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This study compared the relative tolerances of subterranean clover (Trifolium subterraneum L.), balansa clover (T. michelianum Savi.), and gland clover (T. glanduliferum Boiss.) to acid soil conditions. Seed yield, seedling density, herbage production, N2 fixation, and herbage mineral composition of the 3 legumes were assessed when grown on an acid soil (pHCa of 4.3 and 15% exchangeable Al [0-0.10m]) with and without the addition of lime (CaCO3). Annual legume species were sown in a mixed sward together with burr medic (Medicago polymorpha L.), and in mixtures with either lucerne (Medicago sativa L.), chicory (Cichorium intybus L.), or phalaris (Phalaris aquatica L.). Due to drier than average seasonal conditions, none of the perennial species persisted beyond the first summer.Lime increased the herbage production of annual legumes by 18-22% and total pasture production by 14% in both 2002 and 2003. Subterranean clover was the most tolerant of the annual legumes to acid soil conditions, showing no visible toxicity symptoms and no response to lime in terms of seed yield. In contrast, both balansa and gland clovers exhibited visual symptoms of manganese toxicity in the absence of lime, with Mn concentrations in the shoots of 817mg/kg and 626mg/kg, respectively. Both species responded positively to lime with seed yields increasing by 45% and 124%, respectively. Lime increased the proportion of herbage N derived from N2 fixation by subterranean clover from 29 to 40% and by gland clover from 30 to 43%. Lime had no effect on the proportion of N2 fixed by balansa clover (29-31%), suggesting a suboptimal symbiosis of rhizobia with that species. Adding chicory or phalaris to the pasture mix increased sward herbage production in the establishment year by 39% and 21%, respectively. Based on leaf symptoms and herbage yield responses to lime, Mn toxicity was present in lucerne with tissue levels of up to 916mg/kg, but no symptoms were observed in chicory (1129mg/kg) or phalaris (403mg/kg). Chicory and phalaris were more tolerant of acidity and high levels of Mn than lucerne, gland clover, and balansa clover.The study highlighted the value of the small-seeded annual legumes, balansa clover and gland clover, to the production of mixed pasture swards even in drier than average seasonal conditions. Although more sensitive to acid soils than subterranean clover, they set a greater number of seeds and, in the case of balansa clover, a greater weight of seed under moisture stress in the establishment year than the larger seeded subterranean clover. |
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