EFFECTS OF PH AND ALUMINUM ON THE GROWTH OF TEMPERATE PASTURE SPECIES .1. TEMPERATE GRASSES AND LEGUMES SUPPLIED WITH INORGANIC NITROGEN

A flowing solution culture experiment was conducted in which four temperature pasture grasses and 11 temperate pasture legumes were grown in solutions of low ionic strength for 29 days at six aluminium (Al) levels with average Al3+ activities {Al3+} of 1.0, 5.1, 5.9, 9.9, 20.8, 41.6-mu-m at pH 4.5. Growth of these species was also measured at pH 5.5 in the absence of Al. In the absence of added Al, lucerne (Medicago sativa) cv. Hunter River and phalaris (Phalaris aquatica) cv. Grasslands Maru grew better at pH 5.5 than at 4.5. The reverse was so for ryegrass (Lolium perenne) cv. Grasslands Nui and subterranean clover (Trifolium subterraneum) cv. Tallarook. Increasing the pH from 4.5 to 5.5 had no effect on the growth of the other 11 species and cultivars tested. Micromolar activities of Al (< 10-mu-M) markedly reduced the growth of all species. Four groups were tentatively identified in terms of their tolerance to Al. The two cultivars of ryegrass and the two red clover (Trifolium pratense) cultivars, together with Lotus corniculatus cv. Maitland, were relatively sensitive to Al. A 50% reduction in total dry matter (TDM) was associated with {Al3+} of 2-3-mu-m. The four white clover (Trifolium repens) cultivars, tall fescue (Festuca arundinacea) cv. Grasslands Roa, phalaris cv. Grasslands Maru, and lucerne cv. Hunter River were intermediate in their tolerance, with {Al3+} of 3-5-mu-m reducing TDM by 50%. The two subterranean clover cultivars were more tolerant to Al than white clover (50% TDM reduction at 5-6-mu-m {Al3+}. Lotus pedunculatus cv. Grasslands Maku was exceptional in its tolerance to Al. Its growth was unaffected by {Al3+} up to 5-mu-m and 50% growth reduction was associated with {Al3+} of 7-mu-m. The results of this study emphasize the need to conduct solution culture experiments at realistic solution concentrations and ionic strengths when examining the effects of Al on plant growth.