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The importance of nitrogen

Nitrogen is considered one of the most important macronutrients in crop production. Nitrogen is essential, for plant growth and it supports various plant functions. It is also an integral element in many “building blocks” of plant tissue. Approximately 78% of the atmosphere consists of nitrogen. Although there is this abundance around us, this form of nitrogen cannot be used by the majority of plants.

The problem of soil degradation

Years of cropping and unsustainable practices have degraded many agricultural soils to such an extent that they cannot sustain production purely by natural processes. Therefore, current agricultural production is highly reliant on nitrogen-containing fertilisers to supply the crops with their nitrogen needs. Not only is fertiliser costly, but the production thereof is environmentally unfriendly and responsible for significant greenhouse gas emissions. The application of nitrogen fertiliser is further responsible for soil-, water- and air pollution during cultivation. This is due to very low nitrogen use efficiencies by the plants and the reactivity of the product to the natural elements. It is therefore essential to introduce alternative ways of bringing nitrogen into cropping systems.

The value of legumes

A legume can form a relationship with the soil bacteria Rhizobia. The legumes, in association with these soil microorganisms, can then convert atmospheric nitrogen into an inorganic, plant-available form. This process, called nitrogen fixation, occurs through a symbiotic (mutually beneficial) process in the host plant’s roots.

Introducing legumes to your system, and managing them efficiently, introduces various benefits to your farm. The most obvious advantage of legumes is their ability to naturally add nitrogen to the soil. This can reduce fertiliser inputs significantly, thereby reducing production costs. Nitrogen acquisition through biological N fixation is less prone to nitrogen losses compared to the use of fertilisers, and will therefore result in lower on-farm emissions.

Legumes also add diversity to your system. This results in various benefits such as improved soil health, improving the resilience of your cropping system, possibly adding an additional stream of income, and improving the quality of feed for livestock, to name a few. Some species of Rhizobia have even been documented to improve the uptake of other valuable soil nutrients, as well as producing beneficial and growth-promoting plant hormones.

The management of legumes

Legumes need to be managed well to promote fixation and reap the maximum benefits. Management practices that improve soil health and soil life will also promote nitrogen fixation. These practices include:

  • improving soil carbon
  • diversification of the cropping system,
  • minimising chemical fertiliser inputs,
  • minimising herbicide, pesticide and fungicide use, and
  • managing soil moisture.

Elevated total soil nitrogen levels, which are often a problem encountered on dairy farms, is especially known to reduce/inhibit many microorganisms and natural processes in the soil. This is also true for biological nitrogen fixation, and with increasing nitrogen levels, fixation is reduced significantly.

Conclusion

Taking advantage, or greater advantage, of biological nitrogen fixation, seems like a logical option for a chemical nitrogen substitute. Adding legumes to your cropping system, and managing them well, has great potential to add significant amounts of nitrogen to your system, in a more sustainable way than chemical fertilisers. These crops will also help in improving your soils, and adding resilience to your system These are the things we need to move forward in sustainable farming.

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