Prepare a post-harvest nitrogen budget
At the end of the crop, we can do another budget using the actual values and see how well we did.
In this budget, we look at three nitrogen “pools”, what was exported in the crop we harvested, what remains in crop residues left in the paddock (including any weeds!), and what we find remaining in the soil.
In our case we sold 21 t/ha sweetcorn. The book says there is 7.8 kg N/t so 163 kg N was exported.
We estimate 100 kg N/ha in residues, so that’s 80 kg N/ha more than we had at planting. We find we have 40 kg N/ha of nitrate in the soil, which is 10 kg N/ha less than we had before planting.
When we add up what we know we have, we get
163 + 80 – 10 = 233 kg N/ha.
Assuming we kept to our original fertiliser plan we added 168 kg N/ha, so the balance is
233 – 168 = 65 kg N/ha.
We can account for 65 kg/ha more nitrogen in the system than we started with.
In many cases, 65 kg N/ha could be the difference between PAN and mineral N, so we do want to know about it. Time to check some of our inputs, and try using the Potentially Available Nitrogen test instead of mineral nitrate next time!!
NOTE:
There can be a lot of nitrogen sitting in the soil below 15 cm. In some of our sweetcorn trials, the standard 0 – 15 cm soil test showed a need to add fertiliser. But when we looked at the amount of nitrogen sitting below that, but still in the root zone, there was plenty.
Planting a deep-rooted crop like sweetcorn or maize can suck up a lot of nitrogen left after shallow rooted crops like salad greens or broccoli.