Calculation error: Custom SOM causes incorrect soil carbon changes

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

  1. underestimated when bulk density is greater than 1 g/cm3 and
  2. 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





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.