Soil classification

Particle-size limits (Soil Survey Manual)

sand 0.05…2.0 mm;  silt 0.002…0.05 mm;  clay < 0.002 mm

USDA triangle classes (Soil Survey Manual)

sand:             S > 85  and  Si + 1.5·C < 15
loamy sand:       70 ≤ S ≤ 90,  Si + 1.5·C ≥ 15,  Si + 2·C < 30
sandy loam:       (7 ≤ C < 20, S > 52, Si + 2·C ≥ 30)  or
                  (C < 7, Si < 50, Si + 2·C ≥ 30)
loam:             7 ≤ C < 27,  28 ≤ Si < 50,  S ≤ 52
silt loam:        (Si ≥ 50 and 12 ≤ C < 27)  or  (50 ≤ Si < 80 and C < 12)
silt:             Si ≥ 80  and  C < 12
sandy clay loam:  20 ≤ C < 35,  Si < 28,  S > 45
clay loam:        27 ≤ C < 40,  20 < S ≤ 45
silty clay loam:  27 ≤ C < 40,  S ≤ 20
sandy clay:       C ≥ 35  and  S > 45
silty clay:       C ≥ 40  and  Si ≥ 40
clay:             C ≥ 40,  S ≤ 45,  Si < 40

S, Si, C are the sand, silt and clay fractions in per cent by mass. Strict and non-strict inequalities follow the source: one and the same line is shared between neighbouring classes. The twelve sets form a partition, verified over all 5151 integer triples summing to 100 %.

Texture

The class follows from the position of the point (sand, silt, clay) in the USDA triangle; there are twelve classes. This characteristic enters the water balance (FAO-56 table 19, whose heading names the USDA classification explicitly) and the lime requirement.

Soil type

The World Reference Base for Soil Resources distinguishes 32 reference soil groups by diagnostic horizons and properties. Chernozems are defined by a thick dark humus horizon, high base saturation and secondary carbonates lower in the profile. Soil type and texture are independent: a Chernozem is usually a loam or a clay by texture.

Estimating texture from coordinates

The class can be suggested from the SoilGrids 2.0 soil map (ISRIC): global maps at 250 m resolution produced by quantile regression forests. Means and the bounds of the 90 % prediction interval are shown for the 0–5 and 5–15 cm layers; the class is derived from the means alone, because only they are scaled to a hundred per cent, whereas the fraction medians are estimated independently and do not sum to a hundred. Layers are not averaged. The estimate is never written automatically: it is placed into the field by the user's action, and the same confirmation saves it as saves a manually chosen value.

The WRB answer

The map classifies in the WRB 2006 legend, while the app lists the groups of the 2022 edition. The lists agree on 31 names out of 32: the third edition replaced Albeluvisols with Retisols, and the fourth kept the groups unchanged. Groups with a matching name are offered, as is Albeluvisols in the form of Retisols; for other names the reason for having no substitution is given.

Use of the soil type

The soil type is recorded and accompanied by its WRB definition. It enters a quantitative calculation in one case: for Histosols (organic soils) and Calcisols (soils with secondary calcium carbonate accumulation) the lime dose is not taken from the table. For the former it is derived from hydrolytic acidity; for the latter carbonate saturation has already been reached.