INOCULATION OF legumes with rhizobia can deliver substantial nitrogen inputs but inoculants need to be used strategically and with a risk/benefit approach to maximize the nitrogen delivered. Care is also needed where the survival of rhizobia can be compromised, such as when dry sowing, in acid soils and when adding the fertilizers and pesticides. These are the key take home messages derived from a 2013 survey of growers use and understanding of inoculants. While the inoculation of legumes with rhizobia has become a slandered practice, it can often be improved by fine tuning practices. Many growers follow the dictum if in doubt inoculate, but this blanket approach can lead to unnecessary inoculation or skepticism if the results are not as expected.
This is why a target approach to inoculation and nitrogen management is needed. This includes a consideration of soil type, legume species and inoculation history. Also, changing practice, such as trends toward early (dry) sowing in some regions, is taking us into new territory for recommendations about the rhizobial inoculation. Another important practical issue is the compatibility between the rhizobial inoculant, fertilizer and seed applied pesticides additives.
N Fixation benefits
Legumes (crops and pastures) are estimated to fix almost three million tons of the nitrogen in a year. This amount of fixed nitrogen contributes about half of the estimated six million tons of nitrogen that is required annually for grain and animal production. However, the contributions made by the legumes vary considerably with species and with the situation (soil type, seasonal rainfall and crop management). Crop legume, on an average fixes 110 kilograms of nitrogen per hectare annually, including nitrogen in the grain. However, the range is large, varying in individual paddocks from close to zero to more than 400 kg of N/ha depending upon the conditions. Nitrogen fixation generally increase with crop biomass, therefore good agronomic practices leading to optimal legume growth will favor the higher nitrogen inputs. In southern Australia legume growth is strongly influenced by the amount of water that the crop pasture can access. Management practices that optimize water use efficiency and keep soil nitrate levels low will favor legume growth and nitrogen fixation.
When, Where, How?
There is low chance of response to grain legume or pasture inoculation where there has been a recent history of inoculation with the appropriate rhizobia; the soil pH is more than 6 (in CaCl2), grain yields and pasture production have been sufficient.
In these situation, inoculation every four years or so will be adequate because soil rhizobial population will generally be maintained. After four years there is more likelihood of a response to inoculation because lower numbers of rhizobia will remain in the soil, so a top with the potent commercial inoculation strain may beneficial.
If the legume species or another that uses the same rhizobia has not been grown in the previous four years, or soil condition are hostile, then the probability of a response to inoculation is much higher. This is the case where acid-sensitive legumes. In this situation it will be prudent to inoculate every time is sown because rhizobial population tends to diminish quickly under this soil condition. The exception to this acid rule and lupines because both lupines and their rhizobial strain well adapted to acid soil.
Where a crop such as chick pens which have a specific requirement is grown for the first time, is essential as there will be no background presence of suitable. To check for sufficient modulation, growers and consultants encouraged dig several plants over a five meter by five meter area two to after sowing, gently wash out the root system and look at the amount of nodulation. A visual check will show if a reasonable number of nodules is present and well distributed or weather has been a nodulation delay or failure. Carefully breaking open nodules will reveal if they are pink or reddish color shows that nodules are active; whereas green or white nodules are inactive.
Common inoculation issues
Sowing inoculated seed into dry soil is not recommended where a legume crop is shown the first time. However where a legume has been used frequently and the soil is not hostile to rhizobia, the risk of nodulation failure resulting from drying sowing is much less. Rhizobial formulations that are applied in furrow, such as granules or poet suspended commend in liquid, are placed deeper in the soil and will have a better chance of survival.
Rhizobium biologists recommend against mixing inoculate with fertile that are acid .We suggest either separating the inoculation and the fertile or if possible doing small-scale tests if you are considering fertilizers and mixing inoculation micronutrients. Some micronutrient preparations are acidic and therefore likely to cause reduced nodulation effectiveness.
Elements away from rhizobial inoculumn are recommends. If molybdenum is required as a seed treatment (Mo is sometime needed for optimum nodulation especially in acid soil) then molybdenum trioxide or ammonium molybdenum should be used not sodium molybdate which is rhizobia.
The detrimental effects of mixing inoculate and fertilizes are often only realized when a nodulation problem appears in a paddock that is otherwise responsive to inoculation. This situation can then be very difficult to rectify during the seasons.
Some combination of rhizobia with some pickles and pesticides appear to perfume well whereas other kill rhizobia. The time interval between inoculation and sowing should be less than six hours. The mixtures which are NOT compatible with peat liquid and freeze dried inoculation include chemicals containing high levels of zinc, copper mercury, fertilizers and seed dressing containing sodium molybdate, zinc and manganese, fungicides such as sumisclex or rovral and insecticides containing endosulfan or carbofuran.