Nitrate Case Study

On March 15th, 2021, five Zimmer & Peacock 60 cm nitrate rods were installed into several plots of land. During installation, holes were created, the nitrate rods were inserted into the ground and then the surrounding area was packed in with sand. Each plot of land that a nitrate rod was installed into either differs in the type of nitrogen based fertiliser used or the total N applied, Table 1. Total Nitrogen is the sum of nitrate (NO3), nitrite (NO2), organic nitrogen and ammonia (all expressed as N). The aim of this first case study is to determine the accuracy of in-soil nitrate sensors and how rates, timings and types of nitrogen alter the readings.

Nitrate Rod

Fertiliser Type

Total N

1

No Fertiliser

0

4

Solid Based Fertiliser*

180

5

Solid Based Fertiliser *

240

7

Liquid Based Fertiliser**

240

9

Liquid Based Fertiliser ** + Nitrification Inhibitor***

180


Table 1:
Shows what nitrogen based fertiliser and the total N application of said fertiliser has been applied to the plot of land where the respective nitrate rod has been installed.
*Solid Based Fertiliser– Compromises of 34.5% prilled ammonium nitrate.
**
Liquid Based Fertiliser – a 39% w/v liquid fertiliser containing 390Kg N/ 1000 litres.
***
Nitrification Inhibitor – is a unique urease and nitrification inhibitor for liquid fertiliser.

The nitrate rods were allowed to acclimatise for 2 weeks before the first fertiliser application on March 30th, 2021. Kieserite, a magnesium based fertiliser, was applied alongside the first nitrogen based fertiliser application across all plots at a rate of 100Kg/ ha. This is also when the nitrification inhibitor was applied to the plot containing nitrate rod 9.

Graph 1 and Graph 2 display the quantity of nitrate detected by each rod at the depths of 30cm and 60cm respectively, over a period of 3 months. From these graphs it can be observed that zero rainfall occurred during the period of March 26th and May 3rd. Both types of fertilisers used during this case study require the moisture in the soil to move the fertiliser into the soil profile to become available to the plant roots. From the graphs, it can also be noted that during this time of zero rainfall, that the moisture content of the soil dropped at both depths of 10 cm and 20 cm, more drastically at the shallower depth. Due to this it can be determined the fertiliser had not actually had time sufficient time to move into the soil profile, resulting in a decrease of nitrate in the soil at this time.

With the first heavy rainfall occurring on May 4th, there is then observed roughly three weeks of rainfall. With this first day of rainfall, a sudden increase in soil moisture at 10cm occurs. A similar trend is seen in the nitrate concentration at both depths, this is likely due to the increase in soil moisture allowing the distribution of fertiliser throughout the soil profile. It can be noted that the increase in nitrate concentration is higher at a depth of 60cm compared to that at 30cm. This could be due to three possibilities, or a combination of all three. The first is that with the increase in rainfall and soil moisture, the fertiliser is simply moving through the soil profile quickly. Secondly, the sand around the nitrate rod is providing a greater permeability compared to that of the surrounding soil, allowing the fertiliser to move as deep and unevenly distributed as it has. Lastly, a bacterial process in the soil is occurring closer to the surface resulting in the nitrate being converted into nitrogen and leaving the soil.

Three days after the start of the rainfall, on May 7th, the second application of nitrogen based fertiliser was applied.  There were no subsequent additions of either Kieserite or the nitrification inhibitor. This means that the second application of nitrogen based fertiliser occurred whilst the soil was still under the effect of the nitrogen from the first fertiliser application. The second application should have been delayed until the effects of the first application were fully benefited from. This could result in the crops being over fertilised and result in the fertiliser leaching, especially with the amount of rainfall recorded during this time, which is the opposite of what precision farming is trying to achieve. With the second addition of fertiliser, an obvious increase of nitrate concentration is not observed, due to the high levels of nitrate still in effect from the first application. A slight increase can be noted by the sensors at 30cm deep, in comparison, the sensors at 60cm deep appear to have plateaued out during this time. High moisture content in the soil along with the consistent rainfall during the second application provided a steady movement of the fertiliser throughout the soil profile. This enabled the fertiliser to reach the sensors at 30cm. However, the movement of fertiliser was faster during the first rainfall due to the sudden increase in soil moisture, which could have caused the results shown in Graph 1 and Graph 2, resulting in the reading of nitrate for the sensors at 60cm only.

 

The third application of fertiliser occurred on May 26th, again there were no subsequent additions of Kieserite or the nitrification inhibitor. Initially, a drop in nitrate occurs across the majority of sensors across both depths. This is due to a drop in soil moisture and an increase in soil temperature. On June 4th, more rainfall occurs resulting in the soil moisture increasing again. With this increase in soil moisture, the peak in nitrate occurs as the third fertiliser application can now move throughout the soil profile. There is then another section of low rainfall and when coupled with an increase in soil temperature, a rapid drop in soil moisture then occurs resulting in another drop in nitrate content. At this point, it can be observed that multiple sensors at the 30cm depth are starting to plateau out at 10ppm (parts per million).

