The third crop - broccoli

After the next crop is planted, the nitrate drops quite sharply. The blue line now represents the third crop in the sequence – broccoli. Some of the decrease in nitrate is due to crop uptake. However the initial decline is a bit too sudden to be explained by uptake alone. I had to irrigate to get the new crop started and rain following irrigation inevitably leads to some leaching.

The second fallow

Despite all the wet weather, the nitrate comes bouncing back after the wheat harvest. Over 32 days the nitrate jumps from 23 to 109 kg N per ha. In other words the soil organic matter is mineralising at a rate that supplies 19 kg N per ha per week, even faster than we saw after the sweetcorn crop a year ago.

My gardens have had regular applications of compost over the years, which of course is a good thing. But you can’t turn the microbes off, and so in warm wet soil they are having a field day. The implication of this is that the soluble nitrate must now be managed by very careful irrigation. Even a bed managed ‘organically’ can be swimming in nutrients in a highly soluble form.

Nitrate during the what crop

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.