Opterra N Supply Pool Calculator     
Background This N supply calculator is supplied for primarily educational purposes showing the relationship between crop supply pools,  uptake and internal transport efficiencies for cereals and canola.
It is designed to be used in conjunction with the Nitrogen section of the Back Paddock Companys' BSM and ProSoil-Grain training courses.  It is a more configuable form of the common  N budget, allowing the operator to expore the effect of  relative contribution of varies N pools, changes in uptake efficiencies  the N  pools and N application tactics.
Mineral N tab 1. Input soil test values for nitrate-N and ammonium-N
2. Enter depth increments tested
3. Enter soil bulk density (BD) for the depth increment (Tip: hover mouse pointer over  column title for typical ranges for some texture groups)
4. If discountiong NH4-N discount for static background content - enter base value.
5. Adjust the Subsoil layer limitation factor for any know constaints to water uptake form  a layer (e.g. salinity)
6. Select the  Preferred method from dropdown menu to automatically transfer Profile Mineral N vale to the  N Supply Calculator
Soil Organic N tab Method 1 ( winter dominant rainfall areas)
1. Enter soil orgainc carbon % (OC %) for 0 10 cm sample layer
2. Enter Growing Season Rainfall (GSR) for Expected ( Decile 5), Low (Decile 2) and High ( Decile 8) rainfall (mm)

Method 2  (universal)
1. Enter soil OC %
2. Enter soil bulk density
3. Select soil texture fron drop-down menu
4. Locate appropriate  agr ecological zone column in the Monthly Mineralisation Potential table
5. Sum monthly % for months from sampling to sowing and enter in % Annual mineralisation (sampling 2 sowing)
7. Enter selected value in" Input value you want to use in N budget calculation here" and it will automatically transfer  the value vale to the  N Supply Calculator
Residue Organic  N If a previous crop was a pulse there  a possibility that there amy be some N abvaulable form this sources - separate to the humic pool. If the surface soil  has been subject to wetting and drying during fallow period and the site free of weeds) there is a possibility that the potential N form this pools will already be in the  mineral n pool.
1. Select  previous pulse crop  species/variety
2. Enter potentail and actual yield of the  pulse crop
3.  Select harvest index  (hover mouse over input cell for description of options)
4. Select from the drop-down menu if grain was havested (Yes/No)
5. Input  selected value from  Methods 1 to 5 or Average to"Value to use in N budget (kg/ha)"

N Supply calc - cereal


N Supply calc - canola
The  default values for the red editable cells are  the best starting point for calculating Fert N required for a target yield and grain protein range.
The majority of these values have cell comments (hover over) that will provide guidance to parameters and related values  to guide changes
The objective is to  configure the  soil supply factors appropriate to the situation, calculate the  desired yield range ( Yield Calculator).
1. Select Crop
In Yield Calculator pane
2. Enter Fallow Stored moisture estimate
3. Enter GSR (mm)
4. Enter Water Use efficiency (WUE) kg/mm
5. Select a  N fetiliser rate combination  for pre and post sowing N that closely matchs the Yield calculator Potential Yield to the Results section Most Probable Yield (t/ha) for the Target Grain Protein %
© Backpaddock Company P/L 2019  
Disclaimer This calculator  is provided as an education resource and should not be relied upon to provide commercial nitrogen reommendations.The outputs  of the calculator are a guide only, and depend on representative samples being analysed and appropriate parameterisation of the variabales. Additionally environmental and managerial factors influence performannce therefore as suppliers of this calculator we do not accept any liability whatsoever arising from the application of recommendations, for any damage, loss or injury of any nature and the user takes these evaluations and recommendations on these terms. This calculator is provided in good faith, based on the best  information available.  
Depth  Increment Number NO3-N (mg/kg) NH4-N (mg/kg) From (cm)  To (cm)
Typical values
Shallow Deep
Sand 1.3 - 1.5 1.5 - 1.6
Loam 1.1 - 1.3 1.3 - 1.5
Clay 0.9 - 1.2 1.3 - 1.5
Soil BD (g/mL)
NH4-N discount Subsoil layer limitation factor
(100 % = no limitation,
0 % = no roots in this layer)
  Soil Mineral N (kg/ha) Method description          
Method 1 Nitrate only          
Method 2 Nitrate + Ammonium        
Method 3 Nitrate +  discounted Ammonium        
Preferred Method Soil Mineral N (kg/ha)            
Soil Organic N Credits
Method 1 (Ridge - Van Rees) - only calibrated  for winter cropping in temperate and Mediterranean  climates      
This table is provided to help calculate the proportion of humic mineralisation like to occur in the between sampling and sowing, and sowing to harvest for Method 2 only.

1. Identify the appropriate rainfall zone column.
2. Sampling to sowing - sum the monthly mineralisaton potential % for the months fron sampling to sowing and input to appropriate cell. (sampling 2 sowing)
3. Sowing to harvest - sum the monthly mineralisaton potential % for the months fron sowing to 1 month prior to harvest and input to appropriate cell ( incrop).

