Nitrogen and phosphorus nutrition of dryland grain sorghum at Katherine, Northern Territory. 1. Effect of rate of nitrogen fertilizer

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Abstract
Responses of dryland grain sorghum to applied nitrogen were examined in three field experiments at Katherine, Northern Territory, in 1969-70, 1970-71 and 1971-72. Rates of nitrogen up to 89 kg ha-1 in the first season and up to 200 kg ha-1 in subsequent seasons were used. The crops were sampled at floral initiation, mid-elongation, anthesis, and maturity. The three growing seasons were rated as below average, above average and average, respectively, for grain sorghum production. Nitrogen response was strongly season-dependent, Maximum yields (adjusted to 14 per cent moisture) were: 1969-70, 2280 kg ha-1, with 22.4 kg N ha-1 applied (with lower yields at higher rates of nitrogen) ; 1970-71,7730 kg ha-1 with 150 kg N ha-1 applied; and 1971-72,4440 kg ha-1 with 200 kg N ha-1 applied. These represented increases of 6, 50, and 62 per cent, respectively, over the zero nitrogen treatments. Applied nitrogen increased grain numbers per head and individual grain weight, but had no significant effect on head numbers. Applied nitrogen increased yields of nitrogen and phosphorus in plant material, and increased nitrogen concentration in plant parts. Apparent recoveries of nitrogen fertilizer ranged from 0 to 40 per cent, depending on year and rate applied. An approximate balance sheet suggested substantial losses from the mineral nitrogen pool in 1970- 71, a year of above average rainfall. Significant interactions between nitrogen and phosphorus fertilizer occurred only with grains per head and thousand grain weight.

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