What might be killing my Rhizobia in the soil or stopping nodulation from starting?

  • 29 Jul
    By atadmin

    There are numerous factors contributing to Rhizobia population deterioration in Australian soils.
    It should be noted that the factors listed below are recognised as the main contributing factors, there are others. In combination these factors can have a compounding negative effect on the persistence of effective rhizobia strains being available to initiate productive nodulation in forage and crop legume paddocks.
    These factors, listed below, have been ordered top to bottom in order of greatest impact.

    1. Extremes in soil pH that fall outside the tolerable range for the legume-rhizobium symbiosis to function effectively.
    2. Low soil clay content.
    3. Low soil organic matter.
    4. Usage of agrichemical products known to be harmful to rhizobium soil populations – generally higher impact on high pH soils.
    5. Prevalent usage of residual broadleaf herbicides controlling low populations of host legume species during cereal phases – generally higher impact on high pH soils. Plant back windows for Group B mode of action herbicides should be observed with the Sulfonylureas (SUs) sub-group seeming to frequently present symptoms of nodulation suppression . Clopyralid (Lontrel®), a commonly used Group I mode of action product  has increasingly been shown to suppress nodulation in the season following application.
    6. High summer soil temperatures – no stubble/overburden to insulate.
    7. Genetic dilution of elite rhizobial strains. This is a natural evolution whereby key genetic traits transfer to indigenous Root Nodule Bacteria these hybrid types may then ineffectively nodulate the target species.
    8. Long rotational breaks of primary strain hosts