Does high vegetation mean high nitrogen content? OneSoil field experiment results
We develop our applications along with farmers using the results of our scientific experiments. During the 2019 season we have tested various methods of nitrogen application in the Ukrainian fields. In this very article our precision farming specialist Usevalad Henin tells how we did it, and what conclusions we came to.
On the OneSoil web-platform there is an application for the calculation of differentiated application of nitrogen fertilizers. Users often ask why we do not determine the nitrogen application rates by ourselves. We don’t do it because nitrogen is a mobile soil element and in order to determine the rates correctly the farmer has to analyse the field himself. To help do it appropriately we conduct field experiments and share the results with you. This is how the article “How we determine variable rate nitrogen” appeared, which we published in our blog last year.
During the field season of 2018–2019 we also conducted several experiments for the differentiated application of nitrogen fertilizers. Ivan Gumenyuk, head of innovations department of a household located in the central part of the Ukraine, responded to our call. Thanks to Ivan’s support and enthusiasm we managed to organize and conduct a well-designed scientific experiment on two fields with winter wheat. The results of this experiment were unexpected, even for us.
Now let’s start from the beginning: let’s talk about nitrogen application methods, tell about the course of the experiment and data analysis principles, and study the results related to the first and the second field. Finally, let’s decide which nitrogen application method is the most effective.
On nitrogen application methods
There are three popular nitrogen fertilizer application methods.
The first method: heavy nitrogen application in the low vegetation zone
In this case we assume that namely shortage of nitrogen limits the development of plants and therefore the vegetation. By increasing the nitrogen rate we can increase the NDVI vegetation index, and thus the crop yield.
The nitrogen fertilizer rate determination scheme is as follows:
- in the zones with low vegetation a large dose is applied
- in the zones with average vegetation a medium dose is applied
- in the zones with high vegetation a small dose is applied.
The second method: heavy nitrogen application in the high vegetation zone
This method assumes that if you apply a high rate of fertilizers in the zone with high vegetation, than the yield will be even larger. And if we introduce a high rate of fertilizers in the zone with low vegetation, nothing will happen — the crop yield will not increase.
That’s why the following scheme is used:
- in the zones with low vegetation a small dose is applied
- in the zones with average vegetation a medium dose is applied
- in the zones with high vegetation a large dose is applied.
The third one: heavy nitrogen application in the average vegetation zone
According to this method, in the zones with high vegetation index NDVI the increase of nitrogen application rate might lead to crop lodging. It is assumed that in the zones with a low vegetation index it is not nitrogen which limits the yield, but other factors. But in the average vegetation zone the increase of nitrogen fertilizer doses might lead to a significant growth in crop yield.
Therefore the scheme is as follows:
- in the zone with low vegetation a small dose is applied
- in the zone with average vegetation a large dose is applied
- in the zone with high vegetation a small dose is applied.
How was the experiment conducted and how we analyzed the data
We tested two fields located in the central part of the Ukraine, in the Kirovograd region. The soils are black earth, the crop was winter wheat. Non-uniform terrain: there is a significant difference in elevation everywhere, which causes the difference in crop yield.
On field 1 we differentiated only the first nitrogen nutrition and on field 2 we differentiated the first and the second nutritions. The application of fertilizers was performed on February 22 (here’s the full timeline of field operations). For this purpose we used liquid nitrogen fertilizer “КАС 23” (23 kilograms of nitrogen per 100 litres), therefore in the following we will always use litres. The fields were reaped on July 10 using a combine harvester with an on-board computer for the monitoring of crop yield.
The crop yield data from the combine harvester often have various interferences and discrepancies which arise during technical operations. To eliminate their influence on the experiment results, we identified homogeneous regions on the fields which were covered by 2 to 4 runs of the combine harvester, and we calculated average values for these regions. In these regions there were 8 to 16 zones (repetitions) with similar rates in each, and we also took this data into account when analyzing.
Field 1
Field area is 60 hectares, the difference in elevation is from 133 to 152 meters. The field was divided in three zones with high, average and low vegetation which were equal in area. We determined the zones on the basis of the most recent image at the date of nitrogen fertilizer application.
To test differentiated nitrogen fertilizer application methods, the field was divided into 5 parts. We had two control zones and one for each of the three methods of nitrogen application. The two upper parts of the field were chosen as control zones because their vegetation was homogeneous. The remaining zones were determined so that there were parts with high, average and low vegetation in each of them.
Results for field 1
Control zone
The control part of the field with an application rate of 70 litres has lower NDVI index values than the control part with a rate of 250 litres.
The crop yield analysis showed that in the zone with an application rate of 250 litres the average yield amounted to 82 centner/ha, and in the zone with a rate of 70 litres it was 77 centner/ha. The application of additional 180 litres of nitrogen fertilizers provided the increase of 5 centner/ha of the reaped amount of corn. This means that from similar starting conditions the increase in crop yield amounted from 2 to 15 centner/ha, thus the parts with the highest vegetation experienced maximum increase in crop yield.
Low vegetation zone
In this zone we used three nitrogen fertilizer application rates: 96, 132 and 270 l/ha. The zone was located on a slope, and we think that the terrain limited the vegetation.
The NDVI index did not grow with the increase of nitrogen application rate, and this was also confirmed by the crop yield rates:
- in the zone with an application rate of 270 l/ha the crop yield amounted to 78,1 centner/ha
- in the zone with an application rate of 132 l/ha the crop yield amounted to 76,6 centner/ha
- in the zone with an application rate of 96 l/ha the crop yield amounted to 78,2 centner/ha.
