6: Soil Color

Question 1

Black soil

Answer 1

Humified (highly decomposed) organic matter

Question 2

Brown soil

Answer 2

Combination of Fe-oxides, oxyhydroxides, and OM

Question 3

Red soil

Answer 3

Oxidized iron, hematite (Fe2O3)

Question 4

Gray soil

Answer 4

Reduction of Fe3+ to Fe2+ and loss of Fe2+, anaerobic conditions WITH leaching,

Grayish colors indicative of iron depletion

Question 5

Blue-green soil

Answer 5

Reduction of Fe3+ to Fe2+ and retention of Fe2+, anaerobic conditions WITHOUT leaching

Pale bluish or greenish-yellow colors are indicative of prolonged saturation, but reduced iron is not leached away

Question 6

White soil

Answer 6

Carbonate or evaporite accumulation OR strong eluviation leaving non-pigmented light-colored minerals

Question 7

What are dark soils indicative of?

Answer 7

Often found on surface horizons, they are indicative of good fertility and high organic matter content

Question 8

What are brown, orange, and reddish soils indicative of?

Answer 8

Well-drained soils, often very old and/or highly weathered

Question 9

Redoximorphic features

Answer 9

Variegated color patterns of dull gray and bright orange, indicative of seasonal saturation

Causes are perched water, high ground water table, poor soil structure, high clay content, hence poor aeration