Nitrogen residue
The nitrogen residue is a method allowing to adjust fertilization in nitrogen based on the nitrogen remaining in the soil at the opening of the balance sheet. For the result to be reliable, a certain methodology must be followed. Nevertheless, like any method, it remains open to criticism, and this is what we will explore together.
Best practices for a reliable RSH
In the following lines, we will regularly use the acronym RSH for Nitrogen Residue at Winter Exit.
When to sample
Reminder:
- The residue value represents the amount of mineral nitrogen available in the soil at a given time.
- The value evolves very rapidly in the soil (mineralization, biological activity, vegetation, rain leaching...).
Sampling must be done as close as possible to fertilization, in dry conditions, after winter leaching (around January 15th generally).
Factors that can make a result uninterpretable:
- Sampling after a recent nitrogen application less than 2 months ago.
- Sampling on frozen, snow-covered, or very dry soil.
- Sampling after recent soil work (plowing, stubble cultivation,...)[1].
The ammonium (NH4+) content is a good indicator of sample quality. Above 20 kg N/ha in the top horizons, question the quality of the sampling.
High values can also come from the following situations:
- A recent turning of grassland or recent incorporation of nitrogen-rich residues (e.g., beet leaves).
- A naturally rich organic matter soil.
- A saturated or near-saturation water soil.
- A recent application of organic or mineral fertilizer[1].
Depending on the pedoclimatic context of the concerned territory, these values can be kept if they are representative of practices and agricultural context.
How to sample
- Where? : Within the largest homogeneous zone of the plot in a circle of 20 to 30 meters diameter.
- How many samples? : Minimum 14 elementary cores to constitute a representative sample. The graph below clearly shows that below this number, the error is too large to have a reliable nitrogen residue measurement.
What depth
- Shallow soils: minimum 2 horizons.
- Deep soils: minimum 3 horizons[1].
Reminders:
- 1 horizon = 30 cm.
- Analyses on a single horizon are reserved for very specific situations: very shallow soil, crops with a short cycle or shallow rooting.
- Within the framework of the Nitrates Directive, for deep-rooted crops, it is mandatory to perform an RSH on three horizons.
Which laboratory
By an order dated December 22 and published in the Official Journal on December 29, 2021, the Ministry of Agriculture and Food updated the list of accredited laboratories for soil analyses: 28 establishments are accredited for 2023 (Note not all are authorized to perform nitrogen residue measurements).
Find the order here.
Find the list of accredited laboratories directly with links to their sites here.
Effect of post-sampling conditions
As nitrogen mineralization is strongly temperature dependent, it is essential to be vigilant about post-sampling conditions to ensure a reliable result.
The sample must reach the laboratory:
- Refrigerated (4°C): Within 2 to 3 days after sampling.
- Frozen (-18°C): Preferable if possible as ideal to block mineralization.
- In both cases: Preserve the cold chain during transport (no drop-off on Saturday, quick shipping).
It is important to respect the cold chain:
Also beware of freeze/thaw/freeze risks which also cause significant mineralization:
- Avoid freezing samples from frozen soils.
- Respect the cold chain.
Limits of the method
Density of deep horizons
The uptake of soil minerals by plants depends on the ability of plants to root. Thus, if a soil is too dense, roots will not go deep and plants will not be able to take up soil minerals. Trials by the SCARA cooperative show that often the third horizons, from 60 to 90 cm, have a fairly high density, around 1.5.
| Horizon | Density | Moisture | NH4+
(kg/ha) |
NO3-
(kg/ha) |
Mineral nitrogen
(kg/ha) |
|---|---|---|---|---|---|
| 0-30 cm | 1.20 | 21% | 3.10 | 26.00 | 29.10 |
| 30-60 cm | 1.40 | 18% | 6.10 | 10.30 | 16.40 |
| 60-90cm | 1.50 | 14% | 0.30 | 17.00 | 17.30 |
| Total | - | - | 9.50 | 53.30 | 62.80 |
As can be seen from this trial result from MAPAC in Quebec, such density strongly penalizes rooting, especially in very clay soils. One can therefore question the relevance of a third horizon when plants will anyway be very limited in utilizing the nitrogen present there.
| Optimal | Rooting limit | Prevents
rooting | |
|---|---|---|---|
| Sand,
sandy loam, loamy sand |
1.6 | 1.7 | 1.8 |
| Sandy loam,
loam |
1.4 | 1.6 | 1.8 |
| Sandy clay loam,
loam, clay loam |
1.4 | 1.6 | 1.75 |
| Silt, loam | 1.3 | 1.6 | 1.75 |
| Loam silty, | 1.4 | 1.55 | 1.65 |
| Clay (35 to 45%) | 1.1 | 1.5 | 1.6 |
| Clay (>45%) | 1.1 | 1.4 | 1.5 |
Heterogeneity of results within a plot
It is known that plots are heterogeneous on fairly static indicators such as organic matter content or levels of phosphorus or potash. This heterogeneity is even stronger when it comes to very dynamic indicators such as nitrogen quantity.
Here is for example a corn-soybean-wheat rotation in Germany on a 5 ha plot with silty soils. Differences in residues can reach up to 100 UN per zone (from 80 to 180 UN). Depending on where you sample, there will be a strong impact on the result. As can be seen on this map, the nitrogen residue is also strongly inversely correlated with agronomic potential. If fertilization is equal everywhere, more productive zones will take up more nitrogen than less productive zones. Thus, the nitrogen residue will be impacted with less productive zones showing the highest nitrogen residues.
Values vary depending on laboratories
Finally, nitrogen residue values also depend on the laboratory. Here is an example where the same sample was sent to 4 different laboratories, and differences can reach 48 nitrogen units, which is not negligible for the dose that can then be applied within the nitrogen balance framework. Results for barley 2017:
Sources
This article was written thanks to the kind contribution of Martin Rollet, agronomist engineer at the National Center for Agroecology and at Agroleague.
