Increasing the Amount of Nitrogen in a Cropping System
Nitrogen plays a crucial role in the general metabolism of plants. It can be introduced into the cropping system through inputs, a short-term solution, or through organic solutions, which offer a long-term impact.
Cover crops
This graph is based on trials conducted on a farm practicing no-till farming for about fifteen years and working with permanent cover crops for 5 years. Two modalities are presented here:
- In green: wheat on permanent alfalfa cover.
- In orange: wheat on tilled soil without associated cover.
It can be seen that for the same nitrogen dose, the modality cereal + permanent cover has a higher yield.
A very dense cover efficiently valorizes mineralization
- A cover allows to “deprive” plants of CO2: the current atmospheric level is 400 ppm versus an optimal development level around 800 ppm for a crop like wheat.
- Mineralization also concerns carbon that escapes as CO2.
- With a sufficiently developed cover crop, this CO2 is trapped under the canopy where its concentration will increase, benefiting plant productivity.
Cover destruction
The time lapse between the destruction of the cover crop and the planting of the spring crop impacts the nitrogen release.
- The later the destruction, the more nitrogen accumulation is significant, as shown by this graph from Frédéric Rayns' work.
- The earlier the destruction, the less time the legume has to produce biomass.
Example of alfalfa
With an alfalfa cover established for several years, a rapid turnover of organic matter benefits the cereal in place. It has been measured that alfalfa can release up to 50 to 60 units of nitrogen for the crop. These values are contextual. It is a target to aim for.
On the response to nitrogen graph on the right, it can be seen that the “wheat alone” modality is almost always above the wheat under permanent alfalfa cover modality:
- This is explained by the after-effect of alfalfa destruction: the wheat benefits from nitrogen release and has a better yield.
- Alfalfa is a perennial crop: it needs several years to reach its full potential. Generally, 3 to 4 years are counted before nitrogen fertilization can be reduced without yield loss.
Quantifying the economic return of cover crops as fertilizer equivalent
Potential economic gains related to cover crops are envisaged in the short term through input savings and in the long term through improvement of biological fertility and soil water retention.
| Applied doses (mineral N) | Cover biomass (DM/ha) | Nitrogen supplied by the cover | Savings at €1.6/ N unit |
|---|---|---|---|
| 75 N units | 2.1 | 75 | 120 |
| 150 N units | 3.8 | 150 | 240 |
| 225 N units | 5.6 | 210 | 336 |
| 300 N units | 7.3 | 280 | 448 |
Impact of cover crops on mineral elements
The MERCI method (method for estimating returns by catch crops), developed in 2010 by the Regional Chamber of Agriculture Nouvelle-Aquitaine, provides a software that allows evaluation of recycling and availability of mineral elements by cover crops. The software is free and openly accessible at this link.
Organic fertilizers
| Fertilizer equivalence coefficient (Keq) | |||||||
|---|---|---|---|---|---|---|---|
| Crop type | rapeseed | winter crop | spring crop | grasslands | |||
| Effluent type | Application period | autumn application | spring application | autumn application | spring application | autumn - winter | spring |
| Compost green waste | 0.05 | 0.05 | 0.05 | 0.05 | 0.05 | ||
| Herbivore compost | 0.10 | 0.05 | 0.10 | 0.15 | 0.00 | 0.05 | 0.00 |
| Pig compost | 0.15 | 0.05 | 0.15 | 0.15 | 0.00 | 0.20 | 0.20 |
| Vegetable waste compost | 0.30 | 0.20 | 0.30 | 0.40 | 0.40 | 0.40 | 0.40 |
| Herbivore manure, rabbit manure and slurry | 0.20 | 0.10 | 0.20 | 0.05 | 0.30 | 0.10 | 0.05 |
| Pig manure | 0.30 | 0.20 | 0.25 | 0.05 | 0.30 | 0.40 | 0.40 |
| Poultry manure | 0.40 | 0.30 | 0.35 | 0.40 | 0.50 | 0.40 | 0.40 |
| Droppings | 0.40 | 0.30 | 0.35 | 0.40 | 0.60 | 0.40 | 0.40 |
| Cattle slurry, cattle slurry | 0.40 | 0.30 | 0.50 | 0.30 | 0.50 | 0.40 | 0.40* |
| Pig slurry | 0.50 | 0.35 | 0.60 | 0.40 | 0.60 | 0.50 | 0.50* |
| Poultry slurry | 0.50 | 0.30 | 0.60 | 0.40 | 0.70 | 0.50 | 0.50* |
Organic residual products (ORP)
Fertilizer equivalence coefficient of ORP
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To take into account the spreading of organic products (manures, slurries, screenings, etc.) in a fertilization plan, it is important to know their fertilizing value.
