Nitrogen is the most important plant nutrient, and more than half of the world's agricultural crops depend on nitrogen in the form of fertiliser. The fertiliser is not in itself an environmental problem, but the problem is that the efficiency of the utilisation of the nitrogen is often too low. Nitrogen efficiency is the ratio between the amount of nitrogen added to the field and the amount removed again at harvest. This ultimately leads to higher costs and greater loss of nitrogen to the environment.

“As it seems right now, only about a third of the added nitrogen worldwide is removed by harvesting grain crops. And that is a big problem for the waterenvironment in terms of leaching of nitrogen,” explains PhD student Vita Antoniuk from the Department of Agroecology.

Assesses nutrient content in photographs

In an attempt to find new methods for assessing the nitrogen content of winter wheat in order to ensure less overuse of nitrogen in fields, the researchers have been working on a long-term experiment in China. Here, they have investigated three different methods to find the most effective and reliable way to assess nitrogen deficiency in crops, so farmers will know when to fertilize and equally important, when not to:

1. Drone photography with a multispectral camera
2. Nitrogen content of the leaves (SPAD) with chlorophyll meter
3. Smartphone pictures 

To assess the nitrogen content, the researchers used so-called vegetation indices, two of which were selected specifically, as they work well with photographs from drones and smartphones, respectively:

• Green Normalized Difference Vegetation Index (GNDVI)
• Visible Atmospherically Resistant Index (VARI)

“When plants do not get enough nitrogen, it is clearly seen in their structure and in their chlorophyll content. Chlorophyll is a substance that absorbs blue and red light, giving plants their green color. In other words, one can see a difference in the color of the plants. It is a difference that can be seen with the naked eye. When using vegetation indices to evaluate the difference, it becomes even clearer. However, we found that the method is not performing very vell if the crop is under severe lack of other important nutrients such as phosphorus, meaning that other existing conditions should be taken into account,” says Vita Antoniuk.

The farmer's smartphone can become an important tool

The experiment has so far only been conducted in China, where the fields are relatively small and few, making it easier to use a smartphone to assess whether there is a need for additional nitrogen supply. According to the researchers, the smartphone solution may not be the way forward if you have large fields, because there can be large differences in the nitrogen content from one corner to the other, and it cannot be captured on a single smartphone image. 

“In thT case, you will then be able to use drones or satellite. However, they are not failsafe solutions, for example there might be clouds that can prevent the satellite from photographing and the resolution may simply be too low to use. Drones can be used almost as efficiently as smartphones, the only difference being that in every drone images pixel can be mix of crop, soil and maybe weeds that will interfere with the measurement a bit, which can be avoided by using smartphones,” explains Vita Antoniuk.

Maybe an app in the future

So far, the technology has only been tested in the field trial in China, but the researchers are confident, and even if the results cannot be passed directly to other fields, crops or climatic conditions, there is hope for the technology anyway.

“My hope is that an app will be developed, which will allow farmers from around the world to use their smartphone to assess the nitrogen content in their crops. But there is still a long way to go before it will be possible," Vita Antoniuk says.

Behind the research