Understanding nitrogen use efficiency

There are many different types of nitrogen fertilisers available and to boost productivity, it’s important to make sure you are using the most efficient type.

However, even with the most effective product, if plants aren’t using it efficiently, it could lead to sub-optimal performance and subsequent yield losses.

But there is a solution.

Calculating nitrogen use efficiency is a way of improving an understanding of the relationships between the total nitrogen input compared with the nitrogen output and can help growers get more from their applications, explains Mark Tucker, agronomy and business development manager at Yara. “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 plus soil mineral nitrogen).

“On occasions you will see this simplified further whereby nitrogen output is compared with 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 with 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 percentage recovery of fertiliser applied, explains Mark. “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%.

So that’s what it is, but how exactly do you go about measuring NUE?

“Measuring nitrogen use efficiency is all about good data capture and management. The key bits of data required are harvested grain yield (t or kg/ha); nitrogen supply (soil mineral nitrogen / grain yield from zero fertiliser plots, organic nitrogen applications, inorganic fertiliser applications); and harvested grain percentage 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.”

When it comes to improving overall nitrogen use efficiency, nitrogen uptake efficiency, nitrogen utilisation efficiency and nitrogen harvest index can all be looked at and managed separately to highlight where growers could benefit from changing their strategy, adds Mark. “In other words, if you have measured it you can now start to manage it.”

1. 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, explains Mark. “It’s 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 – for example, magnesium, calcium, sodium and potassium.”

The most notable nutrients that influence soil structure are magnesium and calcium, he adds. “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 – or 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, says Mark. “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, he notes.

“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, adds Mark. “The rate and timing also play 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, meaning ensuring that this is aligned with the crop’s demand.”

The correct timing of nitrogen is important to drive overall efficiency right through the growing season.

For example, oilseeds and cereals will have their highest daily demand for nitrogen through the months of March, April and May, points out Mark. “Multiple applications – a minimum of three – help us ensure supply and demand are synchronous, while 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, with a zinc deficiency having one of the greatest impacts – reducing the metabolism of nitrogen by 50%, explains Mark. “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’s important to ensure that these deficiencies are addressed early in the season so that they are in sufficient supply prior to the peak nitrogen uptake that will happen between GS31 and GS39 for cereals.

Managing 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, explains Mark. “Approximately 90% of the nitrogen found in the grain will come from the nitrogen that is in the vegetative plant material before flowering.

“Natural senescence will be triggered soon after flowering and fertilisation has occurred so it’s 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, he adds. “It’s 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.”