Nutrient budgeting simply put is like tracking a bank balance. It is an accounting system to monitor the nutrients in, and the nutrients out in their various forms. For cropping, your nutrients in are the fertilisers, and composts applied, and any nutrients in residues left over from the last crop.  The nutrients out are those in the crop harvested and any other losses, which can be to water (known as leaching) or to air (e.g. volatilisation).

Nitrogen is one of the most highly mobile and form changing nutrients. It changes from organic matter or urea through various stages to ammonia and nitrate which is available for plant uptake. In this example, we focus on the nitrate that will be available for your crop. Remember, the input information starts with a fertiliser report and the Nutrient Management for Vegetable Crops in New Zealand guidelines. Put it together on our template that you can download here.

Let’s say we plan to grow a sweetcorn crop and we expect to get a yield of 20 t/ha.

Our mineral N soil test tells us we have 50 kg N/ha in the top 15 cm of soil (standard soil test depth).

We look up sweetcorn in the nutrient management book and for a 20 tonne crop it tells us we can justify adding an extra 180 kg N/ha.

NOTE: The Nutrient Management for Vegetable Crops in New Zealand book’s Soil Available N is from the test called Potentially Available Nitrogen (PAN). This includes nitrate present in the soil PLUS the nitrate that is expected to become available from soil breakdown during the life of the crop. Using PAN will give recommendation for less additional fertiliser than using just the mineral N value. We’ll see the result of our error in the next lesson 🙁

We decide to add the nitrogen in two applications, some at planting and some as a side-dressing once the crop is established.

We add 150 kg of Crop20 at planting which is
150 x 20% = 30 kg N/ha.

We add 300 kg/ha of urea at side-dressing, or
300 x 46% = 138 kg N/ha.

In total that is 168 kg

We estimate there is about 20 kg N/ha in the residue from the previous crop, and add up the numbers.

Fertiliser 168 kg N
Residue 20 kg N
Total 188 kg N

We want 180 kg N, so the fertiliser plan gives a little bit extra. We could cut back a little.

NOTE: You can use the same template for other nutrients such as potassium where the calculations are the same. But for phosphorous, we need a slightly different approach because we manage it in a different way. We have a different template. That’s another lesson.

Review the process on the video