The Fertilizer Association of Ireland (FAI) has just published a new technical bulletin entitled ‘Improving farm nitrogen use efficiency’ in conjunction with Teagasc.
Nitrogen (N) is an essential crop nutrient, driving both crop yields and quality. However, it must be applied at those times of optimal crop uptake.
If this is not achieved, environmental challenges will arise, including nitrate leaching into watercourses and the emission of nitrous oxide and ammonia.
Both of these gases create major environmental problems for Irish agriculture. Ammonia is a major air quality contaminant, while nitrous oxide is a potent greenhouse gas (GHG).
The new booklet identifies nitrogen use efficiency (NUE) as the key measure, quantifying the effectiveness of applied nitrogen use throughout the growing season.
Improving NUE can help to optimise the use of nutrients on the farm, as well as maintaining and improving soil health, and reducing excessive N build up.
Ultimately, NUE is an indicator of the proportion of N that is recovered in the products being sold, including meat, milk and crops.
The new bulletin specifically summarises the link between N and soil health, plus its interaction with other crop nutrients.
Soil pH
The most fundamental relationship is that between N and soil pH.
Correcting soil pH to the optimal level for a specific crop is the starting point for farmers to secure improved soil N levels.
High levels of soil acidity impede the mineralisation of N from soil organic matter.
Increasing soil pH through the addition of lime has the potential to increase N supply by up to 70kg/ha/year.
Meanwhile, research carried out by Teagasc at Johnstown Castle in Co. Wexford shows that maintaining higher soil pH levels reduces nitrous oxide emissions by up to 39%.
Lime is the main regulator of soil pH values.
Farm fertiliser plan
The new booklet confirms the importance for farmers of completing a farm fertiliser plan, and must be updated annually.
As part of this process, soil samples should be taken every four to five years. This will allow for the accurate determination of soil pH, potash (K) and phosphorous (P) levels.
Maintaining optimal soil fertility levels (Index 3) will increase the efficiency of applied N from 35% on low fertility sites up to 63%, where recommended levels (Index 3) of P and K are concerned.
Incorporating clover into grass swards improves animal performance, while also reducing N requirements throughout the growing season.
Teagasc trial work has confirmed that increasing soil pH from 5.5 to 6.8 for grass/clover swards will deliver crop increases of 13% – from 8.1t to 9.5t/h on a dry matter (DM) basis.
The application of slurry using low emission spreading systems (LESS) in the spring time reduces the requirement for applied chemical N.
According to the new FAI/Teagasc booklet, cattle slurry should always be recycled onto silage ground. This approach ensures that nutrients, such as K, are kept in balance.
N applied under the correct weather and soil conditions can be used more efficiently by crops, while the risk of losses to the environment is minimised.
As a rule of thumb, fertilisers should be applied when the following conditions are met – soil temperatures are above 5.5 °C, ground trafficability is good and no rain is forecast for the following 36 hours.
Nitrogen and sulphur
Sulphur (S) is an essential plant nutrient and a key component of important amino acids.
The nutrient also plays an important role in N fixation within legume plants, including red and white clovers, peas and beans.
As a rule of thumb, soils with 50% or more of sand content and less than 3% carbon are more likely to be responsive to applications of S.
In Ireland, S levels in soils tend to be lower than is the case in other parts of Europe. This reflects the lower atmospheric S levels found in this country.
As a consequence, Irish farmers are likely to see a positive growth response in grassland and crops when additional S is applied.
Recent Teagasc research linked to S confirms that organic sources of the nutrient, such as those found in slurries and farmyard manures, are not immediately available to crops.
Protected urea
Protected urea specifications include the presence of a urease inhibitor. This can be coated on to the surface of the fertiliser granule or actively incorporated within the product during the manufacturing process.
Urease is an enzyme, present in the soil, which controls the conversion of added urea to the ammonium (NH4+) cation.
This is the key step in the conversion of urea to ammonia gas, which is subsequently lost to the atmosphere.
A number of products are recognised by the Department of Agriculture, Food and the Marine (DAFM) as effective urease inhibitors. These are: NBPT, 2-NPT and NBPT plus NPPT.
The use of protected urea can lead to significant reductions in ammonia losses from both grassland and cropping scenarios, compared to standard urea fertiliser.
In addition, the use of protected urea can support a reduction in nitrous oxide losses from grassland, compared with Calcium Ammonium Nitrate (CAN) fertiliser.
The switch to using protected urea as the main source of fertiliser N for agriculture is a key target within the Irish government’s Climate Action Plan.
Ammonia is a pollutant that contributes to poor air quality while nitrous oxide is 298 times more potent than carbon dioxide (CO2) as a GHG.