The crop was close enough to 5 t/ha. Each ton of grain harvested removes about 20 kg N per ha. Let’s say the harvest index was 0.4 (grain / grain plus stubble), then the standing stubble would come in at 7.5 t/ha. This stubble would contain around 0.4% N, making 30 kg. So the total N uptake would be 100 kg (grain) + 30 kg (stubble) = 130 kg N.
The change in soil nitrate between sowing and harvest was 149 kg – 20 kg = 129 kg (the gold line in the graph above). This is almost the same value as the 130 kg N uptake calculated above – but the match is fortuitous. There would have been mineralisation of organic matter after sowing, losses through leaching, and other factors in play.
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