Nitrate-fixing cover crops

What are nitrate-fixing cover crops? 

Nitrate-fixing cover crops are sown in the fall 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 fertilizer. 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. 
   

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 cover crops belong to one of the following plant families: grasses, crucifers, and legumes.

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 organization  (conventional, no-till, organic farming, reduced tillage, etc.). 

• Weed control on the field. 
•  Date and method of seeding nitrate-fixing cover 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.

Examples of cover crops according to objectives 

High-biomass nitrate-fixing cover crops can be produced by mixing two species such as vetch and cereal rye, or Crimson clover and annual ryegrass. 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 brassicas. 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 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 cover 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 cover crops are fall-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. Planting from August onwards makes it possible to till the soil beforehand to improve soil-seed contact.

In practice, you can plant soil cover:  

•  Very early, immediately after harvesting the previous crop, until late-August. It is possible to sow very early with: clover, pea, vetch, turnip, fava bean, sorghum                                                                                                                             
• Between mid and late August: radish, oats, ryegrass, mustard, rapeseed
• 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.  

How to plant nitrate-fixing cover 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 terminate nitrate-fixing cover crops? 

Terminating nitrate-fixing intermediate crops can depend 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. 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. 
• Plowing: 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).