Variable-Rate Fertilizer Application with Winter Wheat: Is It Worth a Try?

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See our experiment results for variable-rate fertilizer application at a partner farm
Variable-rate fertilizer application on wheat_Cover_OneSoil Blog
'Variable-rate' has become a legendary word recently… Indeed, variable-rate seeding and variable-rate application for fertilizer and plant protection products oftentimes bring palpable profits to farms. However, each case is unique, and two fields with similar characteristics may react differently to variable-rate application. This is exactly what happened during my recent field experiment with variable-rate fertilizer application on winter wheat.

I tested my theory in an experiment conducted in two similar fields located in Northern and Central Ukraine.

For your convenience, I'll divide the experiment into steps. You can read a detailed description of each step in the corresponding text block (click or tap "read more" button) or skip it and read on.
Usevalad Henin
GIS and agrochemistry specialist. He’s been developing precision farming tools since 2013. Co-founder of OneSoil.
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Philip Kondratenko_OneSoil Agronomist
Usevalad Henin
GIS and agrochemistry specialist. He’s been developing precision farming tools since 2013. Co-founder of OneSoil.
'Variable-rate' has become a legendary word recently… Indeed, variable-rate seeding and variable-rate application for fertilizer and plant protection products oftentimes bring palpable profits to farms. However, each case is unique, and two fields with similar characteristics may react differently to variable-rate application. This is exactly what happened during my recent field experiment with variable-rate fertilizer application on winter wheat.

I tested my theory in an experiment conducted in two similar fields located in Northern and Central Ukraine.

For your convenience, I'll divide the experiment into steps. You can read a detailed description of each step in the corresponding text block (click or tap "read more" button) or skip it and read on.

Working theory

I’ve now been carrying out field experiments for 10 years. My working theory behind variable-rate fertilizer application is that high-productivity zones require more fertilizer to be applied, while low-productivity zones require less. Nutrient removal in the high-productivity zones is higher because each yield unit needs a specific amount of nutrients to grow. As such, we should apply the most fertilizer to high-productivity zones to replenish nutrients. In low-productivity zones, the chemical and physical properties of the soil oftentimes don’t affect the yield. Nutrient removal is lower here, which is why these zones require less fertilizer.

During my years of experiments, the working theory saw different results. At times, variable-rate fertilizer application significantly increased yield. Other times, it showed an insignificant increase in yield and huge savings on resources, and still other times, it didn’t impact yield but saved resources. Let’s take a look at how the two winter wheat fields reacted to variable-rate fertilizer application.

What are productivity zones?
Read more
Productivity zones are areas of the field that produce the same amount of yield (high, medium, or low) from year to year. If an area in the field produces high yields several seasons in a row, we can consider it to be a high-productivity zone. Medium and low yield zones are medium- and low-productivity zones, respectively.

Why did I choose to apply fertilizers by productivity zone?

  • Conducting a soil nutrient analysis isn't enough to determine the optimal amount of fertilizer to apply. The analysis may give you a wrong idea of the soil's condition (read here to see why). Productivity zones give a clear picture of yield based on the satellite images processed with our algorithms. That's one reason I chose this method.

  • Moreover, if you choose to apply fertilizers by productivity zone, you also save money. Use a yield monitoring system on your equipment or the OneSoil web app to identify your productivity zones. That's reason #2.

Field descriptions

Field #1. We'll call it Jaime.
Locations and soils: Northern Ukraine, sod-podzolic soils. The crop was winter wheat. On September 15, 2020, diammophoska was applied at varying rates: 80, 135, and 190 kg/ha.

Field #2. Let's call this field Cersei.
Locations and soils: Central Ukraine, chernozem soils. On October 1, 2020, diammophoska was applied at varying rates: 80, 135, and 190 kg/ha.

Step 1. Identify productivity zones

Jaime

Cersei

How to identify your field’s productivity zones
(using the Jaime Field as an example)

Read more
To define the productivity zones in the Jaime Field, we need to first analyze NDVI (normalized difference vegetation index) data.

In the OneSoil web app, I can get this data for four successive years for this field. Then, I find images in the key growth stages when the correlation between yield and NDVI data is most obvious.
Combine the maps to find your productivity zones_OneSoil Blog
NDVI data from year to year shares a lot in common. This points to the following conclusions:

a) The productivity zones are stable.
b) There is a limiting factor in the field that has nothing to do with the crops or the weather. This could be the relief, soil acidity, or any other internal factor.

