulation and farmers are just required to pour in and mix the seeds thoroughly with inoculants then they can be sown.

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Commercially available

Startup Requirements
Inoculant production requires industrial microbiology facilities. Mother cultures are stored under refrigeration and their viability and purity tested. Cultures are raised in Yeast Extract Mannitol broth in aerated fermenters. Carrier is placed into a thin plastic bag and sterilized. Mixing mature broth and carrier material requires sterile conditions. Product is cured under low humidity conditions to improve its hardiness. Inner packages are then combined with adhesive and instructions in an outer labelled alumino-laminate package to protect the product during shipping and storage. Skilled microbiologists are required to produce the product.
Production Costs
Establishing a legume inoculants factory costs about $150,000 to build, and another $120,000 to equip with an estimated capacity to produce 16 tons of product per year worth over $500,000. Each 100 g packet is produced for less than $2.20 each and sold for over $1 profit. Isolation, identification and extensive testing of elite rhizobia strains requires significant investment from the public sector and donors, but this cost should not be passed to commercial applications because the resulting elite strains are considered Regional Public Goods.
Customer Segmentation
Seed inoculation with rhizobia is exclusively intended for legumes production in both smallholder and commercial farming systems. Liquid formulation rhizobial inoculants are preferred by commercial farmers where planting operations are mechanized. Inoculants are distributed through agrodealers and farmer cooperatives.
Potential Profitability
Seed inoculation with rhizobia presents a cost-effective and sustainable approach for supplying nitrogen consequently reducing cost of production and increased legume yields. Specifically, farmers save on cost for purchasing nitrogen fertilizer essential for top-dressing, labour cost for top-dressing and time. In comparison, N Fertilizer is the largest single cost ($40) and inoculant one of the least cost (<5% of total) per acre. Future attention should perhaps address tradeoffs in entering legumes’ production (and growing less of something else) and the residual benefits to subsequent crops following legumes in rotation programs.
Licensing Requirements
Rhizobial inoculants are produced and marketed under commercial licenses with the oversight of the relevant Government department in a particular country and the Licensor (Technology owner) for compliance with laid out system requirements.
Innovation as Public Good
Inoculants are considered Regional Public Goods since they are initially evaluated and developed through grants from the public sector and donors


Accompanying Solutions

High-iron bean, Climbing bean with higher nitrogen fixation potential, Chemical seed dressing, Specialized fertilizer blends (e.g. sympal), Low cost staking, Mechanical weeder & herbicide (integrated weed management), Hermetic grain storage, Flour and flour products, Pre-cooked beans.

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