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Annual medics on red-brown earths

J.R. Crosby, M.T. Spencer, G. Sweeney

South Australian Department of Agriculture, Box 1671, Adelaide SA 5001

Although the ley farming system has been widely adopted in South Australia since the 1950s, there is still no annual pasture legume recommended for neutral-acid Red-Brown Earth soils where rainfall is less than 400 mm. Subterranean clover, Trifolium subterraneum, is unreliable at this low rainfall, and annual medics, Medicago spp., do not persist, perhaps due to the inability of the symbiotic bacteria Rhizobium meliloti to colonise this soil type. Howieson and Ewing (1) have identified several acid-tolerant strains of R.meliloti, one of which, WSM540, is now used as the commercial Group A inoculum in Western Australia. This experiment compares the persistence of WSM540 with strain CC169, the commercial Group A inoculum in South Australia.

Methods

The trial was located near Booleroo Centre, South Australia (Latitude 3254'S, Longitude 13824'E), on a hard-setting Red-Brown Earth with pH 6.8 (1:5 H20) and annual rainfall of 380 mm. In 1989 the annual medic cultivars Paraggio, Parabinga and Santiago were sown into a moist seed bed following inoculation with the strains WSM540 and CC169. In a further treatment, seed was uninoculated, and nitrogen was applied during the growing season. Nodulation was scored 13 weeks after sowing (2). In 1990 wheat was sown across the plots, and in 1991 medic regenerated in the wheat stubble. Nodulation was scored 10 weeks after emergence.

Results and discussion

No significant difference in nodule score was found between cultivars in either year. In the year of sowing, the inoculated medics had significantly greater nodule scores than the uninoculated medics, however, there was no difference between the two strains of rhizobium. In the year of regeneration, medics in the plots originally inoculated with WSM540 had significantly greater nodule scores than those in the CC 169 plots, or either of the uninoculated treatments (Table 1).

Table 1. Mean nodule score (average of all cultivars).

Square root transformation of the means showed that treatments in columns followed by a different letter are significantly different (P<0.05).

This study confirms that CC169 may not persist well on hard-setting Red-Brown Earths, and that the acid-tolerant strain WSM540 may be more successful on this soil type. These findings further highlight the need to tailor Medicago-Rhizobium associations for specific soil environ- ments.

References

Howieson, J.G. and Ewing, M.A. 1986. Aust. J. Agric. Res. 37, 55-64. Howieson, J.G. and Ewing, M.A. 1989. Aust. J. Agric. Res. 40, 843-50.

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