Optimizing Nitrogen Inputs

Technical articles - AgroLeague - Fertilization, Productivity, Carbon cycle and GHG

Managing deficiencies in arable crops Balance sheet method Appi-N method Cultural control CAU Nitrogen

Good nitrogen fertilization management helps avoid any imbalance that could cause losses by leaching, the arrival of pests and diseases, and promotes the quality of the final product.

Determining quantities by the balance sheet method

Presentation of the method

The balance is made between:

The needs of the crop (need per quintal/yield) as well as a part of the non-usable nitrogen.
The residual nitrogen at winter exit and the nitrogen that will naturally be mineralized thanks to the soil's biological activity.

The missing balance to balance the equation will be supplied exogenously in mineral or organic form.

Example with soft wheat

The needs to be supplied:
- 3 UN/q for a targeted yield of 70 q/ha, i.e. a total need of 210 UN/ha.
- We count 30 UN non-usable.
- 210 + 30 = 240 UN/ha.
The supplies:
- The residual nitrogen at winter exit (RSH) to be measured.
- The nitrogen supply e that the soil can produce (destruction of grassland, previous crop, residual effect of organic inputs, humus mineralization).
- The humic balance allows calculating the amount of nitrogen that can be expected from our soil for the current crop.
- The difference is supplemented by exogenous nitrogen inputs.

Focus on organic nitrogen

To take into account the spreading of organic products (manure, slurry) in a fertilization plan, it is important to know their fertilizing values. Fertilizer equivalence coefficients have been established: they express the efficiency of an organic fertilizer compared to a reference mineral fertilizer. Only part of the organic nitrogen is usable in the short term by the crop, the rest depends on the mineralization process. The fertilizer equivalence coefficient allows knowing the portion of nitrogen that will be available for the crop during the campaign.

Example

The fertilizer equivalence coefficient N for pig slurry applied on grassland is 0.65. That is to say that 65% of the nitrogen contained in the pig slurry applied will be available for the current crop.
If the goal is to supply 50 UN/ha on a grassland at winter exit (February-March), knowing that pig slurry contains 3.5 UN/m3, it will be necessary to apply 50/(3.5*0.65) = 22 m3/ha of pig slurry.

Residual nitrogen at winter exit

To assess soil supplies, the first thing to do is to measure residual nitrogen at winter exit. This is the measurement of mineral nitrogen content in a soil sample to estimate the amount of mineral nitrogen available for the current or upcoming crop.

It is necessary to be rigorous in sampling to be as representative as possible of the plot and get the most value. Doing the sampling yourself ensures quality work, 15 to 16 samples are necessary:

Economic data

Cost: around €40 per plot.
Savings: assuming savings generated thanks to residual nitrogen are 25 nitrogen units at €2/u/ha on a 10 ha plot:
Value gain on the plot is: 500 - 40 = €460.

Biomass at winter exit and rapeseed nitrogen ruler

According to the Technical Institute Terres Inovia, the biomass of rapeseed is an indicator of the amount of nitrogen absorbed by the rapeseed because this plant has the ability to store nitrogen in autumn and remobilize it in spring.

Measuring biomass allows using the Rapeseed Nitrogen Ruler to calculate the total nitrogen dose to apply to the crop in spring:

Knowing the potential yield and the nitrogen already applied, it is possible to deduce the missing balance to complete the equation.
Applications will be split at different stages according to needs and remaining doses to apply (see table below).

Digital tools to determine needs

There are decision support tools (DST) digital to improve the balance.

The Appi-N method (INRAe)

Researchers at INRA developed the Appi-N method which allows managing all nitrogen inputs according to the different stages.

The management method of nitrogen is based on the nutritional status of the plant (nitrogen nutrition index: INN), calculated from the chlorophyll index. It allows a reduction of quantities used, a delay of the first input (about 20 days) and a reduction of losses (soil and air). It takes into account:

The acceptable deficiencies: deficiency stages that do not impact final yield, especially early in the cycle and can even be beneficial.
The nitrogen use efficiency coefficient: linked to growth rate.
The weather conditions favorable to its assimilation.

The Yara N-tester

This tool focuses on the chlorophyll concentration of the leaf:

About thirty leaves are pinched from main stems between the 2-node and swelling stages, which allows optimizing the last input.
The N-tester provides chlorophyll values and Yara developed a decision support tool (DST) to help calculate nitrogen values to apply available on wheat, winter and spring barley, corn and potato.

