Soils 205-90

Lecture 22 Soil Classification

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A. Concepts

1. "Old" system

(a) mid '30s to mid '60s

(b) based on soil genesis

2. "Soil Taxonomy"

- Comprehensive Classification System

(a) based on quantitative properties of soils

- can be verified

(b) nomenclature

- reflects properties

- Latin and Greek root words

(c) Use diagnostic horizons

B. Diagnostic Horizons

1. Epipedons = surface horizons

x Mollic - thick, dark-color, high base saturation (BSP)

x Umbric - like mollic, except low BSP

x Ochric - light color, low O M, lacks structure

x Melanic - thick, black, high O.M., in volcanic-ash soils

x Histic - organic materials dominate, wet areas, low D_b

2. Subsurface horizons

x Argillic - illuviated high-activity silicate clays

x Natric - argillic + high Na⁺ (ESP), columnar/prismatic structure

x Kandic - low-activity Fe and Al oxide & silicate clays (kaolinite)

x Cambic - altered horizon, Fe release (red-yellow colors)

= not illuviated

x Spodic - illuviated organic matter with Fe and Al oxides

x Oxic - highly weathered (loss of Si), high Fe and Al oxides

x Albic - light-colored, eluviated (low in clay and oxides)

t Pans - impermeable layers

i Duripans - Si-cemented clays, O.M., CaCO₃, ...

- arid, semi-arid climates

i Fragipans - high bulk density, "brittle"

- more humid climates

i Placic - thin layer cemented with Fe/Mn/O.M.

t Various accumulations

i Calcic = lime

- Petrocalcic - cemented calcic

i Gypsic = gypsum

- Petrogypsic - cemented gypsic

i Salic = salt

3. Do not necessarily coincide with genetic horizons

Mollic may be A1, A2, Ba, Bt

Argillic may be Bt1, Bt2, Bw

C. System Categories

Order (12)

Þ

Suborder (~ 65)

Þ Þ

Great Group (~ 250)

Þ Þ Þ

Subgroup (~ 1,500)

Þ Þ Þ Þ

Family (~ 8,000)

Þ Þ Þ Þ Þ

Series (~ 20,000)
(in U.S.)