Unit 3 - Soil Horizons

Chapter 1 - Soil Structure and Color

Introduction to soil horizon nomenclature. The ability to write a soil profile description will be a valuable skill and will determine your understanding of soils and how they develop. In order to accurately describe a soil profile, understanding how the horizons are named is the important first step

{short description of image}

Clues for Horizon Designations - Soil Structure and Soil Color

We will now discover the clues used to name the horizons in the soil profile and will begin with soil structure. "Soil structure" refers to the way these separates are attached together. Soil separates do not act in the soil as individuals, but as partners, or aggregates. Aggregates are the clumps of soil separates. When aggregates are bound together into larger masses they are called PEDS. "Soil texture" refers to the proportion of soil separates (that is, the amount of sand, silt and clay in a soil). Texture was studied in Unit 2.

It is very difficult to change soil texture; however, soil structure modifies the influence of the texture, and soil structure is relatively easy to change or alter especially when the soil is being cultivated. Soil clods can be broken apart by wetting and drying, and by freezing and thawing. Cementing agents which bind soil separates together include microbial gums, iron oxides, organic matter, and clay.

Types of Structure

1. Granular Structure
This photo shows a well-granulated A horizon. Granular structure is the most beneficial form of soil structure for plant growth. Granular structure aggregates are formed by the breaking apart of larger aggregates through the physical processes of wetting and drying, and freezing and thawing. These aggregates are then cemented together by the by-products of the microbial decomposition of organic matter, which are called microbial gums. The more microbial gums, the greater the aggregate stability. The way to obtain microbial gums is by adding organic matter to the soil; thus, plant residues contribute indirectly to better soil structure. 

2. Platy Structure
Platy structure is often found in the E horizon (below the A) where water moves laterally through the soil. Platy structure can be detrimental because it restricts root and water penetration.

3. Blocky Structure
Blocky structural peds are found most frequently in the B horizons. They have been created by the wetting and drying and freezing and thawing cycle of the B horizon. The clay films also act as a binding agent for the blocky aggregates. The B horizon can often be determined in a profile by looking for the location of blocky peds which can be readily seen. Blocky can be either angular (sharp ped edges) or sub-angular (rounded ped edges).

4. Prismatic or Columnar Structure
Where the blocky peds are longer than they are wide, the prismatic or columnar structure is identified (common only in B horizons). They are often the first structure formed in a soil, because their formation only requires vertical cracking in the soil.

5. Structureless C horizons generally lack any structural aggregation. Their lack of structure is termed "massive." Massive structure is hard to break apart and appears in very large clods. Where very sandy soils lack aggregation, or the soil particles don't stick together the structureless condition is termed "single-grained." Single grained always accompanies a loose consistence.

Examples of Soil Structure

Soil Color: What is the color of the soil?

Soil color is a property that will give important information about the soil's characteristics. Note the page from a Munsell color book and the Earth Colors book. Soil color is determined by comparing the color of the soil to the chips in the soil color charts. Soil color consists of 3 parts: hue, value, and chroma

Hue is the dominant spectral color of the rainbow - yellow, reds, orange. The value is the relative darkness or lightness, and the chroma is the purity or strength of the color. In looking at the page on this slide and in your book, hue is given in the upper right hand corner of the Munsell page (10YR) and bottom of the Earth Colors page.
Value is expressed as the numerator of the fraction and is along the left hand margin of the page. Value is the relative darkness or lightness of the soil color.
Chroma is along the bottom, and is the denominator of the fraction. Chroma is the relative purity or strength of the color, low chromas have dull colors, while high chromas have bright colors.
So a color of 10YR 3/2 has a hue of 10YR, a value of 3, and a chroma of 2. Coloring soils, like texturing, takes practice. You also need to be able to note the Munsell notations and determine what kind of soil horizon the color might represent.

The significance of soil color relates to the properties we can infer from certain colors.
This soil has a dark surface color- 10YR 3/2 - which indicates a high organic matter content;
the second layer has a 10YR 3/3 color, which indicates not as much organic matter;
the subsoil is 10YR 4/4, a typical brown color, and the 7.5YR 4/3 is similar, but redder; and
the parent material is 10YR 5/6, which is typical for Des Moines Lobe glacial till.

In the laboratory you will have a chance to practice your "soil coloring"!

{short description of image}

Back to Lab Units

Soil Horizons Chapter 2- Horizon Nomenclature

Soil Horizons

© Regents of the University of Minnesota, 2009. The University of Minnesota is an equal opportunity educator and employer.