Are you getting the full value from your nitrogen fertiliser?

Cost, environmental impact and lowered levels of permitted use are just some of the reasons why dairy farmers need to pay increased heed to the efficiency at which nitrogen fertiliser is used on their farms.

That’s according to Dr. David Wall, a Senior Research Officer and Enterprise Leader at Teagasc Johnston Castle, who detailed the factors impacting nitrogen use efficiency (NUE) as part of a recent Grass 10 webinar on ‘Advancing nutrient management in 2025 with PastureBase Ireland’.

“It is very important that we think about other nutrients in addition to nitrogen,” he explained. “Fertiliser and nitrogen allowances have come down. We have had a cut in the last 12 months and potentially under a nitrates scenario, they will come down a little further.

“Having soil fertility turned up in every field - your pH, phosphorus (P), potassium (K) and sulphur (S) - is going to be more important going forward.”

Shedding light as to how these nutrients affect the efficient use of nitrogen fertiliser, Dr. Wall turned to the bank of research from Johnstown Castle, citing numerous studies which indicated that nitrogen use efficiency is maximised where soil fertility is optimum for pH, and soil P and K indexes.

What’s challenging in this regard is the current levels of soil fertility on Irish dairy farms, as evident from soil test results analysed at Teagasc Johnstown Castle over 2024. Highlighting this, Dr. Wall noted that 39% of the dairy soils tested have inadequate pH (less than 6.2), while 38% and 37% of soils tested are at either index 1 or 2 for either P or K, respectively. All-in-all, this means that just ~40 of soils on dairy farms are at optimum levels for pH, P and K.

Maximising nitrogen use efficiency

The starting point to maximising NUE, he explained, is correcting soil pH. Quoting an on-farm study, he noted that rising soil pH from 5.5 to the optimum of 6.3 can allow for grass dry matter yield increases of 4-5t DM/ha where nitrogen fertiliser applications remain the same.

Correcting soil pH through liming is also a necessary precursor to: ensure the availability of nutrients contained within chemical fertilisers are used effectively, maximise the availability of N, P and K from slurry; and, to allow clover to establish to reduce the farm’s overall dependency on chemical N.

Expanding further as to how soil fertility drives NUE, Dr. Wall presented results of an on-farm study, where samples were collected across 446 fields from 15 farms over two years.

Key results of this study:

• Where soil pH, P and K indexes are low, just 35% of the nitrogen applied was retained by the system;
• Where soil pH was optimum, but P and K indexes were low, 53% of the nitrogen applied was retained by the system;
• Where soil pH and soil K levels were optimum, but P indexes were low, 54% of the nitrogen applied was retained by the system;
• Where soil pH and soil P levels were optimum, but K indexes were low, 57% of the nitrogen applied was retained by the system;
• Where soil pH, P and K levels were optimum, 63% of the nitrogen applied was retained by the system.

To put the above simply, for every 100kg of chemical nitrogen applied, farmers operating at optimum levels of pH, P and K would retain 63kg N for grass production. On the other hand, where soil pH, P and K are at suboptimum levels, just 35kg of N would be retained for grass production – a considerable loss at a time when the efficient use of nitrogen fertilisers must be prioritised.

The importance of sulphur

Dr. Wall also presented data from a study completed by Claire Aspel in 2022 across multiple soil types, which examined the impact Sulphur (S) applications have on NUE and nitrate-N leaching. The results of this study show that grass dry matter production responses are witnessed when S is added to the fertiliser mix on soils varying from light to heavy. The yield response witnessed was greater on lighter soils (+4t DM/ha), as evident in figure 1 below. The inclusion of S into the fertiliser programme also significantly reduces nitrate-N leaching losses, with Dr. Wall showing how N losses could be reduced by up to 64% when S is included as opposed to nitrogen fertiliser applications only.

Figure 1: The importance of balancing sulphur supply with nitrogen

Stemming from this work, he advised farmers to apply 15units/ac of S per year, targeting applications of 8-10 units/ac on silage ground, whereas S applications on grazing ground should be split into approximately three timings of ~5 units/ac, taking place in from April, May and June in this period of higher N fertiliser use.

For further details on the above, Dr. Wall’s full presentation to the Grass10 webinar is available to view below:

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