How to improve nitrogen-use efficiency

Nitrogen use efficiency helps to understand the relationships between the total nitrogen input compared to the nitrogen output. This is complex, involving uptake, metabolism and redistribution, however adopting a complete crop nutrition strategy allows improvements to be made resulting in better efficiency, profitability and sustainability.

What is Nitrogen Use Efficiency?

Nitrogen use efficiency is a way of understanding the relationships between the total nitrogen input compared to the nitrogen output. In its most basic form nitrogen use efficiency can be described from the grain yield (kg) per unit (kg) of total available nitrogen (applied nitrogen + soil mineral nitrogen). On occasions you will see this simplified further whereby nitrogen output is compared to applied nitrogen as the input. This can of course be satisfactory, but it assumes that within a crop rotation soil available nitrogen is a constant. A full and more detailed calculation is a consequence of the efficiency of several other dynamic processes:

1. Nitrogen Uptake Efficiency (NUptE). The efficiency with which nitrogen is taken up by the roots as they grow and explore the soil. This is described by calculating the total above ground biomass nitrogen as compared to the amount available to the crop.
2. Nitrogen Utilisation Efficiency (NUtE). The efficiency with which the plant converts the nitrogen that it has taken up into harvestable grain.
3. Nitrogen Harvest Index (NHI). This accounts for the total nitrogen harvested in the grain as compared to the total nitrogen taken up. Such a calculation therefore considers the efficiency of remobilisation, as the plant moves from being a green, solar radiation capturing canopy, to partitioning dry matter as grain fill starts and ripening begins.

One of the key measures within all of this is the % recovery of fertiliser applied. This is sometimes referred to as Fertiliser Recovery Efficiency (FRE). Globally this value has been calculated as low as 33%. However, research and farm data collection has shown that this value can be as high as 80 – 90 %.

How do we measure NUE?

Measuring nitrogen use efficiency is all about good data capture and management. The key bits of data required are:

1. Harvested grain yield (t or kg/ha)
2. Nitrogen supply:
   a. Soil mineral nitrogen
   b. Organic nitrogen applications
   c. Inorganic fertiliser applications.
3. Harvested grain %N / protein content.

Ensure that the data is all corrected to the same moisture content, and then you are in a great place to start making some calculations.

A worked example:

Nitrogen Output
Grain yield at 100% Dry Matter = 10 t/ha 
Grain Protein at 100% Dry Matter = 13.5%
Grain Nitrogen = 13.5 / 5.7 = 2.37% N 
Total Nitrogen Output = Grain Yield x Grain N % = 10,000 x 2.37% = 237 kg N / Ha

Nitrogen Input
Soil Nitrogen Supply before any nitrogen applications = 80 kg N/ha (Soil mineral nitrogen + crop nitrogen) 
Organic manure N application = 40 kg N/ha
Inorganic Nitrogen applications = 220 kg N/ha
Total Nitrogen Supply = 80 + 40 + 220 = 340 kg N/ha 

Nitrogen Use Efficiency = Nitrogen Output / Nitrogen Input = 237 / 340 = 70 %

How can we improve NUE?

The three areas highlighted i.e. NUptE, NUtE and NHI, can be used to start dissecting your nitrogen strategy and where there may be new considerations towards increasing this value. In other words, if you have measured it you can now start to manage it.

Managing Nitrogen Uptake Efficiency (NUptE)

The starting point for achieving a high nitrogen uptake efficiency is creating an environment that enables the roots of the growing plants to proliferate throughout the soil mass. It is therefore essential where possible to manage your soil such that a good soil structure is achieved through appropriate cultivation and establishment techniques.

Soil structure can be affected by the soil nutrients, and more specifically the cation content (e.g. magnesium, calcium, sodium, potassium). The most notable nutrients that influence soil structure are magnesium and calcium. As magnesium levels increase and dominate, soils disperse losing their ‘crumb’ like structure. Displacing some of this magnesium and replacing it with calcium is a soil management technique that can be deployed to improve soil structure. The calcium ‘flocculates’ the soil giving it a better crumb structure. Increasing organic matter levels not only improves the nutrient holding capacity (fertility) but also the soil structure as it binds soils, increasing the water holding capacity and aeration.