This means that with the increase of fertilizer application rates, the crop yield did not increase. This fact shows that it is not the nitrogen content in the soil which limits the vegetation in the zone with a low index, but other factors. Which factors these are can be determined only using a detailed morphometric and agrochemical field study.
Average vegetation zone
For the zones with average vegetation two nitrogen application rates were used: 168 and 240 l/ha:
- in the zone with an application rate of 168 litres the crop yield amounted to 77,8 centner/ha
- in the zone with an application rate of 240 litres the crop yield amounted to 79,4 centner/ha.
The analysis of separate highlighted field zones (repetitions) showed that the crop yield increased slightly, by 1 to 1,8 centner/ha, with the increase of the fertilizer application rate. In other words, there is a steady trend towards the increase of crop yield.
High vegetation zone
In the high vegetation zone we used three nitrogen fertilizer application rates: 110, 132 and 270 l/ha:
- in the zone with an application rate of 110 l/ha the crop yield amounted to 84,5 centner/ha
- in the zone with an application rate of 132 l/ha the crop yield amounted to 83,9 centner/ha
- in the zone with an application rate of 270 l/ha the crop yield amounted to 87,8 centner/ha.
The analysis of separate highlighted field zones (repetitions) showed that there was no significant increase in crop yield with the change of nitrogen application rate from 110 to 132 l/ha. Thereby with the increase of the rate to 270 l/ha the crop yield has grown. This means that the increase of crop yield can be seen not only when analysing the whole zone but also separate regions thereof (repetitions), and it lies in the range from 2 to 7 centner/ha.
As we see, the most effective method of nitrogen application was the second one when we apply the highest nitrogen rate in the zone with high vegetation. In the zone with low vegetation the crop yield is limited by factors related to the terrain: moisture distribution, granulometric composition of soil, humus content. That’s why the increase of nitrogen fertilizer dose did not have a desired effect. And in the zone with average vegetation the increase of crop yield was rather small, that is why it is doubtful that the increase of the dose of fertilizers is reasonable in this case.
Field 2
Field 2 is located four kilometres away from field 1. The soil and the terrain are similar, the area is 87 ha, the difference in elevation is from 120 to 151 meters. The first nitrogen nutrition was performed in the early spring, the second one was made at the phenological stage 31 BBCH.
As in the previous experiment the field was divided into five zones: two control zones, and three zones of equal area for testing the methods of nitrogen application.
Before the second application of nitrogen fertilizer we divided each of the three experimental zones into two parts. We applied 110 l/ha of nitrogen fertilizers in one of these parts, and 190 l/ha in the other part. The goal was to compare how the change of fertilizer rates influences the zones with equal initial rates. Thus we tested only zones with high and low vegetation, because we didn’t manage to determine the zone with average vegetation due to huge variability of NDVI index rates in some of the parts.
Results for field 2
Control zone
On this field the whole control zone was not homogeneous: within its borders there were parts with high, average and low vegetation. Nitrogen application rates during the first nutrition were 300 and 60 l/ha. For parts with low and average vegetation we didn’t register any increase in crop yield with increase of doses during the first nutrition. On the parts with high vegetation the crop yield increased by 3,1 centner/ha.
For the second nitrogen nutrition we divided our control zone into two parts with application rates of 0 and 150 l/ha. On the parts with low vegetation there was no increase in crop yield. On the parts with average vegetation the increase amounted to 1 to 3 centner/ha, on the parts with high vegetation the increase was from 3 to 6 centner/ha.
The analysis of separate highlighted field zones (repetitions) showed a similar result: for the absolute majority of zones with average and low vegetation the increase of the fertilizer rate did not result in the growth in crop yield. In the zones with a high vegetation index in case of an additional application of 240 litres of nitrogen the increase in crop yield was from 1,5 to 6 centner/ha.
Low vegetation zones
We applied 3 rates: 120, 165 and 300 l/ha during the first nitrogen nutrition, 110 and 190 l/ha during the second nitrogen nutrition.
According to this data there was no significant growth in crop yield with the increase of fertilizer rates. As for field 1, we think that the limiting factor was the terrain, because the low vegetation zone was located on a slope.
Average vegetation zone
We used three different rates: 120, 165 and 300 l/ha. During the second nitrogen nutrition we did not differentiate the fertilizer dose for parts with average vegetation, because we did not see any sense in this action.
One can notice that with the increase of application rate up to 300 litres the crop yield increased by 3 centner/ha.
The analysis of separate highlighted field zones (repetitions) also showed a distinct trend towards the growth in crop yield with increase of nitrogen application rate which was up to 5 centner/ha for some of the parts.
High vegetation zone
We’ve reached the most interesting part of our study. In these zones the crop yield increased with the increase of nitrogen fertilizer rates. The results are provided in the table.
The analysis of separate highlighted field zones (repetitions) showed that in some parts with the increase of rates the crop yield grew significantly by maximum 8 centner/ha.
Thank you all for reading the article to this point! We know it hasn’t been easy. Let’s proceed to the main point, namely conclusions.
What did the experiment show?
It can be concluded that the most effective method of differentiated application of nitrogen fertilizers was the second one described above (“heavy nitrogen application in the high vegetation zone”). But you should understand that this result is valid for black earth soils in a moderate climate zone. On soils with different terrain and in other climate zones, the result of the experiment could be different.
I hope that experiments in differentiated nitrogen application will continue around the world and we will come to a consensus in about five years. And at the moment we recommend the following: before the differentiated application of nitrogen you should analyze your field and determine limiting factors yourself.
Timeline