 

When comparing nitrate rods 4 and 5, the solid based fertiliser application, it can be observed that there is a higher nitrate concentration at 30cm for nitrate rod 5. However, at 60cm both nitrate rods have comparable results. Nitrate rod 5 did receive a greater total N compared to that of nitrate rod 4, meaning that the nitrate rods are producing data that reflects the actual fertiliser application.

 

When comparing nitrate rods 7 and 9, the liquid based fertiliser application, it can be observed that there is a higher nitrate concentration at 30cm for nitrate rod 9. Similarly, to that of the solid based fertiliser, at 60cm both nitrate rods have comparable results. However, unlike the solid based fertiliser, it was nitrate rod 7 that received a greater total N. This shows that the use of the nitrification inhibitor greatly improved nitrate retention. This can further be proven when looking at the third fertiliser application area in Graph 1. During the first decrease in soil moisture, in the third fertiliser application section, all nitrate rods show a decrease in nitrate concentration except that of nitrate rod 9. Nitrate rod 9 has instead plateaued during this period, resulting in a constant nitrate concentration of around 27ppm. This is because the use of the nitrification inhibitor keeps the fertiliser where it is needed for longer, increasing nitrogen efficiency. This trend can also be observed in  Graph 2, across the second fertiliser application where the nitrate concentration stays around 35ppm with minimal fluctuations.

 

When comparing solid fertiliser versus liquid fertiliser at 30cm, it appears that the solid fertiliser with a total N of 240 is comparable to liquid fertiliser plus the usage of the nitrification inhibitor with a total N of 180. Whilst the solid fertiliser with total N of 180 is comparable to liquid fertiliser with a total N of 240. There is roughly a 10ppm difference between these two groups during the second fertiliser application period.

 

When comparing solid fertiliser versus liquid fertiliser at 60cm, during the first and third fertiliser application, all fertilisers are comparable. Although during the second application, with the increased soil moisture, solid fertilisers average 5ppm higher than liquid fertiliser.  However, the solid fertilisers are more sensitive to the lack of soil moisture compared to the liquid fertilisers. The nitrate levels drop considerably more during the dry spells, in the third application stage, for the solid fertilisers. This means that the liquid fertilisers are providing more stable results due to fewer fluctuations regarding soil moisture, especially with the use of the nitrification inhibitor. From this it can be concluded that the application of a liquid fertiliser in tandem with the use of a urease and nitrification inhibitor is the best option to use.

 

Nitrate rod 1 was placed in a plot where no nitrogen based fertiliser was applied, yet it is displaying similar trends to the plots that have received nitrogen based fertiliser. Although the results shown are not necessarily to the same magnitude or stabilisation as the others. This could be due to the fertiliser leaching into the soil of this plot and nitrate rod 1 is picking this up. Nitrate rod 1 is placed in a plot that is next to the plot containing Nitrate rod 4. Graph 3 and Graph 4 show colour plots displaying nitrate concentration, allowing potential leaching and cross-contamination to be observed. It can be presumed that this is the effect of leaching being displayed, due to the response time of the fertiliser applications being slower than that of the other rods and the concentration of nitrate is also lower. This is because the fertiliser simply has further to travel through the soil profile. During the second application phase there was an increase in rainfall. This is where the leaching can be observed, which is more prominent at the depth at 60cm.

 

 

For future case studies and tests, there are a few things that could be done differently or improved on. The next step would be to repeat this test but using the next generation of sensors, developed by Zimmer and Peacock. Another thing that could be improved on is the area surrounding the nitrate rod should not be filled in with sand, instead the same soil that from that plot should be used. The plots of land being monitored should be further apart so to reduce cross contamination between different fertiliser types and the amount of fertiliser used. This would allow for more accurate results, regarding nitrate rod 1 which received no additions of fertiliser, and a better baseline. Finally, the collected data would be used to determine when to apply further applications of fertiliser because the benefit of the nitrate application is only observed when the soil moisture increases. This would allow the crops to benefit completely from the first application of fertiliser and reduce the risk of over fertilisation. 

Graph 1: Shows the data sets of all 5 nitrate rods (ppm) for the sensors at a depth of 30 cm, alongside Rainfall, Soil Moisture (%) and Soil Temperature (°C). The start of a new coloured section represents a fertiliser application. 

Graph 2: Shows the data sets of all 5 nitrate rods (ppm) for the sensors at a depth of 60 cm, alongside Rainfall, Soil Moisture (%) and Soil Temperature (°C). The start of a new coloured section represents a fertiliser application. 

Graph 3: Shows the data sets of all 5 nitrate rods (ppm) for the sensors at a depth of 30 cm, alongside Soil Moisture (%) and Soil Temperature (°C) expressed as a colour map. 

Graph 4: Shows the data sets of all 5 nitrate rods (ppm) for the sensors at a depth of 60 cm, alongside Soil Moisture (%) and Soil Temperature (°C) expressed as a colour map.