Soils 205-90

Lecture 21 Soil horizons

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A. Master Horizons

O, A, E, B and C

B. Horizon sequence

O

A

E

EB

BE

B

C

 Note:  not every soil will have one of each master horizon; but when they are present, they generally fall in this sequence

C. O horizon

1. organic materials above mineral soil (litter layer)

2. Oi - slight decomposition

Oe - medium decomposition

Oa - highly decomposed

D. A horizon

1. humus accumulation in mineral soil

2. darkened color

E. E horizon

1. zone of maximum eluviation; loss of organic matter, clays, and/or Fe-oxides

2. no humus accumulation = lighter color

F. B horizon

1. illuviated

- accumulation of clays (silicate, Fe & Al oxides), O.M., CaCO₃, ...

- usually, from above (A or E)

some from sides or below

2. complex in highly developed soils

- numerous subdivisions

color, lime, structure,...

G. C horizon

1. unconsolidated material

2. little affected by weathering processes

H. Transition zones

AB - between A and B horizons, more like A than B

BE - between E and B horizons, more like B than E

BC - between B and C horizons, more like B than C

I. Subordinate distinctions

1. subdivisions of master horizons (table 2.4)

2. lower case letters

examples:

c - concretions or nodules	o - Fe and Al oxides
g - gleying (mottling)	p - plowing disturbance
h - illuviated O.M.	s - illuviated Fe., Al, & O.M.
k - carbonate accumulation	t - silicate clay accumulation
m - cementation	w - distinctive color/structure

Bk - lime accumulation in B horizon

Bt - silicate clay accumulation in B horizon

Ap - depth of plowing

Bx - fragipan

J. Truncated profile

B at surface

- results from erosion of A

K. Old system

O, A₁, A₂, A₃, B₁, B₂, B₃, C

O = same, except O₁ and O₂

A = A₁

E = A₂

AB or EB = A₃

BA or BE = B₁

B = same as B₂ (B_21ir, B_2ca, etc.)

- except boundary

(cemented and lime horizons are C in old and B in new)

BC = B₃

C = same, except boundary (see B above)