Calculation error: Custom SOM causes incorrect soil carbon changes
A bug has been found when using the custom SOM function in CFT 2.0 of the tool. SOM is not multiplied by the bulk density to convert it from cm to t/ha. Also, SOM is entered in the tool as a percentage [0,100], but is not converted to a proportion [0,1].
The effect of this bug is that the magnitude of the change in soil carbon is
- underestimated when bulk density is greater than 1 g/cm3 and
- overestimated when bulk density is less than 1 g/cm3.
To correct this, you need to multiply the management change value by the bulk density (measured in g/cm3). This error will be fixed as part of migration to Cool Farm Platform and upgrade to CF 3.0.
EXAMPLE CROP ASSESSMENT
| Soil | Management practice changes |
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|
|
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SOM | Years ago | Field % | Land use | Tillage | Inputs |
0.600 (measured 7 years ago) | 11 | 12 | Native Grassland >> Cultivated | No-till >> Reduced | High C Input with manure(No change) |
Fill the crop details (maize, 1ha, 1 tonne), and the soil and carbon tab (other tabs are empty). In the soil tab, indicate custom value of SOM. In our example we entered 0.600 SOM measured 7 years ago - this gives us emission reductions from "management changes" on 12% of a 1 ha field of -28.69 kg CO2e. This number is incorrect and should be multiplied by the bulk density g/cm3. The correct result for bulk density of 1.65 g/cm3 would be -47.34 kg CO2e.
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