Soil treatment, plant species, and management effects on improved pastures on a solodic soil in the semi-arid subtropics: 3. Improving cool season plant and cattle productivity

Seasonal analyses of grasses and legumes in a long-term experiment on a solodised solonetz soil (Natrustalf) in southern Queensland showed that the large liveweight losses of steers in the cool months (June-August) appeared to be broadly correlated with low plant nitrogen (N), phosphorus (P), and potassium (K) concentrations. Annual variation in the N, P, and K percentages of Rhodes grass (Chloris gayana) and volunteer grasses [medics (Medicago spp.) and siratro (Macroptilium atropurpureum)] as affected by soil and plant treatments were examined by fitting a Fourier series to the data. Cool season liveweight losses of steers ranged from 391 glday with sown grasses alone to 11 g/day with sown grasses and medics plus lime and gypsum. The N content of volunteer and sown grasses ranged from 0.56 to 0.70% in the cool months, which is below the accepted N requirement of 1%. The P concentration of volunteer grasses was only 0.08%, but where P fertiliser was applied, concentrations in Rhodes grass were 0.10-0.11%. Potassium levels were also low in the cool months, with values of 0.23-0.43% in grasses. Exchangeable magnesium (Mg) and sodium increased from 1.3 and 0.48 mLJ100 g at 0-10 cm depth to 8.5 and 5.2, respectively, at 40-70 cm. Exchangeable calcium decreased from 2.5 mL/100 g at 0-10 cm to 0.40 at 40-70 cm. The Mg levels in siratro (0.58-0.60%) were higher than those in Rhodes grass (mean 0.12%). Medics responded markedly to lime, resulting in a 54% increase in steer liveweight gains on Rhodes grass-medic pastures. The use of lime on solodic soils for reducing both surface soil acidity and exchangeable aluminium increased plant and steer productivity. The cost of lime transportation can be high in inland areas, but small limestone deposits have been identified on the Darling Downs and these merit closer attention.