In soils with a resident population of symbiotically effective rhizobia in
sufficient number that legume inoculation is not a requirement for successful
legume cropping, greater flexibility may be exercised in the use of legume
seed dressings that are toxic to rhizobia. Inoculating crops antecedent to
legume crops has been suggested as a method for pre-establishing effective
inoculant rhizobia in soil. The extent to which this strategy
(pre-inoculation) would remove the need for inoculating legume seed
(conventional inoculation) was tested for Bradyrhizobium
sp. (Lupinus) and
Rhizobium leguminosarum bv. viciae
with crops of lupin (Lupinus angustifolius) and faba
bean (Vicia faba), respectively.
In the glasshouse, in pasteurised sand and red kandosol, the numbers of
B. sp. (Lupinus) in the
rhizospheres of wheat (Triticum aestivum), canola
(Brassica campestris) and clover
(Trifolium subterraneum) increased 300–10000-fold
over a 14-week period, reaching numbers similar to that achieved on
L. angustifolius. These increases were not greatly
affected by chemical seed dressings commonly applied to the crops: on wheat,
Vincit C and Baytan C; on canola and subterranean clover, Lemat. In the sandy
soil, the nodulation of lupin following pre-establishment of rhizobia, drying
and mixing of soil, was not improved by conventional inoculation; in the red
kandosol nodulation was increased only marginally by conventional inoculation.
The results with the glasshouse pot trials warranted further investigation in
the field.
Under field conditions, when B. sp.
(Lupinus) was pre-established with wheat, on a red
kandosol in south-western New South Wales, the number of these bacteria
surviving in the dry soil at the end of the wheat phase was much lower than in
the glasshouse study. In the following season, the nodulation of lupin sown
without inoculant, and dependent only on pre-established rhizobia, was
significantly reduced, as compared with that on conventionally inoculated
lupin. An exception occurred where the lupin was dry-sown 3 weeks before rain,
in which case nodulation was comparatively poor even with conventional
inoculation. Reduced nodulation was generally consistent with initially fewer
rhizobia in the lupin rhizosphere. However, the numbers of rhizobia were
eventually similar to those found with conventional inoculation. In the third
year, in autumn, B. sp. (Lupinus)
was abundant in the soil in all treatments and there were no differences in
lupin nodulation between treatments.
Similarly, introducing R. leguminosarum bv.
viciae on wheat, in an acidic red kandosol in
south-western New South Wales, failed to provide as much nodulation of faba
bean as was achieved with conventional inoculation. The maximal dry matter of
the bean crop was also significantly lower with pre-inoculation as compared to
conventional inoculation.