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What is Hydric Soil? Understanding Waterlogged Earth

Hydric soil is soil that formed under conditions of saturation, flooding, or ponding long enough during the growing season to develop anaerobic conditions in the upper part. This prolonged saturation leads to specific chemical and physical properties that distinguish it from well-drained upland soils and are crucial for supporting wetland ecosystems.

Defining Hydric Soil

Hydric soils are more than just wet dirt; they are living ecosystems shaped by the constant or frequent presence of water. The term itself is derived from “hydro,” referring to water. The definition hinges on the duration and frequency of saturation during the growing season. This saturation forces oxygen out of the soil, creating an anaerobic environment, which in turn, alters the soil’s chemistry and morphology. The United States Department of Agriculture’s (USDA) Natural Resources Conservation Service (NRCS) provides the technical criteria and indicators used to identify hydric soils. These criteria focus on observable features like mottling, gleying, and the presence of a sulfidic odor, all indicative of oxygen-depleted conditions.

Why Hydric Soils Matter

The existence of hydric soils is paramount for several critical reasons:

Wetland Delineation: Hydric soils are one of the three key parameters (along with hydrophytic vegetation and wetland hydrology) used to delineate wetlands, areas that are legally protected and ecologically vital.
Water Quality: Wetlands with hydric soils act as natural filters, removing pollutants and sediment from runoff before it enters waterways. This helps to improve water quality for downstream users and aquatic ecosystems.
Flood Control: Hydric soils have high water-holding capacity, allowing them to absorb and store large volumes of water during storm events. This helps to reduce flooding and erosion.
Wildlife Habitat: Wetlands provide critical habitat for a wide variety of plant and animal species, including many that are threatened or endangered. The unique conditions created by hydric soils are essential for the survival of these species.
Carbon Sequestration: Wetlands are highly effective at storing carbon dioxide from the atmosphere, helping to mitigate climate change. Hydric soils play a crucial role in this process.

Identifying Hydric Soils

Identifying hydric soils in the field requires careful observation and analysis. The NRCS provides detailed field indicators that are used to determine if a soil meets the definition of a hydric soil. These indicators are based on the presence of specific morphological features that are indicative of prolonged saturation and anaerobic conditions.

Key Indicators of Hydric Soils:

Mottling: These are spots or blotches of different colors within the soil matrix. They are caused by the oxidation and reduction of iron and other elements due to fluctuating water tables. High chroma mottles (brightly colored spots) are particularly indicative of hydric conditions.
Gleying: This refers to a soil matrix that is dominantly gray or bluish-gray in color. Gleying is caused by the reduction of iron in the absence of oxygen.
Sulfidic Odor: A rotten egg smell (hydrogen sulfide) indicates the presence of anaerobic bacteria that are using sulfur as an alternative electron acceptor in the absence of oxygen.
Organic Matter Accumulation: Hydric soils often have a high organic matter content, particularly in the surface layers. This is because anaerobic conditions slow down the decomposition of organic material. The presence of a histic epipedon (a surface layer with a high percentage of organic matter) is a strong indicator of hydric soil.
Iron and Manganese Concretions: These are hard, rounded masses of iron and manganese that form in the soil due to fluctuating water tables.
Reduced Matrix: This refers to a soil matrix that is low in oxygen and is typically gray or greenish-gray in color.

FAQs about Hydric Soils

Here are some frequently asked questions about hydric soils, along with detailed answers:

1. How is hydric soil different from just “wet soil”?

Hydric soil is specifically defined by the duration and frequency of saturation that leads to unique chemical and physical properties. Wet soil simply refers to soil that is currently saturated or moist. A soil can be wet temporarily without being hydric, especially if the saturation is short-lived and does not occur regularly during the growing season. Hydric soils exhibit long-term evidence of anaerobic conditions.

2. What tools or resources can I use to identify hydric soils?

The USDA NRCS Web Soil Survey is an invaluable resource for identifying potential hydric soils in your area. This online tool provides detailed soil maps and descriptions, including information on hydric soil classifications. In the field, a soil auger, a Munsell soil color chart, and a thorough understanding of hydric soil indicators are essential. Consulting with a qualified soil scientist or wetland specialist is often recommended for accurate identification.

3. Can hydric soil be present in seemingly dry areas?

Yes, hydric soils can exist even in areas that appear dry on the surface at certain times of the year. This is because the defining characteristic of hydric soil is the prolonged saturation during the growing season, which might not be consistently visible. These soils may experience seasonal flooding or high water tables that create anaerobic conditions even if they are dry during other periods.

4. What is the growing season, and how does it affect hydric soil determination?

The growing season is the portion of the year when soil temperatures are warm enough to support plant growth. The specific definition varies by location and is determined by local climate data. The duration and timing of saturation during the growing season are critical for determining if a soil is hydric. Saturation outside the growing season has less significance.

5. What are “indicators of hydric soils,” and why are they important?

Indicators of hydric soils are observable soil characteristics that demonstrate the presence of prolonged saturation and anaerobic conditions. These indicators, such as mottling, gleying, and sulfidic odor, serve as direct evidence that a soil meets the technical definition of hydric soil. They are essential for accurately identifying and delineating wetlands.

6. How does the presence of hydric soil affect land development?

The presence of hydric soil is a significant factor in land development because it often indicates the presence of wetlands. Wetlands are protected under federal and state laws, and any development activities that impact wetlands may require permits and mitigation measures. Understanding the distribution of hydric soils is crucial for avoiding regulatory issues and minimizing environmental impacts.

7. What is the role of plants in hydric soil identification?

While hydric soil identification primarily relies on soil characteristics, the presence of hydrophytic vegetation (plants adapted to wet conditions) is another key indicator of wetlands. The combination of hydric soil and hydrophytic vegetation provides strong evidence of a wetland ecosystem. Certain plant species are specifically adapted to anaerobic soil conditions.

8. Can hydric soils be artificially created or altered?

Yes, hydric soils can be artificially created through impoundment or irrigation that leads to prolonged saturation. Conversely, they can be altered or destroyed through drainage or filling activities. Such alterations can have significant ecological consequences and may be subject to regulatory oversight.

9. What is the difference between a histic epipedon and other types of surface horizons?

A histic epipedon is a thick, dark, and organic-rich surface horizon that forms under saturated conditions. It is a key indicator of hydric soils and wetlands. Unlike mineral surface horizons, a histic epipedon is composed primarily of organic matter that has accumulated due to the slow decomposition rates in anaerobic environments.

10. How does drainage impact hydric soils?

Drainage systems remove excess water from the soil, reducing the duration and frequency of saturation. This can transform a hydric soil into a non-hydric soil over time by altering the soil’s chemistry and morphology. Drainage can also lead to the loss of wetland habitat and the degradation of water quality.

11. Are all dark-colored soils considered hydric soils?

No, not all dark-colored soils are hydric. While high organic matter content, often associated with dark colors, is common in hydric soils, dark color can also result from other factors. The key is to look for other indicators of hydric conditions, such as mottling, gleying, and a sulfidic odor, in conjunction with the dark color.

12. Where can I find more information about hydric soil regulations and identification?

The USDA NRCS is the primary source for information on hydric soils. Their website provides detailed technical manuals, field indicators, and online tools. State and local environmental agencies also offer guidance and regulations related to wetlands and hydric soils. Consulting with a qualified soil scientist or wetland specialist is also highly recommended.