With the fertiliser season almost upon us, it is important that growers decide on an approach to cope with the higher costs without risking over exposure and future productivity.

This will be different for every grower and every field but the following is what I would do based on soil test results.

It is important to remember that it is nitrogen rather than phosphorus and potassium that has the biggest price increase. This will mean a different approach to the different nutrients.

The cost of nitrogen influences its optimum rate, which is the relationship between its price and the value of grain produced for each additional kilo of N applied.

The price of the grain produced also influences this relationship.

In crop production, the yield response to applied nitrogen is higher at lower rates and lower at higher rates. When this is graphed, it eventually levels off to a plateau and even higher N rates can cause it to fall again due to the impact of lodging.

Nitrogen responses

Nitrogen is the big additional expense (up 2.5 fold or more), so optimum rate is directly proportional to the combination of nitrogen cost, yield potential and crop value.

Fields with lower yield potential should have a lower economic optimum but they also have higher risk.

Recent Teagasc analysis, based on previous nitrogen response trials, suggest about a 15% nitrogen rate reduction for winter wheat and barley but less for spring crops.

There is little scope for nitrogen rate reduction on malting barley because nitrogen management is critical to achieving the protein range required to access the price premium.

P and K

I would not consider excluding phosphorus and potassium as a way of cutting costs. We might get away with it for one year but the cost increases for these nutrients are much lower and they may not be any cheaper next year.

That said, I would reduce application rates, where possible, to curb total farm expenditure on fertiliser. This means having soil test results for every field and ensuring that soil pH is adequate.

Save a little on Index 3 soils. These only require maintenance levels, which depend on the crop. I would pull back rates here by up to 20%, especially in fields at the high end of Index 3. Some savings here would help decrease overall farm expenditure on fertiliser. Fields at the lower end of Index 3 would still get 90% to 100% of maintenance.

For Index 2 fields, I would stay with maintenance levels at the very least. Soils at Index 2 may already have limited yield potential so there is even greater need to watch total N rates on these.

Fields at Index 1 should still get maintenance levels plus nearly all the additional recommended level to help build soil fertility. This is critical on owned land as yield potential will always be influenced by soil fertility, as well as soil health.