Monthly Mineralisation Potential ( % of annual)
OC % (0-10 cm)     Month
Areas where long term records indicate majority of effective rainfall is received in summer e.g Dalby, Qld
Areas where long term records indicate majority of effective rainfall is not seasonaly dominant summer or winter.e.g. Dubbo, NSW
Areas where long term records indicate majority of effective rainfall is received in winter e.g. Donald, Vic
Areas where long term records indicate majority of effective rainfall is received in winter e.g. Merredin, WA
  GSR Estimated  N    Jan 15.0% 11.0% 10.0% 8.0%  
Expected   Feb 17.0% 13.0% 8.0% 6.0%  
Optimistic   Mar 14.0% 12.0% 11.0% 2.0%  
Pessimistic   Apr 6.0% 5.0% 4.0% 6.0%  
        May 6.0% 4.0% 4.0% 15.0%  
        Jun 6.0% 8.0% 11.0% 10.0%  
Method 2  - generic       Jul 4.0% 6.0% 12.0% 15.0%  
OC %     Aug 3.0% 6.0% 8.0% 7.0%  
BD     Sep 4.0% 7.0% 6.0% 5.0%  
Soil texture
    Oct 5.0% 8.0% 10.0% 13.0%  
Soil texture factor     Nov 8.0% 9.3% 8.0% 3.0%  
% Annual mineralisation (sampling 2 sowing)     Dec 12.0% 10.7% 8.0% 9.0%  
% Annual mineralisation ( in-crop)                
Input value you want to use in N budget calculation here kg/ha              
  Residue Organic N Credits
Chris Dowling:
Select a pulse species by clicking the down arow
Winter pulses, GRDC Southern Adviser Update (Peoples 2018)
Results from Other Generic Calculations 
  Potential Grain Yield (kg/ha)
Chris Dowling:
Insert potential yield of pulse based on biomass grown if affected by late stress.
  Method 1 (season after pulse only) 0.28 x [54+(30 x  legume grain harvested (t/ha))]      
  Actual yield (kg/ha)   Method 2 (season after pulse only) 15 kg mineral N/ha per tonne of legume grain harvested    
  Actual % of Potential yield   Method 3 (season after pulse only) 28% of legume above- & below-ground residue N      
  N% in DM   Method 4 (season after pulse only) 9 kg mineral N/ha per tonne shoot residue DM      
  Crop N (kg/ha)   Method 5 (season after pulse only) universal process          
  Pulse Crop Harvest Index
Chris Dowling:
Typical - Actual yield >75 % of potential
High -Actual 100 % of potential butyield better than it looked
Low - poor yield for the size of the crop. Actual yield <75 % of potential
  Average ( method 1- 5) kg/ha          
  Crop Harvest Index   Fallow  rainfall adjustment ( harvest to sampling) 0.13 kg mineral N/ha per mm fallow rain (rate of release)     
  Crop DM (t/ha)                  
  N % in Grain                  
  Grain N removal (kg/ha)              
Chris Dowling:
If green or brown manure crop select NO
  Actual  Fallow Rainfall YTD ( harvest to sampling)                  
  Net Fallow adjusted N  (kg/ha)                  
  Value to use in N budget (kg/ha)                  
            Fertiliser N  
  Crop N Source   Total  (kg/ha)
  Seasonal N Pools Supply (kg/ha)  
  N Pool  Crop Uptake Efficiency
Suggested range
75 - 85%
Suggested range
65 - 80 %
Suggested range
Majoity of soil N
Evenly distributed 10-60 cm - 70 %
Dominanlty below 60 cm - 50 %
Dominantly 0-10 cm - 60 %
Suggested range
All fertiliser N applied preplant
Stranded in dry surface soil (poor in crop rainfall) - 10 - 20 %
Stranded above full profile (avarage in crop rain) - 20 - 30 %
Well distributed down profile by fallow moisture - 30 - 40 %
Suggested range
Incrop - winter cereal
Z14 -25, 30 - 50%
Z25 - 32, 40 - 60%
Z37-50 , 30- 10%

  Weighted N Pool Uptake Efficiency  
  N Pool Grain N efficiency    
  Source Weighted Nitrogen Uptake Efficiency = Fert N Contribution to Total  Crop N Supply
  Crop Nitrogen Utilisation Efficiency =
Suggested ranges
55 -70 % - dryland
60 - 80 % - high rainfall/ suppl. irrigation
75- 85% - fully irrigated
Grain protein target
Grain protein target <10 % = 85 -70 %
Grain protein target 10 - 12 % = 60 -70 %
Grain protein target >12 % = 60 - 40 %

Based on French and Schultz equation
Yield Calculator 
  Apparent Grain Nitrogen Use Efficiency = Fallow stored water (mm)    
    Soil  Pool Weighted N Supply (kg/ha) =   Target (Decile 5)  Low (Decile 2)  High (Decile 8)
          GSR (mm)
    WUE (kg grain/mm)
          Potential Yield (kg/ha)
[No canvas support]
[No canvas support]
    Results Match yields above and below  by adjusting  fertiliser N rate    
Most probable yield for the soil and fertiliser N , target grain protein range below.
Most Probable Yield (t/ha)
          Target Grain Protein  (%)
10.5 to 12.5 % is the typical grain protein target range for hard wheat, feed barley.
12.5 - 14 % is the typical grain protein target range for PH and durum wheat
9.5-10.5 % is the typical target grain protein range for crops such as sorghum maize, soft wheat, milling oats and malting barley.
          Fertiliser  N Rates kg/ha    
          Adjust Fertiliser N Rates 0                      100                   200    
          Presowing (kg/ha)
          Post Sowing (kg/ha)
          Total Fertiliser N kg/ha)