Nutrient Use Efficiency: Where Does the Nitrogen Go?

A field that receives 150 kilograms of nitrogen does not necessarily deliver 150 kilograms to the crop. Part is taken up by the crop, part remains in the soil in forms that may be used later, and part is lost from the system altogether. Nutrient use efficiency is the share of the applied nutrient that the crop takes up in a season, and it is one of the simplest ways to judge a nitrogen programme.

Efficiency is not a single number that can be moved with one change. It depends on the crop, the soil, the weather, the source and the timing, and each of these interacts with the others. A grower who wants to improve efficiency therefore needs to know where the nitrogen goes on their own farm, and that requires a clear picture of the loss pathways.

Three loss pathways

Ammonia volatilization happens when urea or ammonium fertilizer sits on the soil surface and the pH around the granule rises. The ammonia escapes to the air, and the nitrogen is lost. The risk is highest in warm, moist, alkaline conditions, particularly when the fertilizer is not incorporated or covered by rain soon after application. Crop residues on the surface can increase the loss, because they contain urease.

• Ammonia volatilization from surface-applied urea and ammonium products, highest in warm, moist, alkaline conditions and where the fertilizer is not incorporated.
• Leaching of nitrate below the root zone in wet soils and sandy soils, particularly when heavy rain follows a large application.
• Denitrification, where waterlogged soils convert nitrate to nitrogen gas and nitrous oxide, with the largest losses in warm, wet periods.

Leaching is the movement of nitrate with drainage water. Nitrate is not held firmly by the soil, so it moves with the water and can leave the root zone. Sandy soils and soils with high rainfall are most affected. Leached nitrate is lost from the field, and it can also contaminate groundwater and surface water, which is one reason why regulators pay attention to nitrogen management.

Denitrification happens in soils that are waterlogged or very wet, where oxygen is scarce. Microorganisms use nitrate as an alternative to oxygen and release nitrogen gas or nitrous oxide to the air. Nitrous oxide is also a powerful greenhouse gas, so denitrification losses have both agronomic and environmental costs.

Measuring NUE

A simple NUE measure is the crop nitrogen uptake divided by the nitrogen applied, expressed as a percentage. Crop uptake is estimated from the yield and the nitrogen content of the harvested product, plus any nitrogen left in the stover or straw. The measure gives a clear comparison between treatments on the same field, even if the absolute numbers vary with the season.

Track NUE over several seasons, because a single year can be distorted by weather. A wet spring may cause large losses in one year and small ones in the next, so a three-year average gives a more reliable picture. Recording the rate, source, timing and weather alongside the NUE makes it possible to see which factor drove the change.

Improving NUE

Matching the rate to the crop's need is the first and most effective step. Applying more nitrogen than the crop can use increases losses without increasing yield. Splitting the application so that nitrogen is available when the crop needs it reduces the amount exposed to leaching and denitrification at any one time.

Incorporating surface-applied fertilizer, or using a urease inhibitor or a stabilized product, reduces ammonia volatilization. Timing applications before a period of expected rain, rather than before a dry spell, reduces the exposure of urea on the surface. Drainage and irrigation management also change the loss pathways, so the water plan should be part of the nitrogen plan.

Setting a NUE target

Set a target NUE for each crop based on local research and on the farm's own records. A realistic target differs by crop, soil and climate, and it should be reviewed each season. If NUE falls, look first at timing and rate, then at losses from waterlogging, surface application or leaching, before increasing the rate. Changing one thing at a time makes it possible to see what works.

Efficiency gains often come from simple changes: splitting a dose, incorporating surface fertilizer, avoiding application before heavy rain, and matching the rate to the crop's expected uptake. These changes cost little and are easy to record, and they often give a larger return than a new product.

Linking NUE to the business

Efficiency matters to the business because every kilogram lost is a kilogram paid for and not used. Improving NUE from a low level usually gives a clear financial return, because the cost of the lost nitrogen is avoided. Improving from an already high level is harder and gives smaller gains, so the target should reflect where the field sits today.

• Calculate the cost of the nitrogen applied each season, and the value of the yield it supports.
• Compare the NUE with the regional benchmark, where one exists, to see whether there is room to improve.
• Choose one change to test each season, and record the effect on NUE and yield.

Comparing efficiency across seasons and fields

Comparing NUE across seasons is useful, but only if the other factors are recorded. Rainfall, temperature, the crop variety and the soil test all change the amount of nitrogen the crop can take up, so a change in NUE may reflect the season rather than the management. A simple record that notes these factors next to the NUE makes the comparison more meaningful.

Comparing NUE across fields is also useful for identifying where losses are highest. A field with a low NUE and a history of waterlogging may need a drainage improvement before any change to the fertilizer programme. A field with a low NUE and a sandy soil may need a split application or a controlled-release product.

Treat the NUE as a diagnostic tool rather than a score. A low value tells you that something is limiting the crop's use of nitrogen, and the other records help you find out what it is.

Reporting NUE to buyers and regulators

Some buyers and regulators now ask growers to report nitrogen use. A simple, consistent record of rates, sources, timing and yields makes this easier, and the NUE can be calculated from that record when required. Reporting should be honest about the method and the uncertainty, because the figure depends on the assumptions used.

Common questions

Is a high NUE always good? A very high NUE can mean the crop is short of nitrogen, because the plant takes up all it can get and the yield is limited. The aim is a balance, with the crop well supplied and losses kept low, rather than the highest number possible.

Do losses happen on every farm? Losses occur wherever nitrogen is applied, but their size depends on the soil, the weather and the management. Some farms have small losses with good practice, while others lose a large share of the nitrogen they apply.

Can NUE be measured on a single field? Yes. Compare a treated and an untreated area on the same field, or compare two rates, and calculate the crop uptake from the yield and tissue data. The result is most useful when it is repeated across seasons.

Recommendations depend on local soil, climate and crop conditions. Follow the product label and local regulations, and consult a qualified agronomist for field decisions.