Dropping a hundredweight of nitrogen per hectare on spring crops could see yields diminish by nearly half a tonne according to trials by SAC consulting.

The college's research shows that spring crops are far more vulnerable to reduced nitrogen than winter cereals. The same 45kg reduction on winter cereals would seen only a 350kg fall in yield per hectare.

As fertiliser dry up across most of the UK and the price rockets above £1000/t, many farmers are taking a fresh look at application rates this summer to stretch suppliers as far as possible. However with cereal prices rising to above £300/t for wheat, a half tonne drop in yield per hectare will cost you £150 compared to the £45 saving in nitrogen if purchased at £1000/t.

In Scottish trials, spring cereals are consistently more dependent on fertiliser for strong yields than winter cereals. So cutting application to spring barley will see a higher drop in yield than in a winter barley. The impact of dropping nitrogen applications can be seen in the attached table.

Professor of Crop Physiology at SRUC, Ian Bellingham, is keen to point out to farmers that the relationship between nitrogen and yield is not a simple equation. He said: "Nitrogen response is highly variable and differs from site to site and year to year."

Spring barley trials in East Lothian in 2018 showed that yields rose with increased application of nitrogen above 200kg per hectare. Whilst in 2019 yields dropped off when more than 200kg of N per hectare was applied to plants.

Read more: Fertiliser supply dries up

Prof Bellingham said: “The variability stems from several factors including differences in the amount of nitrogen supplied by soil from mineralisation of soil organic matter and the ability of the crop to capture and utilise it. This soil nitrogen supply is notoriously difficult to predict with any certainty. As such we have to work with 'average’ responses when making nitrogen recommendations, but accept that there will be an appreciable margin of error.”

Variety doesn’t appear to be a major factor when assessing the impact of reduced fertiliser on yields. Modern higher yielding spring barley varieties have the same fertilizer N requirement as older varieties. This is because their N use efficiency has increased with breeding for yield.