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The N-tester score (linked to chlorophyll rate) is proportional to the nitrogen nutrition index (INN).

The higher the chlorophyll rate of a plant, the more efficiently it is fed with nitrogen.
This is illustrated in the adjacent graph.

The INN corresponds to the ratio between the total nitrogen content of aerial parts and the critical total nitrogen content, determined from the biomass of aerial parts.

This critical nitrogen content corresponds to the minimum nitrogen content necessary to maximize dry matter growth of the plant.
According to Arvalis, INN is “the best indicator' to characterize nitrogen nutrition of cereals, and especially their deficiency level’”.

There is thus a reference curve defining the critical nitrogen content according to wheat growth level. The target is an INN close to 1:

INN < 1: under-dose
INN > 1: over-dose

Satellite tools

Sentinel (free)

Regarding biomass estimation, the NDVI ("normalized difference vegetation index") is used, offered free by Sentinel satellites.

NDVI is a simple index to quantify the plant biomass present on a plot. It normalizes the scattering of green leaves in the near-infrared wavelengths and the chlorophyll absorption in the red wavelengths.

This tool thus allows visualizing areas of the plot that would need more nitrogen because there is lower biomass.

Sentinel is freely accessible and completely free. Caution: as it is a free tool, the precision is not sufficient to observe differences between small plots, or on a sprayer width for example.

Spotifarm (paid)

The NDVI gives an idea of plant biomass on a plot, but not a precise calculation of the nitrogen dose to apply. Some DSTs offer this: this is the case of Spotifarm, a paid tool based on Sentinel data.

The algorithm uses satellite data from Sentinel to indicate precise zones where to apply nitrogen and how to modulate it.

Comparison of different DSTs on winter soft wheat

A study was conducted by the CNRS and AXÉRÉAL in 2020 at Chaumoy to compare the efficiency of different nitrogen modulation methods on the final result. The study was updated with current fertilizer and wheat prices:

Determining the application date by the “double density” strip technique

The “double density” strip (DDS) is a simple, reliable and visual indicator to know when to trigger the first application.

Principle of the method

Choose a representative area of the plot and double the sowing density on a strip 30 m long and 3 to 6 m wide at sowing.
Doubling the number of plants causes a higher and earlier nitrogen consumption on this strip than on the rest of the plot.
The appearance of a yellow discoloration from late January on the strip indicates an early deficiency and allows anticipating nitrogen application. As soon as discoloration is observed, it is recommended to make the first application within 7 days.

Example on soft wheat

1^st application: 40 UN is triggered when this DDS discolors compared to the rest of the plot. It is done at the latest at the 1 cm ear stage.
2^nd application (X – 80) is triggered 20 to 25 days after the 1^st application (200 degree days). It is done at the latest at the 1 node stage.
3^rd application (40 units) is done at the latest at the last leaf unfolded - swelling stage.

If the double density strip discolors at a stage close to the 1 cm ear stage, skipping the 1^st application can be considered.

Rationing inputs

Description of the technique

Apply a nitrogen dose leading to a deficit balance between nitrogen supplies and the needs of the crop to reach the yield allowed by the plot potentialities, the rotation and the climatic context, and avoid conditions favorable to the development of certain pests (diseases especially foliar).

The effect on pests of the total nitrogen dose applied is not independent of the timing of applications and splitting. Thus, on winter soft wheat, applying the dose reduction on the first application by delaying it as much as possible helps avoid a dense cover favorable to pests and diseases, while minimizing the impact of nitrogen deficiency on yield and quality. On wheat, a dose reduction recommended by the balance sheet method of about twenty units applied on the first two applications helps limit the risk of foliar diseases notably (Appi-N method).

Practical caution

Soil type: Generalization sometimes delicate. On soils with low mineralization (chalky soils…), nitrogen dose reduction quickly leads to yield losses.

Example on winter soft wheat

It is useless to apply large doses of nitrogen at the first application because at the tillering stage, wheat cannot use them so losses will be significant.

Tillering: apply nitrogen if the residual on the first horizon is less than 60 UN. Apply max 40 UN (beyond that nitrogen will not be used).
2^nd application: frame the "1 cm ear" stage if more than 100 UN are applied.
3^rd application: 40-60 UN (depends on variety) especially for protein content.

