The optimum amount of Nitrogen that a crop needs varies significantly, not only depending on soil type and yield potential, but also from year to year.
Every year I encourage growers to experiment with N-Rich strips; these involve selecting a couple of tramlines, where more N is applied than the field average, and a couple of tramlines where less Nitrogen is applied. Typically I would look to apply 25% more, and 25% less than the field average.
Remember that fertiliser spinners work on a double overlap, so at least two tramlines need to be used to get an even width that the yield meter on the combine can measure.
A reasonably accurate yield monitor is required to measure the difference, which then allows the yield response to be compared to the additional cost of Nitrogen applied.
The results help to define the appropriate Nitrogen dose required in future years for that crop.
The above example is a trial in barley that I did a few seasons ago based on an EM38 soil conductivity map. Not only was I keen to find out whether we were using the optimum amount of N, but I also selected a field with two different soil types to see how each area responded.
The heavier textured soils are the darker areas of the field, and were historically lower yielding than the yellow areas. The field had 150kg/ha of N applied, apart from two tramlines side by side (the red strip) where we applied 200kg N in total, and two tramlines in blue where we applied just 100kg N in total.
The results showed two interesting observations summarised below. In the higher yielding areas, cutting the nitrogen was false economy as it caused a slight reduction in yield. On the lower yielding areas, there was no yield penalty from cutting the nitrogen rate which means we were applying too much.
|Lower Yielding heavier soil (left hand side of field)||Higher Yielding area (yellow, right hand side of field)|
|No yield improvement seen||No response above the field average of 150kg N|
|No reduction in yield||Slight yield reduction|
Essentially the yield maps told us that the higher yielding part of the field required around 150 kg/ha of N for optimum yield. The lower yielding part only needed around 100-125kg of N.
But….! As always there are complications. In some years we have found that when soil nitrogen levels from Deep N tests are very low, the crop responds to more nitrogen. In low rainfall seasons, the opposite is seen.
Cool temperatures can also influence how much nitrogen is taken up by the plant. In these instances measuring the Green Area Index with aerial images, or using a hand held reflectance sensor elps to estimate the amount of N in the crop.
Using the tools available all help to increase the accuracy of nitrogen dose rates. You never get it right every year, but the aim is to collect more information on your own farm to better inform your fertiliser decisions.