Soil Horizons | Soil Genesis and Development, Scenario 3 - Soils and Salts

The photographs below show these three soil profiles. Look at these photographs and note as many similarities and differences as you can. Don’t worry whether what you see is significant or not – just list things. We will focus on a few observations later in this lesson.

Soil series of saline wetland site.Images courtesy of USDA-NRCS¹ Photo reprinted with permission of the Soil Science Society of America from the Marbut Collection² Photo courtesy of R. Hammer, USDA Natural Resources Conservation Service

Quiz

Question

Question 8: You probably noticed the dark topsoil, called the A horizon, in soils 1 and 2. What do you think darkens the A horizon‏?

clay

Clay minerals can have a variety of colors, from bright red to almost white.

organic matter

Soil organic matter (also called humus) is what colors the soil A horizon.

water

While a moist soil is usually darker than a dry soil, the color difference is usually small. It would also be unlikely that the soil surface is moister than the subsurface, except after a rainfall or irrigation.

soil minerals

There are some dark minerals found in soils, but their concentrations are usually too small to dominate the color. Where such minerals are found, though, they are very distinctive.

Looks Good! Soil organic matter (also called humus) is what colors the soil A horizon.

To review this concept click on the link: Principles Lesson 4.3

Other horizon differences are harder to see reliably. The lighter subsoil horizons in soil 1 are all different kinds of B horizons. In the soil 2 series, the lighter subsoil is called the C horizon. Soil 3 has A, B, and C horizons, too, but their colors are not very distinctive. We will see why these differences occur later. Soil horizons can be described in even more detail, with numbers and other letters following A, B, and C. The capital letters indicate master horizons, which will be our focus here.