Soil Science 100 Instructor William Horwath

Fall 1997

Reading: Text Chapter 1

Learning Goal: Introduce some soil terminology. What is a soil? Key concepts; profile, pedon, horizon.

Use your web browser to check out the SSC100 web site at http://qavis.ucdavis.edu/soils/classes/soils100/. It has all the soils of the day, other soils, announcements, class lecture notes, and other stuff.

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Brief outline:

1.0 What is a soil?

1.1 Naming of parts. (horizons mostly)

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1.0 What is a soil?

Figure 1-1 in the text represents some of the concepts of soil held by different members of society. I want to distinguish between a soil and soil. A soil refers to the entire three dimensional body of material with all its chemical, physical, and biological properties.

Soil in the generic sense may be almost any part of a soil be it in a bag or under your fingernails.

A soil is anisotropic. That's a fancy way to say that it is vertically differentiated into layers that soil scientists call horizons. To fully understand soils, you need to understand the soil scientists concept of the soil.

2.1 Naming of parts.

A soil is a three phase system, containing solid, liquid and gas phases at all times.

Another important concept to appreciate about the study of soils as natural bodies is that of scale. We will consider scales that vary by 1012; 12 orders of magnitude. When we study soils, we study processes at mineral surfaces at the nanometer (10-9 m) and smaller scale. When we discuss interpreting soils or soil genesis, we may discuss them at the landscape scale of kilometers (103 m).

Soil may be a sample, but a single sample never represents an entire soil individual. Soil individuals occur on landscapes. To fully understand a soil, we need to know something about where it lives.

Some different definitions:

Geologist - unconsolidated material at the earth's surface that covers up the really interesting rocks beneath the soil.

Engineer - anything that can be moved without blasting

Agronomist - a growth media for plants - a friable unconsolidated mixture which hold nutrients, water and organisms and allows for exchange of gases.

To a soil scientist, the three-dimensional soil individual is represented by the pedon. In your text we build and interpret a pedon to help you learn about soil components and how soils "work".

Figure 1-3 is "our" pedon (transparency). Here in its most simple form and showing its minimum dimensions. The pedon is just large enough to contain all the properties of the soil individual. The minimum size is 1 m2 by sufficient depth to include all layers and parent material

This pedon is empty, but of course soils are not empty. They are composed of mineral and organic particles, holes filled with air and fluid. Living organisms occupy space in the pores and on the surfaces of mineral and organic particles. Before describing these components in detail, a few more naming of parts.

Figure 1-4 (transparency) is the same pedon, but now it has layers that we soil scientists call horizons. The horizon designations have special meanings and convey much information. It is important for you to be able to recognize horizons and to understand their function and origin in soils.

Soil horizons are roughly parallel with the soil surface but may tongue downward or undulate due to perturbations by animals and plants. The boundaries of vertical movement of oxygen, water and heat are roughly parallel to the surface since these alter parent material, the horizons are also roughly parallel to the surface and intensity varies with depth.

Your text lists the master horizons in Table 1-1. Learn this nomenclature. I will use it throughout the quarter. In figure 1-4 several master horizons have subordinate horizons shown as lower case letters and numbers. If you want to peak ahead, page 320 of your text lists all of the subordinate horizon designations.

Define and describe the Master Horizons.

- Organic vs Mineral horizon (organic carbon of 20%)

- may be divided into three kinds based on the degree of decomposition (i<e<a).

- eluvial horizon (loss of Fe, Al and clays)

- may be mixed by cultivation (Ap horizon)

- thicker A horizons in grassland soils due to incorporation of root tissue.

- eluvial horizon

- light colored due to removal of iron and organic matter

- Horizons formed beneath an A, E or O horizon and is a zone of accumulation (illuvial horizons). How do you recognize a B horizon ?

- Color - red due to iron accumulation

- Have greater clay than the overlying horizons

- No rock structure remains

- Development of soil structure

- B Horizons have many differing subdivisions.

Concept of Parent Material (more on how soils form next lecture).

- unconsolidated material (not hard and solid like a rock)

- beach sands, material deposited by river or glacial till

- Hard bedrock (can not be dug with a shovel).

-examples: granites of the Sierra Nevada

Cr horizon -- rock-like (has the structure of the rock but can be dug by a spade)

Soils can have transition horizons or horizons with characteristics from different horizons such as AB, EB vs BA, BE

Soils can have mixed horizons, such as A/B, E/B, B/C.

Not every soil contains all the master horizons

- O horizons usually missing or minimal under grass

- Eroded surfaces may have no O or A horizons

- A horizons may be missing in some forest soils

- B horizons may be missing in very young soils

Other popular or colloquial terms

Soil Regolith

- the unconsolidated material overlying rocks

- the zone of active soil formation; comprised of the A, E, and B horizons

Topsoil

- the surface layer of the soil (the layer manipulated by tillage and sometimes called the furrow slice) soil influenced by incorporation of organic matter usually darker (A horizons)

Subsoil

-the soil layers underneath the topsoil (does not include the C horizon) (therefore the B horizon)