Special Topics on Soils

1. Water Movement in Soils
2. Effects of Saturation by Water on Soils
3. Fragipans

a. Water Movement in Soils

1. Forces that Control Water Movement

Water movement in soil takes place in response to a force, or potential. The types of forces that act on water moving through a soil include:

• positive forces, such as gravity and pressure
• negative forces, such as suction and osmosis

These forces, in conjunction with the water content of the soil, dictates whether the soil water moves through the soil by “saturated” or “unsaturated” flow.

Saturated flow:

• driving force is positive (gravity or pressure)
• soil water is conducted by all pores
• the rate of flow is greatest through the large connective pores, so saturated flow is higher in sandy soils

Unsaturated flow:

• driving force is suction
• soil water is conducted only by the smaller pores; large connective pores are filled with soil air
• soil water tends to only occupy the smaller pores, so unsaturated flow tends to be higher in clayey soils

2. Soil Physical Properties that Control Water Movement

In addition to the above forces, water movement in some soils is controlled by the type of soil structure present within the soil horizons. Water moving downward vertically through a soil profile is transported in the pore space that exists between the structural aggregates, or “peds”, that make up a particular soil horizon. The shape and arrangement of the peds dictates the size and continuity of the pores. Typically, angular blocky, subangular blocky, and granular structure create large pores that allow water to move through the soil. Prismatic and platy structures have long but narrow pores that tend to slow down vertical water movement.

Some soils contain a hydraulically restrictive horizon, such as a fragipan or clay layer, that can impede the vertical movement of water down through the soil. Water that is held up by a hydraulically restrictive horizon is called a “perched water table”. Soils that do not have a hydraulically restrictive horizon present are considered to have a “regional water table” that extends vertically without interruption.

References: Buckman, N.G. 1990. The Nature and Properties of Soils. Macmillan Publishing Company, NYC.

Hillel, D. 1980. Fundamentals of Soil Physics. Academic Press Inc., Orlando, FL.

b. Effects of Saturation by Water on Soils

Under ideal conditions, the pore space of a soil would be half filled with soil water and half filled with soil air. When adequate soil air is present for certain plant and microbial activities, the soil environment is “aerobic”.

Aerobic soil conditions promote:

• plant root respiration
• aerobic microbial reactions
• combustion or consumption of organic matter

A soil that has its pore space completely filled with water is said to be “saturated”. Prolonged periods of saturation result in the depletion of oxygen by plants and microorganisms in the soil. If a soil remains saturated long enough for all the oxygen to be depleted, the soil environment is “anaerobic”.

Anaerobic soil conditions promote:

• transformation of elements from an oxidized to a reduced form (oxidation-reduction reactions)
• anaerobic microbial reactions
• accumulation of organic matter

In mineral soils, anaerobic conditions bring about a sequence of oxidation-reduction (electron transfer) reactions. The microbial breakdown of soil organic matter is an oxidation-reduction process. Under aerobic conditions, organic matter is oxidized (loses electrons), and oxygen (O2) is reduced (gains electrons) and combines with hydrogen to form water. The ultimate product of aerobic degradation is CO2. When the soil becomes saturated, the amount of oxygen is decreased; with continued breakdown of organic matter the oxygen can be all used up, and the soil becomes anaerobic. Biodegradation of organic matter now continues under different conditions; different groups of microbes go to work using different electron acceptors instead of oxygen. These processes are not as efficient or as complete as the aerobic one. Nitrates, manganese oxides, iron oxides, and sulfates are soil compounds that are used as electron acceptors in anaerobic microbial reactions, in a specific order. After the removal of oxygen, nitrate is the first soil compound to be reduced; then manganese, iron, and eventually sulfate are reduced. These transformations bring about the translocation and/or accumulation of these elements, which can result in morphological features useful in the identification of saturated zones in soil.

Nitrogen transformations in saturated soils can make the nutrient less available for plant uptake. However, excessive amounts of nitrate (NO3-), the mobile form of nitrogen, can be reduced to prevent leaching losses.

Manganese transformations are similar to iron in that manganic (Mn+4) compounds are reduced to more soluble manganous (Mn+2) forms. Re-oxidized and re-deposited manganic oxides appear as black films or coats on soil particles.

Oxidized or ferric (Fe+3) iron compounds are responsible for the brown, yellow, and red colors in soil. When iron is reduced to the ferrous (Fe+2) form, it becomes mobile, and can be removed from certain areas of the soil. When the iron is removed, a gray color remains, or the reduced iron color persists in shades of green or blue. Upon aeration, reduced iron can be re-oxidized and re-deposited, sometimes in the same horizon, resulting in a variegated or mottled color pattern. These soil color patterns resulting from saturation, or “redoximorphic features”, can indicate the duration of the anaerobic state, ranging from brown with a few mottles, to complete gray or “gleization” of the soil. Soils that are dominantly gray with brown or yellow mottles immediately below the surface horizon are usually hydric.

Sulfates in soils are reduced to sulfides when soils are nearly permanently saturated. The presence of hydrogen sulfide can be detected by the “rotten egg” odor, which is used as a hydric soil indicator. Sulfides can be toxic to microbes and plants, and upon re-oxidation, can lead to extremely acid conditions in soils when sulfuric acid is formed.

Hydric Soils of the US

This website includes a listing of New Jersey hydric soils and Field Indicators of Hydric Soils in the US, A Guide for Identifying and Delineating Hydric Soils.

Reference: Mitsch, W.J. and J.G. Gosselink. 1986. Wetlands. Van Nostrand Reinhold, NYC.

c. Fragipans

Properties

• firm, brittle subsurface horizon that restricts plant root penetration and downward water movement
• responsible for “perched” water tables in some soils
• not cemented by any of the common soil cementing agents (calcium carbonate, iron, or silica)
• identified in soils descriptions with the horizon designation suffix “x”; examples are Bx, Btx, Btxg
• tend to form in transported materials such as till, outwash, alluvium, colluvium, and marine sediments
• the vertical extent of a fragipan can be continuous, where it gets more dense and has less structure with depth; or discontinuous, where it “dies out” or terminates abruptly, and can be underlain by friable and/or permeable material

Morphology

• commonly has prismatic structure with visible prisms
• can also have platy, blocky, or massive structure
• prisms typically have high chroma matrix colors (chroma > 2); prism faces commonly have high chroma redoximorphic features and the spaces between prisms commonly are low chroma soil colors (chroma ≤ 2)

Identification in the field

Peds or pieces broken out of a fragipan should have the following characteristics:

• firm or very firm moist consistence
• brittle manner of failure when moist; peds or pieces “explode” rather than deform
• “slake” (disintegrate or dissolve) when immersed in water; this test verifies that the horizon is not cemented

In Soil Taxonomy, a soil horizon is considered a fragipan if more than 50 percent of air dry fragments of the horizon slake in water. Also, 60 percent or more of the horizon must have a firm or firmer rupture-resistance class, a brittle manner of failure, and virtually no roots present. An additional test is that the layer must not effervesce in dilute HCl, which is evidence that the horizon is cemented by calcium carbonate.

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Last Modified: May 27, 2008