Evaluation of the symbiotic nitrogen-fixing potential of soils by direct microbiological means |
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Authors: | John Brockwell Rosemary A Holliday Ann Pilka |
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Institution: | (1) Division of Plant Industry, CSIRO, 2601 Canberra, ACT, Australia |
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Abstract: | A method for estimating the nitrogen-fixing capacity of a population of rhizobia resident in soil is presented. legume test
plants, growing under microbiologically-controlled conditions in test tubes packed with a vermiculite substrate moistened
with a nitrogen-free plant nutrient solution, are inoculated directly with a suspension of the soil under examination. Rhizobia
in the soil nodulate the test plants, and the amount of foliage dry matter produced in the 28 days after inoculation is regarded
as an index of their effectiveness. An inoculum of at least 30, and preferably 100, rhizobia is needed to ensure that nitrogen
fixation is not masked by delayed nodulation. The new method is tentatively described as the ‘whole-soil inoculation’ technique.
Appraisals were made withTrifolium subterraneum L. andRhizobium trifolii and withMedicago sativa L. andR. meliloti. Soil-borne pathogens did not interfere with plant growth. The whole-soil inoculation technique was less tedious and time-consuming
than an alternative method which involved extracting representative isolates from the soil and testing their effectiveness
individually, and appeared to give more realistic values for the nitrogen-fixing capacity of the soil as a whole. Used in
association with a field experiment, the whole-soil inoculation technique confirmed microbiologically that there had been
an agronomic response to surface application of inoculant to poorly-nodulatedT. subterraneum pasture.
It is submitted that this technique for determining the effectiveness of rhizobia in soil, combined with a plant-infection
method for counting rhizobia, can be a reliable guide to the need for inoculation in the field. |
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Keywords: | inoculation Medicago sativa nitrogen fixation nodulation Rhizobium meliloti Rhizobium trifolii Trifolium subterraneum |
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