Nitrate-fixing intermediate crops

Where does nitrogen pollution in water come from? 

Water is naturally high in nitrates, but the concentration is found to be much higher in most waters today. This pollution is due to nitrogen leaching on agricultural fields in autumn. During this period groundwater reserves, such as water tables, are replenished. At the same time, plants absorb less water, and winter rain continues to fall. The excess water infiltrates the soil. Nitrates, the nitrogen ions absorbed by plants, are highly soluble and percolate into the soil solution before leaching out. 

At the beginning of the 2000s, many action plans were implemented around the world to limit nitrogen pollution. In Europe, for instance, the Nitrates Directive legislates in this area. It requires sensitive areas to be identified, mandatory action plans to be created and monitored for effectiveness, as well as for nitrate concentration in the water. It also requires a code of best agricultural practices to be drawn up, and farmers to be informed and trained. 

What are nitrate-fixing intermediate crops? 

Nitrate-fixing intermediate crops are sown in the autumn between two main crops. They can be considered as intercrops with a main purpose of fixing nitrogen through root uptake. 
In addition to fixing nitrates, this type of plant cover has other agricultural benefits:  

• It can be used as green fertiliser. Once destroyed, it releases organic nitrogen which can contribute to the development of the next crop. This reduces the amount of nitrogen input required for the crop. 
• It improves the structure of the soil: it protects against slaking and soil erosion. It also enriches the soil with organic matter. 
• It improves biodiversity in terms of micro-organisms, by increasing organic matter, as well as macro-fauna by providing refuge and food. 
• It improves crop protection. It interrupts pest cycles and reduces weeds. 
• In the case of a catch crop, it provides additional forage with no competing crops and no additional land requirement.  
• It can also be used as a Biofuel-Producing Cover Crop (CIVE: from French « Culture Intermédiaire à Vocation Energétique ») to supply a methanizer. 

We more commonly talk about limiting the leaching of nitrates. The idea is to reduce nitrate leaching by trapping nitrogen in the crops. 

One type of cover crop or a mix? 

In general, species used as nitrate-fixing intermediate crops belong to one of the following plant families: grasses, crucifers, legumes, hydraphyllaceae.The choice depends on the farm and its agronomic specificities.

The following factors influence the choice:  

• Soil and climate conditions in the region. 
• The period and duration of intercropping. 
• The purpose of soil cover.
• Depends on working methods and organisation  (conventional, no-plough, organic farming, soil conservation, etc.). 
• Weed control on the field. 
• Date and method of seeding nitrate-fixing intermediate crops, as well as the length of the cycle and destruction techniques (weather conditions such as frost, the capacity of the soil to break down the cover crop, etc.). 
• Other crops in the rotation. 
• Machinery available on the farm (shredder, roller, stubble cultivator, different seed drills, etc.). 

One species can be sown or several species mixed. A mix means you get the benefits from different species. It is not always recommended to use legumes on their own. In practice, mixes are often preferable. They are less effective when it comes to fixing nitrogen in the soil because they absorb atmospheric nitrogen, but they are better as a green manure. In Europe, Ecological Focus Areas (EFA) are included in the regulations. The aim is to provide additional environmental benefits. At least 2 species from different botanical families must be sown in these areas, over a 3-month period, and no chemical plant-protection products used.  

Examples of cover crops according to objectives 

High-biomass nitrate-fixing cover crops can be produced by mixing two species such as mustard and oats, or even phacelia. They are suited to a range of different soil and climate conditions. If nitrogen content is high, to avoid losing it, it is possible to opt for species that develop more quickly, such as crucifers. Otherwise, shorter-cycle grasses work well. 
It is worth adding a nitrate-fixing legume to the mix for green manure. Example mixes are: oats, vetch and peas, or phacelia and clover. 
If the desired result is to structure the soil, it's best to opt for mixtures of species with different root systems, such as radish with phacelia and sunflower or vetch.  
In order to produce catch crops for animal feed, opt for species with high nutritional values, like oats and legumes, or ryegrass, or clover.

When to plant nitrate-fixing intermediate crops? 

Nitrate-fixing intermediate crops are autumn-grown intercrops. They should be seeded as early as possible when the temperatures are still mild so that the biomass has time to develop and fix nitrogen better. Be careful that they don’t go to seed, though, if planted too early (e.g., mustard). Very early seeding is suitable for species that require more light and heat. Sowing from mid-August onwards makes it possible to till the soil beforehand to improve soil-seed contact. 

In practice, you can sow soil cover:  

• Very early, immediately after harvesting the previous crop, until mid-August. It is possible to sow very early with: moha, buckwheat, clover, lentil, pea, vetch, rye, turnip rape, faba bean, sorghum
• Between mid and late August: rapeseed, radish, phacelia, oats, ryegrass. 
• Late, from the end of August to mid-September: mustard. 

Watch out for problems with plant-protection-product residues with the emergence of certain species (e.g. sulfonylureas on crucifers or legumes). 

Some regions have mandatory planting dates which may also be subject to an interval after harvest. 

How to plant nitrate-fixing intermediate crops: seeding possibilities 

Planting methods depend on the chosen species. Some crops are more demanding than others, so require different machines and techniques. Non-demanding crops, such as oats or mustard, can be broadcast. Crops with moderate demands, such as vetch or rye, can be sown with a stubble cultivator, while other species require row seeding or direct seeding. 

For a cover-crop mix with different-sized seeds, it is possible to sow in two stages. For example, broadcast mustard and sow field beans in a row due to their larger seeds and higher demands. 

How to destroy nitrate-fixing intermediate crops? 

Destroying nitrate-fixing intermediate crops can be more or less easy depending on how sensitive they are to different techniques as well as the tools available on the farm:  

• Some species are destroyed easily by frost, such as vetch which is very sensitive and freezes at 0°C. Shredding is sometimes advisable, if there is a lot of biomass, to speed up decomposition. 
• Cover crops can be shredded, sometimes followed by stubble cultivation or burial, before seeding. A cutter roller can have the same effect.  However, this technique is less effective for the grass family which commonly produces regrowth. 
• Stubble cultivation not only destroys the cover crop, it also works the topsoil which prepares the field for the following crop, or works as a mechanical weeding method. 
• Ploughing. Make sure to remove the skimmers so as not to bury the residues at the bottom of the furrow. Shredding is still very useful, if there is a lot of biomass, for faster decomposition. 
• Chemical destruction is also very common (e.g., glyphosate + 2.4D). 

Make sure you are aware of your local regulations relating to the destruction of nitrate-fixing intermediate crops.

What other measures can prevent water pollution by nitrates? 

Planting nitrate-fixing intermediate crops prevents nitrate pollution. Other measures can also be implemented, particularly preventive measures, such as:  

• Having sufficient storage capacity on the farm for manure. 
• Fertilising at key moments in the plant cycle, by dividing fertiliser input into smaller doses. 
• Getting expert advice from local technical institutes and agricultural chambers. 

Source: Fighting water pollution from agricultural nitrates | EUR-Lex (europa.eu)