Can yield predict the nitrogen rate?

It is interesting to look at the nitrogen trial results this year and see, yet again, a wide variation in the optimum rate. The average optimum rate of nitrogen on winter wheat in 2016 was 237kg N/ha only slightly higher than the long term average of 230kg N/ha. However, this average hides a huge variation as over the last three years the optimum rate of nitrogen at individual sites has varied from 155 to 290kg N/ha. 

What is the reason behind such variable response to nitrogen ? 

Nitrogen use efficiency is a measure of how well the crop is able to use applied nitrogen and includes the crops ability to take up nitrogen from the soil as well as its ability to utilise the nitrogen effectively once it has been taken up. 

A crops ability to take up nitrogen from the soil depends on how well the root system is able to interact with the soil and is influenced by factors relating to the soil itself, such as soil structure, compaction or drainage. If these are not optimum they should be investigated and the best starting point is generally a spade. Nutrition can also influence this, a crop lacking available phosphate or growing in an acidic soil will already have a compromised root system and will struggle.

Factors within the plant also restrict its ability to utilise nitrogen, particularly deficiencies of other nutrients such as sulphur. This was also demonstrated in trials when the application of sulphur at the, same rate of nitrogen, increased both the crop yield and the nitrogen use efficiency. 

Yield alone cannot predict the optimum rate of nitrogen

So, although there is a mathematical link between nitrogen requirement and yield there is no clear relationship and it is inadvisable to predict the optimum rate of nitrogen based on yield alone.

It is possible to compensate for a low nitrogen use efficiency by simply applying more nitrogen and conversely if the crop has a particularly high nitrogen use efficiency then less nitrogen can be applied: Both of these alternatives have cost implications.

The solution is to firstly ensure that no other factors are restricting nitrogen use efficiency and then to apply the correct rate of nitrogen based on the crop requirements since the crop itself will always give the best estimate of its needs.