Measurements were made of soil pH,
frequency of occurrence of annual species of Medicago
(medics) and populations of Rhizobium meliloti at 84 sites
on 7 dominant soil groups of the Macquarie region of
central-western New South Wales. Over all sites, soil pH(0-10 cm; 1 : 5 soil: water) ranged from 5.26 to 8.07,
medic frequency from 0 to 100% and most probable
numbers of R. meliloti from undetectable to 675 0001g soil.
There was a highly significant (P<0.001) relationship
between soil pH and number of R. meliloti. AbovepH 7.0, the mean soil population of R.
89000Jg; below pH 6.0, it was 37/g. Medics occurred
most frequently on the more alkaline soils and with least
frequency on the more acid soils, but the relationship
between soil pH and medic frequency was weaker than
between pH and R. meliloti number. Medics were more
tolerant of low soil pH than their rhizobia were; at
2 sites, of pH 5.49 and 5.35, medics occurred at 100%
frequency but R. meliloti was undetected.
There was an indication of some acidification in
these soils over a period of 35 years but this remains to