How to determine variable rate nitrogen: Theory and Practice
Nowadays, precision farming technologies allow us to determine correct fertilizer rates for a field. Let’s dig deep (no pun intended) into the methods of differentiated application of nitrogen fertilizers. Also, we will see how the OneSoil Nitrogen app works and offer recommendations for a specific field.
The two most popular methods
Nitrogen is one of the main chemical elements necessary for the growth and development of plants. It is essential for nutrition, photosynthesis, and forming of new cells. Nitrogen is contained in the atmosphere and in the organic matter of soil (humus), but most plants cannot use it directly. In nature, nitrogen turns into a form accessible to plants during the decomposition (mineralization) of humus.
To control the growth and development of crops, farmers are using different nitrogen fertilizers. But a lack of fertilizer reduces the yield, and its excess accumulates in plants, fruits or is being washed away from soil into groundwater. It is dangerous for both humans and the environment. However, it is difficult to determine the rate of nitrogen fertilizers for a particular section of a field for several reasons.
First, nitrogen content changes significantly with time. Usually, farmers apply nitrogen fertilizers either before or after sowing. Some plants, like rapeseed, maize, and cereal crops, require from one to four additional feedings. Second, various types of plants need different amounts of nitrogen. For instance, rapeseed is one of the most demanding crops, and flax is one of the least.
Differentiated application of nitrogen helps to solve this problem. In precision agriculture, two main methods are used to determine the nitrogen variable rates.
1 Modeling the nitrogen cycle
To apply this method, you need to know:
– The spatial distribution of humus
To determine this indicator, you need to conduct an agrochemical survey of soil. Then you should send selected samples to the laboratory for analysis and the humus distribution is mapped afterward.
In the experience of OneSoil, there were fields where soil organic matter varied from 0.7% to 5% at the area of 120 hectares. This means the amount of nitrogen available for crops varied greatly within different zones.
– Granular composition of the soil
To know the relative content of particles of varied sizes, you need to use soil survey maps on a scale of 1:10 000 and below.
– Soil temperature and moisture
These indicators may vary greatly within the boundaries of a field, so it is necessary to consider the relief of the land and the data from sensors.
– Nitrogen uptake by plants
You can estimate this indicator by the vegetation index for a field area. It is calculated using satellite images.
Using this information and special paid software, a farmer can map out recommendations for nitrogen variable rates for each field zone.
This is an accurate method that does not require the physical measurement of nitrogen available in plants in the soil. Hence, the farmer can decide on the necessary fertilizer dose right from his office. The main disadvantage is that measuring the distribution of humus, as well as soil temperature and moisture, is an expensive and lengthy procedure.
2 Modelling by crop performance
There are two sources of information that help to assess the actual state of plants:
– Satellite imagery
To get an image of their field from a satellite, farmers use paid software or contact specialized companies. On the basis of these pictures, the vegetation index (NDVI) is calculated for different field zones, and then a special map or a file with the task for agricultural equipment is created.
This method allows a farmer to assess the general state of his field. The main drawback is the dependence on cloudy weather. The photographs are relevant for two weeks, however, sometimes it is impossible to measure vegetation and build zones because of the cloudy sky throughout this period.
– Sensors on tractors or mineral fertilizer distributors
This method allows you to obtain up-to-date data for each part of the field. It helps to determine the necessary fertilizer rate accurately. Its main drawback is the high price of such sensors.
The OneSoil solution
Our team developed a free web app OneSoil Nitrogen that automatically calculates nitrogen variable rates. Using a satellite image of the field, the app determines three relative zones with high, average and low vegetation indices. On the basis of this information, OneSoil Nitrogen creates a file with a task for the onboard computer of agricultural equipment. A farmer should only upload the information to the USB drive, transfer it to the tractor computer and go to the field.
When developing this application, we used the results of our field experiments conducted since 2015. In 2018, the OneSoil team studied more than 1,000 hectares of fields with soil in which the humus content did not exceed 5%. Generally, we made tests on cereal crops during the second and third nitrogen nutrition cycle.
As a result, we made the following conclusions:
1 High vegetation index — fewer fertilizers
Often a high vegetation index indicates the high content of nitrogen in the soil. To avoid lodging and lowering the release of nitrogen into the environment, we recommend reducing the fertilizer rate to 10−30% of the average rate.
2 Average vegetation index — more fertilizers
A zone with an average vegetative index can have a heavy yield, but often there is not enough nitrogen for plants in the soil. In such cases, we recommend increasing the dose of fertilizer up to 20−25% of the average rate.
3 Low vegetation index — look for the cause
The most difficult and interesting zones are those with a low vegetation index. It is very difficult to distinguish a lack of nitrogen from a lack of water or sulfur. Therefore, we cannot say whether an additional application of nitrogen fertilizers will help to improve the condition of such a site.
In our practice, there were cases when an increased fertilizer rate helped to a to produce a good yield. But there were also fields where even a twofold increase of the dosage has no effect on the situation. Further research showed that plants lacked water in such areas.
To determine the cause of low vegetation index, we recommend using our free OneSoil Scouting app.
The case study
In 2017, the OneSoil team conducted an experiment during the second nitrogen nutrition of winter wheat (BBCH 31 stage, exit in a tube). The average field rate was determined by the farmer and amounted to 100 kg/he of nitrogen fertilizers. For the experiment, the field was divided into three equal zones with high, average and low vegetation indexes. Zones were allocated using a satellite image from April 21st.
Each of the field areas was divided into three parts. For each one, the rate of 50, 100 and 150 kilograms of nitrogen in the active ingredient was prescribed. Fertilizers were applied by the OneSoil map on April 22nd.
Then we observed the change of the state of crops in each zone. To assess the dynamics of the vegetation index, we used satellite images from May 14th and control measurements of yields on June 26th.
After our observations, the following conclusions were made:
Zones with a high vegetation index didn’t react to high nitrogen fertilizer rates. The index did not change for subzones with high (150 kg) and low (50 kg) doses. This indicates that high and medium fertilizer rates were ineffective — nitrogen was not used by plants. In addition, we recorded a lodging of wheat in these areas during the harvest.
Zones with an average vegetative index reacted to a high nitrogen fertilizer rates with an increase of the index. Later we found that the areas with an average vegetative index and a high dose of nitrogen fertilizers yielded better than those with medium and low doses of nutrition.
Only 45% of zones with a low vegetation index reacted to a higher nitrogen fertilizer rates with an increase of the index. Mainly, these were areas located in the lowlands. Crops there were flooded or were in poor condition after wintering. For the areas that were located on the hills, we didn’t record an increase in the vegetative index after changing the fertilizer rate. The amount of harvested grain only confirmed this observation.
We measured soil moisture by our sensors located on the hills, and they showed that crops were experiencing moisture deficiency. Thus, they did not react to higher nitrogen rates.
Conclusion?
The differentiated application of nitrogen fertilizers allows the farmer to save money and increase the yield. The fastest method of calculating the variable nitrogen rates is based on satellite imagery. Special programs allocate zones with different vegetation indices on the field. This allows the farmer to set a dosage of fertilizer for each particular area.
Still, it is worth taking into account several recommendations:
- If the vegetation index is high, the fertilizer dose should be reduced to 10–30% of the average rate.
- If the vegetation index is average, the fertilizer dose should be increased up to 20–25% of the average rate.
- If the vegetation index is low, you need to determine the cause of it first.
To calculate nitrogen variable rates, use our OneSoil Nitrogen web application. All you need is a computer and access to the internet. The program is free — for everyone, forever.
