As with any living organism, plants need food to stay healthy and grow. Nitrogen plays a key part in this process for plants, as one of the basic elements of plant nutrition.
Nitrogen fertilisers are used to enrich soil, in order to obtain high yields from edible crops and to help ornamental plants thrive. To determine the best product for your garden, you need to consider the ratio of nitrogen to other ingredients, the method and time of application, and the needs of the particular plant.
This article explores the role of nitrogen in plant health, and how to spot signs of nitrogen deficiency.
Signs of nitrogen deficiency in plants
Symptoms of nitrogen deficiency in plants include:
- The plant turns lighter in colour compared to other plants;
- The middle and upper part of the plant produces larger leaves faster;
- The lower leaves of the plant slowly lose their colour, turning yellow or white before falling off;
- The stems of smaller leaves start turning purple;
- Vertical purple stripes start appearing on the plant’s stem;
- The plant is shorter, stems and leaves thin, less and smaller leaves form;
- The plant’s middle and upper leaves also start turning yellow;
- New leaves look green, but are nowhere near as green as they should be with normal nitrogen levels.
Even with high levels of nitrogen in the soil, some factors influence the availability and uptake of nitrogen by plants. These include cold weather, compacted and cold soil, poor microbiological activity, and lack of moisture.
Nitrogen fixation as a part of the nitrogen cycle
The air that all living things breathe is composed of mostly nitrogen, but air nitrogen is not available to humans, animals, or plants. Microorganisms come to the rescue, converting airborne nitrogen into a form that is available to all plants. After the plant dies, all the nitrogen-containing substances in the plant debris are recycled by the microorganisms. Protein and amino acids are converted by the microorganisms into ammonia and ammonium compounds, which, in turn, are converted into nitrite and nitrate, after which the following plants use up the nitrates via their root system.
Unspent nitrates are used in a process called denitrification, during which nitrate is converted into nitrogen gas and returned to the atmosphere. During thunderstorms and at high temperatures, nitrogen and oxygen combine to form nitric oxide, which in turn combines with air moisture in the form of nitric acid solution and falls to the soil surface during rainfall.
In other words, plants cannot absorb nitrogen from the air, but microbes can do it for them. Nodules, for example, convert nitrogen into a form that plants can assimilate: this is called nitrogen fixation. Legume families play a major role in this process. In symbiosis with nodule bacteria, having satisfied their need for nitrogen, legumes enrich the soil with it.
All nitrogen fixers are divided into three groups:
- Symbiotic nitrogen fixers. These interact with cultivated plants, enter into symbiosis with them, and form additional organs on the plant roots, e.g. in legumes this is formation of nodules.
- Associative nitrogen fixers. These microorganisms are either on the surface or inside the plant, or live on the surface of the root system.
- Non-symbiotic or free-living nitrogen fixers. These microorganisms do not interact with higher plants, they live freely in the soil and convert nitrogen to ammonia.
Tracking nitrogen in plants
Given the importance of nitrogen in plant health, it’s no wonder there are plenty of ways to track nitrogen levels in soil and crops. These include ground sensors, drones, and satellite imagery. For example, a software called EOSDA Crop Monitoring is a precision farming platform that uses satellite imagery analytics with the help of AI to enable growers to manage fields effectively and remotely. One of the possibilities the tool offers is tracking of chlorophyll content in crops, which is a great indicator of nitrogen level.
Apart from being important for photosynthesis, chlorophyll is also responsible for “painting” the plants green. Therefore, healthy vegetation that contains enough chlorophyll is bright green. On the contrary, plants that lack chlorophyll would turn pale green and yellow, signalling crop health deterioration, possibly due to nitrogen deficiency.