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 available short-term for the crop; the rest depends on the mineralization process. The fertilizer equivalence coefficient allows knowing the proportion of nitrogen that will be available to the crop during the season.
A table, from Comifer's work updated in 2019, presents average composition values of the main categories of organic residual products. Transformations may occur during storage. It may therefore be recommended to analyze organic products before the spreading date.
Carbon to nitrogen balance C/N
C/N ratios of some amendments [1]
| Product | C/N ratio |
|---|---|
| Urine | 0.7 |
| Manure leachate | 1.9 - 3.1 |
| Mixed slaughterhouse waste | 2 |
| Blood | 2 |
| Green plant material | 7 |
| Humus, black soil | 10 |
| Manure compost after
eight months of fermentation |
10 |
| Grass | 10 |
| Comfrey | 10 |
| Poultry droppings | 10 |
| Domestic animal excrements | 15 |
| Mature manure compost,
4 months, without soil addition |
15 |
| Farmyard manure after
3 months storage |
15 |
| Legume hay | 15 |
| Alfalfa | 16 - 20 |
| Fresh manure low in straw | 20 |
| Kitchen waste | 10-25 |
| Potato haulms | 25 |
| Urban compost | 34 |
| Pine needles | 30 |
| Fresh farm manure with
abundant straw |
30 |
| Peat, black | 30 |
| Tree leaves (at fall) | 20-60 |
| Green plant waste | 20-60 |
| Blonde peat | 50 |
| Cereal straw | 50 - 150 |
| Oat straw | 50 |
| Rye straw | 65 |
| Ramial chipped wood (chipped branches) (depending on wood type and chipped diameter) | 60 - 150 |
| Bark | 100-150 |
| Wheat straw | 150 |
| Paper | 150 |
| Decomposed wood sawdust | 200 |
| Deciduous wood sawdust (young leaves) (average) | 150 - 500 |
- According to the Chamber of Agriculture of Pyrénées-Orientales, horse manure would systematically cause nitrogen hunger during the first year after spreading[2], possibly because it often contains a lot of straw. However, there are not enough studies on the subject.
- Composting materials too rich in carbon before spreading has the effect of lowering their C/N ratio.
Example of use in market gardening
- Mix 2 wheelbarrows of grass (C/N = 10) with 1 wheelbarrow of chipped branches (C/N = 70).
- Average ratio (Rm)= (2 * 10 + 1 * 70)/3 = 90/3=30
- Note: this calculation is only valid if the dry matter content (without water) is similar for the considered wastes. If not, weighting must be done based on dry matter.
- To generalize:Rm = (n1*R1 + n2*R2 + n3*R3)/(n1+ n2 + n3). With Rm the average ratio, n1 and n2 the respective quantities of components and R1 and R2 the C/N ratios of these components.
C/N ratio of the cover crop
The amounts of nutrients actually returned by the cover crop to the following crop depend on many factors: pedoclimatic conditions, cultural practices, cover destruction method and vegetative stage of the cover at destruction.
- The more the cover is at an advanced stage, the more it lignifies, the higher its C/N ratio increases.
- The higher the C/N ratio the slower the nitrogen release.
- C/N > 30: 160 days after incorporation, only 20% of nitrogen is mineralized.
- C/N < 15: 160 days after incorporation, 50% of nitrogen is mineralized.
| C/N | % of N
released |
|---|---|
| < 15 | 50 |
| 15 to 20 | 40 |
| 20 to 25 | 30 |
| 25 to 30 | 25 |
| > 30 | 20 |
Annexes
Sources
- ↑ https://fr.wikipedia.org/wiki/Rapport_C/N
- ↑ Anne Sandre, Chamber of Agriculture of P-O, Viticulture Service https://po.chambre-agriculture.fr/fileadmin/user_upload/Occitanie/073_Inst-Pyrenees-Orientales/IMAGES/PRODUCTIONS_TECHNIQUES/VITI/Fertilisation_d_entretien_du_sol_en_viti/Fiche-AZOTE_nv.pdf