Crops with a large root mass and prolific architecture will have a better ability to scavenge for soil nutrients. This root size and architecture is compromised when soils are water logged, resulting in it being shallow and small. Nutrients can be used to manipulate the roots which introduces the ‘4R Nutrient Stewardship’ concept – Right Source, Right Rate, Right Time, and Right Place.

Firstly, the choice of nutrient and its source is important. Apply nitrogen in the nitrate form and phosphorus in the phosphate form to build a bigger root mass that has a larger surface area for nutrient uptake.

These sources of nutrients are also the preferred form for plant uptake. The rate and timing also plays a key role in that, for example, with wheat early, higher rates of nitrogen increase root numbers through the influence it has on tillering. Timing of the phosphate application is also important to improve efficiency i.e. ensuring this is aligned with the crop’s demand.

The correct timing of nitrogen is important to drive overall efficiency right through the growing season. All the key arable crops (oilseeds and cereals) will have their highest daily demand for nitrogen through the months of March, April and May. Multiple applications (minimum three) helps us ensure supply and demand are synchronous, whilst also allowing rates of application to be adjusted according to the prevailing field and seasonal conditions. The right rate in any one field can be fine tuned by adopting latest technology such as the N-Tester BT, and hitting the optimum rate across the field can be managed through variable rate technology such as N-Sensor ALS2 and AtFarm.

Trials in recent years have demonstrated that utilising this technology can enable farmers to achieve nitrogen use efficiencies of 70 – 80%.

Managing Nitrogen Utilisation Efficiency (NUtE)

Once the plant has taken up the nitrogen it needs to be utilised by the plant as efficiently as possible as it incorporates it into plant proteins. Studies have investigated the impact of other nutrients on the nitrogen metabolism. Zinc deficiency has one of the greatest impacts with studies revealing it reduced the metabolism of nitrogen by 50%. Manganese, copper and molybdenum deficiency have all been shown to have a negative impact on nitrogen metabolism and therefore nitrogen utilisation efficiency. These deficiencies are very common in UK arable crops, but are easy to manage with foliar products such as YaraVita Gramitrel and YaraVita Molytrac. It is important to ensure that these deficiencies are addressed early in the season such that they are in sufficient supply prior to the peak nitrogen uptake that will happen between GS 31 and GS 39 for cereals.

Managing the Nitrogen Harvest Index (NHI)

Following on from the plant having taken the nitrogen up, metabolised it into plant proteins, the final bit of the Nitrogen Use Efficiency equation is to remobilise this protein nitrogen into the developing grain. Approximately 90 % of the nitrogen found in the grain will come from the nitrogen that is in the vegetative plant material (leaves, stem, roots) before flowering. Natural senescence will be triggered soon after flowering and fertilisation has occurred so it is important to ensure that early, unnatural senescence hasn’t started prematurely as the movement of nitrogen into the grain will be impaired. This movement is an active process, requiring energy, in the plants phloem vascular system. It is essential that this is not damaged so maintaining the crops architecture – upright stems and leaves – is important. Phosphorus and potassium have specific roles to play, with the former key for energy storage in the plant and the latter for controlling stomatal opening and closing that controls water loss and thus wilting of plants.

Clearly Nitrogen Use Efficiency is a complex process involving uptake, metabolism and redistribution, but through adopting a complete crop nutrition strategy and considering the ‘4R’s’ in nitrogen management improvements can be made resulting in better productivity.

Why are nitrate-based solutions more efficient?

• Produce more crop with less. Efficient use of nitrogen is essential to achieving maximum crop growth and a greater return on each kg of fertiliser you invest in.
• Preferred by plants. Nitrogen is a vital nutrient, and nitrates are the preferred nitrogen form for most crops.
• Better safeguard against the weather. Nitrates have efficient uptake even in conditions that are cool and wet or hot and dry, driving higher yields than alternative fertiliser types.
• Grow healthier roots. Early applied nitrates promote the growth of well-developed roots, which are important for capturing nutrients and is a precursor to higher nitrogen use efficiency.