The accuracy of the maps (random forests part of the model) was assessed using 5fold crossvalidation, with model refitting, and reported using RMSE and % variance explained. The latter is defined as 1MSE/sigmasquare, where MSE is the mean square error at crossvalidation points and sigmasquare is the variance of the target variable. Some soil properties were logtransformed prior to prediction using regression kriging. In such cases the maps are the backtransformed regression kriging maps, but the % variance explained was derived in the transformed (transGaussian) space which differ from the % variance explained in the original space. The figure and table below shows scatter plots and summary statistics of crossvalidation results.
Example of a mapping accuracy assessment plot for soil organic carbon, pH and bulk density (obtained with 5fold crossvalidation).
Table: Mapping accuracy for AfSoilGrids250m maps assessed using 5fold crossvalidation (using only random forests models).
Code

Soil property name

Units

Sample size

Range
(99% prob.)

RMSE

% variance
explained

ORCDRC

Soil organic carbon concentration

g kg^{1}

64,010

0.9–42‰

10.6

61.3

PHIHOX

Soil pH in H2O



68,458

4.4–8.7

0.67

66.9

CRFVOL

Coarse fragments volumetric

cm^{3} cm^{3}

39,206

7–90%

18.4

20.3

SNDPPT

Soil texture fraction sand

kg kg^{1}

55,578

7–94%

15.9

61.1

SLTPPT

Soil texture fraction silt

kg kg^{1}

54,164

1–47%

8.3

56.1

CLYPPT

Soil texture fraction clay

kg kg^{1}

54,167

3–73%

13.7

52.4

BLD

Bulk density (fine earth)

t m^{3}

8732

0.9–1.9

0.14

70.4

CEC

Cation Exchange Capacity (fine earth)

cmol+/kg

47,875

1.2–57

7.9

66.3

NTO

Total nitrogen

g kg^{1}

50,997

0.1–3.1‰

0.69

61.0

ALUM3S

Aluminium concentration

ppm

18,055

150–1800

160

86.3

EACKCL

Exchangeable acidity

cmol+/kg

24,242

0–6.4

1.30

77.3

ECAX

Exchangeable Calcium

cmol+/kg

65,158

0.1–46

12.7

67.2

EXKX

Exchangeable Potassium

cmol+/kg

64,518

0.01–2.4

0.60

58.6

EMGX

Exchangeable Magnesium

cmol+/kg

63,261

0.04–15

2.5

60.0

ENAX

Exchangeable Sodium

cmol+/kg

42,986

0–8.3

3.6

46.7

EXBX

Sum of exchangeable bases

cmol+/kg

64,270

0.31–66

11.0

68.8