Effects on the sustainability of the cropping system

"Environmental" criteria

Effect on air quality: Nitrogen fertilization rationing reduces the risk of ammonia emissions by volatilization proportionally to the dose reduction performed. Nitrogen fertilization rationing reduces greenhouse gas emissions linked to fertilizer manufacturing, as well as nitrous oxide emissions in the field, proportionally to the dose reduction performed.

Effect on water quality: Nitrogen fertilization rationing reduces the risk of nitrogen transfer to water: limiting losses by leaching.

Effect on fossil resource consumption: Nitrogen fertilization rationing reduces energy consumption linked to mineral fertilizer manufacturing proportionally to the dose reduction performed.

"Agronomic" criteria

Productivity: Applying a nitrogen dose that does not balance crop needs and available nitrogen results in a yield decrease varying according to the dose reduction, crop response to nitrogen dose, and soil nitrogen supplies. But costs are lower.

"Economic" criteria

Operational costs: Nitrogen fertilization rationing results in reduced fertilization costs.

Mechanization costs: Nitrogen fertilization rationing can lead to reduced mechanization costs if it reduces the number of passes.

Margin: The impact of nitrogen fertilization rationing on profitability is variable depending on the balance between the yield decrease caused and the cost reduction of fertilization.

Optimize the apparent nitrogen use coefficient CAU

The apparent nitrogen use coefficient (CAU) corresponds to the fraction of total nitrogen in a fertilizer (mineral or organic) that is absorbed by crops until harvest.

Here is a graph from Hazzar Habbib’s thesis studying the impact of cropping systems on nitrogen use efficiency in wheat and corn, under the influence of nitrogen input dose, soil tillage and cover crops in intercrop.

Several lessons can be drawn:

The apparent use coefficient is higher for treatments without mineral nitrogen input.
Physiologically, plants in deficiency better utilize the little nitrogen they find in the soil.
Interesting from a research point of view, this option is obviously not feasible in the field given the low yields obtained.
Around 160 UN, the apparent use coefficient is highest in no-till alone and no-till under cover crop systems. As soon as soil is not tilled and organic nitrogen is supplied by cover crops composed of legumes, the need in UN/q decreases.

Influence of other factors

Weather conditions

To properly time nitrogen, it is important:

That the application is followed by rain so it is well utilized. 15 to 20 mm of rainfall within 15 days after application are necessary for nitrogen to pass into the soil solution and be absorbable by the crop. For example: make applications at times when the probability of recording 15 to 20 mm of precipitation in the following 15 days is high and intervene in cool and humid weather.
Sparse cover crop.
To avoid windy conditions.
To avoid temperatures > 15°C.

Otherwise, volatilization causes losses in gaseous form.

Trace elements

The nitrogen solution can be adjuvanted to improve its utilization. Here is a mixture recommended by the team of agronomists from AgroLeague to stabilize nitrogen:

1 unit of sulfur (or 3 units of sulfate) per 10 units of nitrogen via a thiosulfate (a reducing agent that allows to stabilize nitrogen in the solution). If there is not enough sulfur, the microbial life will not be able to stabilize all the nitrogen and may consume soil carbon to obtain the sulfur necessary to transform nitrogen.
3% molasses (3% of the slurry): molasses will stimulate bacteria and cause a transformation of nitrogen into amino acids.
1 L/ha of humic-fulvic acid: allows chelation and stabilization via organic acids. This makes nitrogen less leachable.
20 g/ha of molybdenum: Mo acts as nitrate reductase, transforming nitrogen into amino acids.

This mixture helps to limit volatilization, to chelate nitrogen by organic acids and make it more available to the crop. By transforming nitrogen, it limits nitrification and leaching.

Aim for the economic optimum

For performance purposes, it is important to consider the notion of economic optimum.

Arvalis and Terres Inovia have produced summaries on wheat and rapeseed with:

Horizontally: the crop price (in €/t);
Vertically: the price of the nitrogen unit (in €/NU).

These tables are interesting because they allow establishing a technical-economic optimum corresponding to the context.

Source: Arvalis
Source: Terres Inovia

Video

For more information

Impact of crop management on sunflower diseases: Mestries E. (CETIOM), Desanlis M. (INRA), Seassau C. (EIP), Moinard J. (DRAAF), Debaeke P. (INRA), Dechamp-Guillaume G. (ENSAT). Agronomic innovations, peer-reviewed journal article, 2011.