Combine the images, and you end up with productivity zones.
The resulting map shows the productivity zones_OneSoil Blog
Green areas indicate high-productivity zones, orange areas are medium-productivity zones, and red areas are the least productive zones in the field.

Step 2. Find the limiting factor

Jamie, relief

Jamie, soil brightness
Cersei, relief

Cersei, soil brightness
To avoid misinterpreting the experiment results, we have to find the limiting factor. To do that, we have to compare the productivity zones with the relief and soil brightness.

How to find the limiting factor
(based on the Jamie Field)

Read more
Combine the terrain with soil brightness_OneSoil Blog
Relief. The dark spots on the map are low-lying areas; the bright spots are higher grounds.


Combine the terrain with soil brightness_OneSoil Blog
Soil brightness. Blue spots indicate dark soil rich in humus.
Yellow spots show light soils with low humus content.
Combine the productivity zones and the soil brightness map. It turns out that the high-productivity zones are the dark-soil areas rich in humus, while the low-productivity zones are located in light soils.

The relief has no significant impact on the distribution of humus across the field. I hypothesized that the yield in this field is impacted by soil acidity, and the field scouting done in 2021 confirmed this hypothesis.

Step 3. Variable-rate fertilizer application

Jamie

Cersei
Now that you know all the productivity zones and the limiting factor, put together a prescription map for equipment. Check out our Guide to learn how to do#nbsp;that in the OneSoil app.

How to use the prescription map to interpret the results
(based on the Jamie field)

Read more
Variable-rate fertilizer application was done in Jamie using the prescription map below:
How to start variable-rate fertilizer application_OneSoil Blog
In the high-productivity zones, the fertilizer application rate was increased to 190 kg/ha, while it was reduced to 85 kg/ha in the low-productivity zones. We left control strips in the high and low-productivity zones to help us interpret the results.

We need control or test strips to confirm the working theory. Using the control strips with the standard rate of fertilizer application, we check whether variable-rate application affects the yields in this specific zone. For example, in the high-productivity zone, we apply 190 kg of fertilizer. In the same productivity zone, we keep a control strip, where we apply 130 kg of fertilizer. It's highly probable that we'll get objective results at the end of the season.

Find more about control strips and other things you should know to conduct the ideal field test in our blog post.

Now back to Jamie. Let's check the field's NDVI data during the season:
Vegetation maps_OneSoil Blog
The maps show that by mid-season and at the end of the season, NDVI is higher in high-productivity zones that had an application rate of 190 kg/ha. The zones with an application rate of 130 kg/ha don’t develop as actively.

July 25, 2021, was the big day for Jamie: the harvest season began. Combines with yield monitoring systems were used to gather the harvest. After that, we combined the yield map from the combines with the prescription map and got the following results:
Yield maps_OneSoil Blog
The yield map points to the strong impact of fertilizers in high-productivity zones.

Now it’s time to do some manual work. Look at the yield map to find replications, i.e., areas where the yield matches the application rate in the specific productivity zone. To get a reliable result, we have to compare the average yield for the replications in the high-, medium-, and low-productivity zones. Also, for the validity of the experiment, we need to get rid of all the noise in our data.

Find the replications and fill out the data in the chart (see Step 4).

Step 4. Analyze the results

Jamie

Cersei
Jaime

In high-productivity zones, the wheat yield increased by 0.53 t after the fertilizer application rate was increased. In low-productivity zones, no impact from the fertilizer application rate on the actual yield was noted.
Average yield, t/ha
Cersei

In the low- and medium-productivity zones, no impact from the fertilizer application rate on the yield was noted. In the high-productivity zones, yield increased but insignificantly (0.16 t).
Average yield, t/ha

Conclusion

Both fields saw an increase in yield in high-productivity zones. Although Jamie demonstrated better results, it’s not upsetting that Cersei didn’t react to the variable-rate application as impressively as Jamie did. Why? It means that the farmer can save tons of fertilizers in Cersei Field’s low-productivity zones.

The fact that the two fields with similar input data at the same farm reacted differently to the variable-rate fertilizer application is nothing extraordinary, but we need to conduct more detailed research to find out why. At the very least, soil acidity tests should be conducted.
The general conclusion for both fields is as follows: winter wheat reacts positively to variable-rate fertilizer application in high-productivity zones.
However, to see how well a specific field will react to variable-rate fertilizer application, you need to conduct field experiments in that field’s particular soil and climatic conditions.

If you want to discuss this article and share your own experience, please, join our Telegram or WhatsApp!
Usevalad Henin
I'd love to try to do some experiments, too!
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