The response of several tropical legumes, grown with Panicum maximum cv. Gatton, to an initial application of molybdenum as molybdenum trioxide was studied over a five year period at six sites in south-eastern Queensland. The most responsive legumes were Glycine wightii cv. Tinaroo and Desmodium intortum cv. Greenleaf, followed by Macroptilium atropurpureum cv. Siratro and Medicago sativa cv. Hunter River, with Lotononis bainesiicv. Miles and Stylosanthesguianensis cv. Cook being least responsive. Sites differed markedly in magnitude of legume response. For example, the most responsive site required 200 g ha-1 molybdenum over five years for maximum growth of Siratro whereas there was no response of Siratro to molybdenum application at another site. There was no difference between surface-applied molybdenum trioxide, molybdenum trioxide applied to the seed pellet and surface-applied sodium molybdate in their residual effects on legume growth. Response of the grass to molybdenum treatment was generally similar to legume response and nitrogen concentrations in legume and grass increased with yield.
Nitrogen, phosphorus and potassium concentrations were determined in plant tops harvested every four weeks from nine tropical grasses fertilized with nitrogen on ten occasions at eight week intervals. Nitrogen was applied at three rates-25, 50 and 100 kg/ha (N25, N50, N100)-on each occasion. Grasses were rain-grown without grazing at a site near Gympie with an average annual rainfall of 1160 mm. Overall, nitrogen concentration in the tops increased and potassium concentration decreased with increasing nitrogen application. Phosphorus concentrations were constant at N25 and N50, but were lower at N 100. Mean nitrogen concentration was highest at 2.4% in Pennisetum clandestinum cv. Whittet, and lowest at 1 .9% in Chloris gayana cvv. Callide and Katambora, with intermediate concentrations occurring in Digitaria decumbens, Setaria sphacelata var. sericea cv. Narok, Panicum maximum cvv. Gatton and Makueni, Brachiaria decumbens cv. Basilisk and Paspalurn plicatulum cv. Bryan. Phosphorus concentration was highest in Whittet at 0.38% and lowest in Bryan at 0.25%. Potassium concentration was highest in Whittet at 2.53% and Narok at 2.29% and lowest in Katambora at 1 .48%. The implications of these concentrations and calculated nutrient uptakes are discussed in relation to animal and plant nutrition.
Nine tropical grasses were fertilized with 25, 50 and 100 kg N/ha on ten occasions at two-month intervals. No nitrogen was applied when conditions were too cold and/or dry for pasture growth. Samples (> 10 cm height) were taken at four-week intervals to estimate dry matter yield. Growth patterns of all grasses were similar, with a warm season peak and cool season trough. Nitrogen treatments did not greatly alter the growth pattern, although all species showed a yield response to increasing levels of nitrogen. Cumulative dry matter yields of Chloris gayana cvv. Callide and Katambora, Setaria sphacelata var. sericea cv. Narok, Digitaria decumbens and Panicum maximum cv. Makueni did not differ at N25 (P < 0.05), each producing about 10 t/ha. This significantly exceeded 5 t/ha from Pennisetum clandestinum cv. Whittet and Panicum maximum cv. Gatton. The same high-yielding group produced 15-16 t/ha at N50, while Gatton yielded 11.4 t/ha and Whittet 8.4 t/ha. At N 100, Callide, Katambora and Makueni were the highest yielding grasses (24-26 t/ha), and Whittet the lowest (15 t/ha). Brachiaria decumbens cv. Basilisk and Paspalum plicatulum cv. Bryan died out before the final harvests. Species responsiveness is discussed in terms of increment in dry matter yield per unit of applied nitrogen.
Phosphorus topdressing experiments (rates to 60 kg P ha-1) on 18 commercial Desmodium intortum cv. Greenleaf/grass pastures were conducted over a 4-year period in south-east Queensland. The aim was to determine whether yield responses, which occurred only in the Greenleaf component at six sites, could be predicted using soil or plant chemical tests. Acid-(0.005 M H2SO4) and bicarbonate-(0.5 M NaHCO3) extractable tests of phosphorus status in 0-10 cm soil samples each explained about 60% of the variance in Greenleaf relative yields. The residual variance was not significantly reduced by the inclusion of terms for total soil nitrogen, total soil phosphorus, exchangeable calcium and pH into the independent variable. These empirical soil phosphorus tests had higher predictive value than plant tests based on phosphorus concentrations in tops and diagnostic samples of Greenleaf. With both acid- and bicarbonate-extractable phosphorus, yield responses are likely in the Greenleaf component when phosphorus levels in most soils are below 22 ppm. Above 29 ppm, no response would be expected
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