The risks of nitrate leaching into water, fluctuations in the global prices of granulated urea and the high level of energy consumption in the production of ammonium nitrate are all reasons that urge us to seek alternatives to the sole use of chemical mineral fertilisers as a source of nitrogen fertilisation for plants.
Enriching soil and crops by adding organic nitrogen provides production systems autonomy from mineral nitrogen fertilisers, while maintaining the level of productivity, both in terms of quantity and quality. In the long term, inputs of organic nitrogen will even have a significant effect on improving soil fertility.
Sources of organic nitrogen
The development of organic nitrogen is emerging an alternative to the use of mineral fertilisers rich in nitrogen. This organic nitrogen can come from two complementary sources:
- Introducing leguminous plants into the cropping system
- Applying organic matter which can take different forms such as green manure, animal manure, and organic waste products from different industries. For the latter, their sources and the standards that govern them must be checked.
There are two ways in which leguminous plants can be introduced into a crop rotation.
A leguminous plant can be planted as the main crop, so that it fully plays its role of “previous crop”.
It can also be introduced as an intermediate crop between two main crops so that it restores the fixed and accumulated nitrogen for the next crop.
Conversion of organic nitrogen in the nitrogen cycle
Whether organic nitrogen is restored to the soil in the form of plant debris or by adding organic matter, it can only be used by plants after mineralisation into nitric nitrogen or nitrates.
Only these elements can be assimilated directly by plants.
Nitrogen in organic form must be converted into ammonia and then into nitric nitrogen by soil bacteria to be used subsequently by plant roots. This mineralisation of organic nitrogen into mineral nitrogen is directly linked to the presence of a large number of bacteria.
Note that these are the same bacteria that will convert the organic nitrogen naturally present in the soil as humus into nitrate. As inappropriate tilling causes the mineralisation of this humus, it is one of the causes for the progressive decline in soil fertility.
In the nitrogen cycle, we must also take into account the populations of filamentous fungi responsible for converting organic matter into humus, a source of fertility.
How to manage the organic nitrogen stock
The management of organic nitrogen in the soil must ensure several balances between:
- The bacteria and filamentous fungi present in the soil;
- The stock of carbon-bonded organic nitrogen in the form of humus and the inputs of organic matter intended for mineralisation;
- Tillage and mineralisation which can lead to the leaching of nitrates into water.
Overall, actions such as maintaining a permanent soil cover with nitrate-fixing intermediate crops, such as leguminous plants, between the main crops, restoring plant residues, adding organic matter and managing soil pH effectively will allow:
- The distribution of carbon-bonded organic nitrogen between storage and mineralisation
- The maintenance of microbial populations between fungi and bacteria
A balanced nitrogen cycle is a source of long-term soil fertility and will ensure the productivity and proper nutrition of crops in agriculture.