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0kg N applied, making use of the soil N legacy produced a 3t wheat crop [See how]



Nitrogen decisions can be hard to make on the fly...


Having data and evidence to decide how much to spread, and where to spread, can help take the emotion and guess work out of these decisions. It's even more critical to make accurate and informed decisions in a high input cost environment, like we're seeing in 2022.


Readily accessible precision ag data layers such as yield maps and satellite imagery, paired with deep N (nitrogen) soil tests can form a solid strategy for matching nitrogen to the potential of the crop.


Let’s look at the ‘Ruin’ Paddock at Breezy Hill as an example:


Location: Booleroo Centre (Wepowie), SA

Annual Rainfall: 300mm

Rotation: Canola, Wheat, Barley


What was the ‘N’ strategy in 2021?

In 2021, there had been a late break followed by solid rain in June and July. The field was in a wheat rotation, coming out of a 'vetchola' in 2020 (vetch/canola mix). It was difficult to know whether there would be fixed nitrogen in the soil from the previous season, or whether there would be a net deficit, given the combination of the ‘N hungry and N fixing’ crops.


The questions?


1. How much urea was required to meet the yield potential of the crop?


2. Was a variable rate map an appropriate option given the field has significant yield variability?

Step 1 – Determine Representative Zones to Test



Figure 1The landscape change map on the left, and a simple Google Earth capture on the right, and the two chosen soil core sites placed in representative zones.


The soil types in this field are largely driven by topography. There are three distinct ridges that are heavily eroded, this is confirmed by a landscape change map layer (created from the elevation map).


A simple ‘Google Earth’ image also gave clarity to the two distinct soil zones. It was decided that two Deep N tests should provide enough information.

Step 2 - Identify the Yield Potential for Each Zone



The yield potential in the field was 5.2t/ha, as calculated through the Angus and Sandras model (updated French and Scultz). Realistically, we know that we don't commonly achieve yields this high due to various environmental limitations.


When considering seasonal rainfall to date, the long term season forecast, and the historical yield in each zone, it was decided that the yield potential was not the same at core 1 as it was at core 2. The core 2 site historically yielded higher, so this was given a yield potential of 3.5 t/ha. Core 1 was matched to a yield potential of 2.5 t/ha, due to hostile soil conditions.


Core 1 Yield Potential: 3.5T/ha


Core 2 Yield Potential: 2.5T/ha



Step 3 - Calculate Units of N already available in the soil


The Deep N results provided the information for this part of the equation. It's important to consider the available nitrate in the upper and lower horizons and pair this with crop rooting depth.


Core 1: 0-30cm = 7 mg N/Kg

30-60cm = 12 mg N/Kg


Core 2: 0-30cm = 9 mg N/Kg

30-60cm = 9 mg N/Kg



Step 4 - Estimate mineralisation for the rate of the season


Factors that fed into the mineralisation calculation, included; the organic N within the soil, soil organic carbon, soil temperature, previous crop rotations and moisture availability. We fed the information into a program called Back Paddock, which calculated the estimated N mineralisation at each depth at each core.

Core 1

Core 2

Analyte

0-30cm

30-60cm

0-30cm

30-60cm

Nitrate mg/kg

7

12

9

9

Estimated N Mineralisation

41

42

34

56

(If viewing on mobile swipe right to see the full table --->)


Step 5 - Calculate Remaining N Requirement to meet the yield potential


The last step was to calculate the gap between the amount of available nitrogen in the soil, and the requirement to meet the yield potential at each core site.


The N requirement at Core 1 (yield potential 3.5t/ha) is 52kg of N. With 83kg of N available between the two depths, the urea requirement to meet yield potential at this site was 0kg/ha.


The N requirement at Core 2 (yield potential 2.5t/ha) is 37kg of N. With 90kg of N available between the two depths, the urea requirement to meet yield potential at this site was 0kg/ha.


Core 1

Core 2

Analyte

0-30cm

30-60cm

0-30cm

30-60cm

Nitrate mg/kg

7

12

9

9

Estimated N Mineralisation

41

42

34

56

Estimated Remaining N Requirement

​52kg/ha

​52kg/ha

​37kg/ha

​37kg/ha

(If viewing on mobile swipe right to see the full table --->)

So, to answer our questions….


How much urea was required to meet the yield potential of the crop in 2021?


0kg/ha


Was a variable rate map an appropriate option given the field has significant yield variability?


In 2021, a variable map was not required, given that the total N requirement to meet the yield potential in each zone had already been met. However, what the exercise did highlight, was that the yield potential in the different zone varies, as does the amount of available N. A variable rate N application will certainly be considered in the future to confirm this hypothesis.



The outcome?

2.94t/ha


After the decision was made to not spread the ruin paddock in August, the season began to shut off, with the next significant rainfall event coming in late October. We were thrilled that despite the dry finish, and no top dressed nitrogen applied, we were able to produce a wheat crop that averaged 2.94t/ha. In the future, we'll utilise the knowledge gained from this exercise to spread from an N removal map, using the protein and yield data layers from the harvester.


The fact that we had all our data layers in one place - PCT AgCloud, allowed us to give spatial relevance to the deep N tests. It also allowed us to easily correlate 'cause' and 'effect' layers, to make evidence based decisions around our fertiliser input decisions.




Figure 2 - The 2020 Vetchola Crop left a solid soil nitrogen legacy for the wheat crop the following season



 


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