How to calculate nitrogen fertilizers for VRA_Cover_Blog OneSoil

How to Calculate Nitrogen Fertilizers for Variable-Rate Application

Reading time — 7 minutes
Discover why plants need nitrogen, why it should be applied by vegetation index zones, and how to create a prescription map in the OneSoil web app.
Philip Kondratenko_OneSoil Agronomist
Usevalad Henin
Usevalad is an expert in GIS and agricultural chemistry. He has been developing precision farming tools since 2013. He is also the co-founder of OneSoil.
Nitrogen is essential for plants' growth and development and helps increase yield several-fold. It's a mobile nutrient. That means its quantity in the soil constantly changes in different field areas. Farmers use variable-rate application to fight nitrogen excess or deficiency, increase yield, or save on fertilizer used. Variable-rate application means applying different rates of fertilizer in each part of the field.

Read on to learn how to calculate nitrogen rates and create a prescription map for variable-rate application.

Or, if you're not much into reading, watch our
webinar on how to use variable-rate technology with the OneSoil app.
Nitrogen is essential for plants' growth and development and helps increase yield several-fold. It's a mobile nutrient. That means its quantity in the soil constantly changes in different field areas. Farmers use variable-rate application to fight nitrogen excess or deficiency, increase yield, or save on fertilizer used. Variable-rate application means applying different rates of fertilizer in each part of the field.

Read on to learn how to calculate nitrogen rates and create a prescription map for variable-rate application.

Or, if you're not much into reading, watch our webinar on how to use variable-rate technology with the OneSoil app.
Usevalad Henin
Usevalad is an expert in GIS and agricultural chemistry. He has been developing precision farming tools since 2013. He is also the co-founder of OneSoil.
Get all new articles straight to your inbox!

Why plants need nitrogen

Nitrogen is one of the most important nutrients for plants' growth and development. It plays an important role in photosynthesis, cell formation, and providing nutrition to those cells. Nitrogen is present in humus, the organic component of soil, but most plants can't directly absorb it. Nitrogen becomes available for plants when humus is decomposed in a process called mineralization.

Nitrogen fertilizers are applied in the soil to control crops' development. While nitrogen deficiency can lead to a drop in yield, excess nitrogen accumulates in plants and washes off from the soil to groundwaters, which is harmful to people and the environment.

Types of nitrogen fertilizers

There are two types of fertilizers: organic and mineral. An example of organic fertilizers is animal manure, which has a small nitrogen quantity 0.5–2.5%.

The level of nitrogen content in mineral fertilizers is much higher. For example, there is 34-35% nitrogen in ammonium nitrate, 46% in urea, and 20% in ammonium sulfate. Variable-rate technology is most often used when applying mineral fertilizers. Because of the high concentration of nitrogen, these fertilizers can acidify the soil.

When to apply nitrogen fertilizers

Nitrogen is applied either prior to seeding or immediately after. For some crops, like rapeseed or corn, fertilizer is applied 1 to 4 more times after the primary fertilization.

Fertilizers can be applied in the soil or simply spread over the surface. The application method depends on the type of fertilizer used (granules, liquid, or powder).

How to conduct variable-rate nitrogen application

When starting variable-rate application, it's important to define zones where the fertilizer needed is practically the same and calculate rates for each zone.

At OneSoil, we developed a free tool for variable-rate fertilizer application, and we suggest applying nitrogen by vegetation index zones. Vegetation index (NDVI) zones are built based on satellite images. They reflect plants' condition at the moment the image is captured. Vegetation index zones are the easiest and fastest way to understand how much nitrogen plants need right before applying the fertilizer.
Nitrogen Calculator in the OneSoil web app_OneSoil Blog
Here's what the variable-rate nitrogen tool looks like in the OneSoil web app
How to use the OneSoil variable-rate nitrogen tool
1
Register in the OneSoil web app. It's absolutely free to do so.
2
Open the 'Fertilizers' section and click the 'Nitrogen' tab. The app will build three zones in the field with high, moderate, and low vegetation indices using the latest satellite image.
3
Enter a nitrogen rate for each of these zones. In just a few seconds, the app will automatically create a prescription file for the equipment's onboard computer.
4
Download the prescription file in your preferred format.
Done! Now upload the information to a USB drive, transfer it to the tractor computer, and head out to the field.

How to calculate nitrogen rates for variable-rate application

The rule of thumb. When calculating variable fertilizer rates, it's important to look at how much potential different zones have and to know the general fertilizer recommendations for certain crops. What does that mean? Let's say you're applying fertilizers in a rapeseed field. Take the farm's standard nitrogen rate for rapeseed and build a vegetation zones map in our app. For some zones, increase the farm's standard rate by 25%–30%, and decrease it for other zones by 25%–30%.

Field experiment. How different vegetation index zones react to fertilizers depends on the soil type and climate conditions. That's why a field experiment is a perfect way to find out in which zones you should increase the rate and in which zones you should decrease it.

Field scouting. Well, this method isn't perfect. Go to the field, measure soil moisture and nitrogen content in the soil and leaves in different vegetation zones. Use this data to choose the best fertilization strategy.

Apply more nitrogen in low vegetation index zones if these areas have a high moisture level and low nitrogen content in the soil and leaves. Apply less nitrogen if the moisture level is low and the nitrogen content is high. In the latter case, nitrogen doesn't limit the yield. The problem is the lack of moisture.

You should also be careful with high vegetation index zones. If you decide to increase the rate in these zones, it's important to check the plants' condition and take care of plant growth regulators.

Do you have any experiment results to back this up?

Sure! We've been conducting field experiments on variable-rate fertilizer application since 2011. Here are a few experiments in which we tested different methods of calculating the nitrogen rate and proved that variable-rate nitrogen application is efficient.
Build a prescription map for nitrogen application!
More Articles
Have you ever tried variable-rate nitrogen application?
Usevalad Henin
People, fields and technology
Get all new articles straight to your inbox! You can read them even in the field.