The Field

In Soil Biology – Part 1, I discussed how to manage soil biology across a landscape to promote denitrification to reduce the amount of nitrogen in stormwater or groundwater.  In Part 2, I will discuss how soil biology and soil wetness cause changes in the colors of the soil profile.

Why are these color features important to landscape architects?  If you want to design planting or restoration plans that are in harmony with nature and produce the results you want, you need to understand the spatial pattern of soil wetness across a landscape.  You can do this by viewing the color features in the soils.  In addition to the spatial pattern, it allows you to know the rise and fall of a shallow water table under your site.

When the soil profile lacks any oxygen for an extended period of time (which normally occurs when the soil is saturated with water) a biochemical change occurs which is called reduction.  When there is a sufficient food source for the bacteria and a sufficient soil temperature, any small amount of remaining oxygen within the soil is depleted by actively respiring bacteria. The reduction process will take place in about 2 to 3 weeks in the south and about 4 to 6 weeks in the northern states.

The history of reduced conditions in a soil manifests itself as color changes in the soil profile.  The soil profile color can be evaluated at any time during the year and it will tell you the history of soil wetness conditions on the site.  Knowing the depth to a seasonally high water table is essential to accurately determine where to locate plants requiring wet conditions, as well as, plants requiring drier conditions, and plants requiring moist but not wet conditions.  You can construct a soil wetness map for your site that is much more detailed and accurate than using a county soil survey.  The soil saturation in conjunction with soil biology creates the color patterns referred to as redoximorphic features or redox features.  The following simplified procedure will explain how to recognize redox depletions and redox concentrations.

Simplified On-Site Procedure for Landscape Architects

The procedure for loamy, silty, and clayey soils is to dig a soil pit to about 36 to 48-inches deep.  Make the hole large enough so you can get in and view the soil layers (horizons).  The easiest way is to use a back hoe.  Next clean off one side of the soil pit with a spade to expose the soil horizons.  You will see a variety of soil colors.  The dominant soil color in a horizon is called the matrix color.  Colors occupying less than 50% of the horizon are referred to as mottles.

Identify the horizons where the matrix colors are brown, red, or yellow.

• In these horizons you may likely see gray mottles which represent areas where iron or manganese has been removed by the reduction processes.
• These features are called redox depletions, because the brown, red, or yellow colors have been removed.  The gray mottles at some time in the past were brown, red or yellow in color, but are now gray.  The seasonal water table has come up to the upper elevation of these gray mottles.
• In the past as the water table receded after the wet season, bright brown, red, or yellow mottles have formed in the lower horizons (iron and managanese have precipitated in the horizons; these features are referred to as redox concentrations).  The lowest elevation of these bright brown, red, or yellow concentrations is the location of the normal water table for major portions of a typical year.
• Thus we now know the top and bottom elevations of the water table fluctuation in the portions of soil profile which are dominantly brown, red or yellow.

Identify the horizons where the matrix color is gray.  A dominant gray color represents very wet conditions.

• Sometimes gray matrix colors are located from the bottom to the top of the soil profile.  In this situation if the A-horizon is black or very dark gray, the water table frequently comes up to the surface of the ground.
• You may find light gray mottles in a horizon where the matrix is dark gray in color.  The lighter gray colors are referred to as redox depletions. The upper elevation of these lighter gray mottles represents the top of the seasonally high water table.
• Below the horizons with redox depletions, you may find brown, red, or yellow concentrations within horizons where the matrix is gray and the lower elevation of these concentrations represents the normal water table depth.
• Thus we now know the top and bottom elevations of the water table fluctuations in the portions of the soil profile which are dominantly gray.

There are exceptions to these general rules.  A soil scientist can prepare a more exacting map of different drainage classes for your site.

Simplified Procedure for Landscape Architects Using County Soil Survey

If you do not conduct an on-site investigation, and only use a county soil survey, the narrative description for each soil mapping unit will use the following drainage class names (prevailing wetness conditions for extended periods of time in a typical year unless artificially drained; or referred to as seasonally high water table).  The drainage class names and brief simplified descriptions are:

• Subaqueous – Water is above the ground surface; suitable for obligatory aquatic and wetland plant species.
• Very Poorly Drained – Water is at or near the surface and typically within the topsoil horizon; suitable for obligatory plant species.
• Poorly Drained – Water comes to the base of the topsoil horizon or approximately 8-inch depth; suitable for facultative wet plant species.
• Somewhat Poorly Drained – Water is shallow typically below the topsoil horizon or about 12-inch to 18-inch depth; suitable for facultative plant species.
• Moderately Well Drained – Water drains slowly and is typically more than 24-inches deep; suitable for facultative upland plant species.
• Well Drained – Water is typically more than 40-inches deep; suitable for upland plant species.
• Somewhat Excessively Well Drained – Water is typically more than 60-inches deep; suitable for upland plants species.
• Excessively Well Drained – Water drain rapidly and is more than 80-inches deep; suitable for upland and xeric plants species.

Now with this information you can arrange your plant palette for this specific site respective to moisture regimes based upon the depth to seasonally high water table.

Summary

Soil biology is integral to selecting plants that are adaptive to particular soils.  The soil redoximorphic features are a product of the interaction of soil bacteria, carbon food source, soil temperature, and water table fluctuations.  If you understand the spatial and vertical expression of soil redox features across your site, you will be able to understand both the natural micro-ecological sequences and the often odd sequence across disturbed soils and sites.

by Barrett L. Kays, Ph.D., PLA, FASLA, Landscape Architect and Soil, Hydrologic and Environmental Scientist