How to Calculate Nitrogen Fertilizers for Variable-Rate Application

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.

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.

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).

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.

Done! Now upload the information to a USB drive, transfer it to the tractor computer, and head out to the field.

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.

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.