Why inoculation is important for your crops.

Nitrogenous fertilizer is the most expensive of plant fertilizer.

The reduction of elemental nitrogen to ammonia is an energy intensive reaction. In the Haber-Bosch reactor nitrogen and hydrogen react at high temperature (350⁰C) and great pressure (300-500 atm) and a catalyst as follows:

N2 + 3H2 = 2NH3

For every kilogram of NH3 formed 2 to 2,5kg diesel is consumed.

Likewise biological nitrogen fixation is an energy intensive process involving 2 to 2,5kg of photosynthate (glucose) for the formation of 1kg NH3. If nitrogenous fertilizer is available, the plant would rather use this than fixing nitrogen from the atmosphere.

In biological nitrogen fixation, 2,2 to 2,5kg of glucose is needed to produce 1kg of NH3 (this is also expensive, but these costs are not covered by the farmer).

Furthermore, this process takes place under normal temperatures and pressures, by means of an enzyme system that includes molybdenum, iron and sulphur (therefore, molybdenum is essential for nitrogen fixation).

Large quantities of nitrogen are fixed annually through effective symbiosis between legume and rhizobia.

The correct, selected bacteria are then either applied directly to the seed before planting or added in a liquid form to the seeds in the plant furrow during planting.

In order for this symbiosis to be effective, the correct bacterial symbiont is essential.

Was inoculation successful?

The effectiveness of inoculation can be evaluated by looking at the number, position and colour of the nodules.

Number and size of nodules:

It is determined by the size of the plant and the nitrogen requirements of the host at the time.

Position of the nodules:

The position of the nodules on the roots indicates when nodulation took place. A few large pink nodules high on the root crown indicate early and effective nodulation. The lower the nodules are on the root system, indicates later nodulation.

Colour:

The colour of a nodule that is cut open also indicates effectiveness and activity. Pink nodules indicate an effective strain of rhizobia that nodulated the plant. Later in the season the nodules turn green, indicating nitrogen fixation had stopped. A large number of small, white nodules over the whole root system indicate an ineffective strain was used to inoculate the plant or that resident rhizobia have nodulated the plant.

There are also various methods for inoculation.

  1. The powder inoculant can be mixed with water and a sticker and applied as a slurry to the seed:

    • It must be thoroughly mixed.

    • Keep it out of direct sunlight after inoculation since ultraviolet rays kill bacteria (exposure for a short period is not hazardous).

    • Plant as soon as possible after inoculation since dehydration also kills bacteria.

  2. The liquid inoculant can be directly applied to the seed or in the plant furrow during planting.

What is an effective inoculant?

Main Points:

  1. Contains the correct rhizobial strain.

  2. Must contain at least the required effective minimum number of viable rhizobia/gram (five hundred million bacteria / gram inoculant).

  3. It must have a shelf life of at least six months and still maintain the minimum requirements in 2.

  4. Must be free of contaminants that may inhibit the rhizobia.

Important details to remember:

The correct organism is of great importance. Some rhizobia that, for example were used in the past to nodulate soybeans were not that effective, but have now colonised the soil in which it survives well and out-competes other selected inoculants. This is why it is essential to use only the right, inoculant strains selected and approved by the ARC. Ensure that the correct type of inoculant for the specific crop is used, i.e. lucerne inoculant for lucerne seed and soybean inoculant for soybean seed.

The correct number of viable cells per seed is important, and that is why inoculants should not be used after the expiry date. The shelf life of Stimuplant inoculants on the white carrier is 9 months and 6 months on the black carrier.

Not all contaminants are harmful, but actinomycetes (that smell like wet or tilled soil) are harmful. Stimuplant maintains a high level of quality control internally and batches are regularly tested and approved by the Plant Protection Research Institute (PPRI) of the Agricultural Research Council (ARC). The advantage is that should there be a complaint about no or poor nodulation, the ARC records can be checked. Tests on inoculants about which a complaint was recorded, showed that the viable counts of the remaining bags had a higher count of the right organism than when initially tested.

Which are the correct bacteria to use?

They are specific rhizobia bacteria that are selected for each specific legume based on the following characteristics:

Competitiveness:

Most soils contain indigenous rhizobia and other soil micro-organisms that may compete with the correct rhizobia before and during infection of the legume plants. Therefore a strain that can outcompete other bacteria is crucial.

Infectivity :

The bacteria must be able to effectively penetrate the root hair and then establish in the root cortex to form a root nodule.

Effectivity:

The rhizobia must be able to effectively fix nitrogen within the nodule. An effective nodule is large and pink as a result of the leghaemoglobin that is produced by the plant. Ineffective nodules are smaller and white/grey in colour.

What is an effective strain?

An effective strain complies with all of the above criteria.

Guidelines for inoculation

  • Use a good quality commercial inoculant that is specific to the legume to be planted

  • Note and record the batch number

  • Store inoculants in a cool, dry place (not in the fridge)

  • Do not use the inoculant after the expiry date

  • Use a higher dosage under poor planting conditions. An overdose inoculant is not harmful, in contrast to an overdose insecticide or herbicide. The legume will nodulate according to its need with the available rhizobia

  • Do not let the treated seed dry out too much, since the rhizobia on the seed surface will die off over time. Do not keep treated seed in the planter overnight, otherwise re-inoculate the seed again the next day, unless a preserver is used

  • Do not expose treated seed surfaces to insecticides or herbicides for prolonged periods of time

  • Observe the recommended dosage on the instruction pamphlet since it is different for each crop

  • Plant seed as soon as possible, preferably within 3 hours of treatment in moist seedbeds

The correct organism is of great importance