TSSH Part 630

Special Projects, Studies, and Inventories

A major role of the resource soil scientist is to promote the use of soil survey and technical soils information. Along these lines, special projects, studies, and inventories can be conducted to address specific soil characteristics useful in solving natural resource issues. The resource soil scientist may be involved by developing a field tool to evaluate the suitability of a site for a given purpose, identifying site characteristics that may affect the proper implementation of a farm practice, or promoting the use of soil survey information as a planning tool. The resource soil scientist may be involved with special studies to identify issues involving soil characteristics that affect use and management. For example, a special study or thematic report may be created to identify areas that contain clays with a high shrink-swell potential. A map could be produced that would provide assistance for planning purposes to a county or State. Similarly, the resource soil scientist could produce a map of soils that have a gravelly substratum, which would assist in the identification of potential locations for new gravel pits.

General information (630.00)

When field data are collected for special projects, studies, and inventories, it is important to select sites that are representative of the planned purpose. When possible, sites that represent mapped soil series and/or map units should be selected. (This may not be applicable if the project is site specific or the objective is to evaluate sites that do not represent current soil series and/or map unit components.) The sites should be representative not only in terms of the soil but also in terms of the plant community, geomorphic position, and other site characteristics. For example, unless the project is land use dependent, you should not sample in an herbaceous community if the typical community is forested. All field data should be accompanied by a complete soil description; the description and as much data as possible should be captured in NASIS through the use of Pedon.

Sampling for full characterization or dynamic soil properties (630.01)

In some special projects, studies, and inventories, partial or full characterization sampling for lab analysis at the Kellogg Soil Survey Laboratory may be needed to establish classification of the soil and to assess characteristics of the soil that are of interest. The Soil Survey Laboratory Methods Manual and the Soil Survey Laboratory Information Manual outline sampling methodology for collecting samples to be submitted and analyzed. When the sampling is done, it is important to choose a representative site, complete a full pedon description of the soil and enter it into NASIS, and follow the guidelines in the lab manual to ensure that samples collected are representative and will yield all necessary data.

The resource soil scientist must be knowledgeable of the soil properties that vary under different uses and under different management systems. If there is uncertainty regarding what properties may be affected by use and management, guidance can be obtained from staff at the National Soil Survey Center (NSSC). Obtaining such guidance is important in ensuring that samples are collected under representative conditions and are sensitive to the problem at hand.

If your project, study, or inventory involves sites with a variety of uses (farmland, woodland, grassland, etc.), you may want to use sampling methodology developed for evaluation of soil change or soil quality.

Use of geophysical methods (630.02)

Ground-penetrating radar has been used to evaluate the taxonomic composition of soil map units and the depth to bedrock or restrictive layers. This technology has also been used to develop bathymetric surveys of reservoirs, chart groundwater flow patterns in coarse textured soils, detect human burial sites, and detect buried ordnance.

Electromagnetic induction has been used for high-intensity soil surveys, precision agriculture, salinity and nutrient loading appraisals, and wetland and ecological site assessments and to confirm the structural integrity of earthen structures. This equipment is available in several States and at the NSSC. Technical assistance is available from the NSSC for local projects, and training can be scheduled. Also see TSSH Part 621 (“Soils and Cultural Resources”).

Long-term soil data collection (630.03)

Long-term data collection identifies changes in the resource through the orderly collection, analysis, and interpretation of quantitative data. It must be conducted over time at permanently marked locations and include baseline data if it is to ascertain the trend of the change in the functional status of the resource. Long-term data collection is often designed so that measurements can be made consistently by more than one observer. Reference data or standards may be used to establish management goals and aid in interpretation of the monitoring results.

Long-term soil data may be needed to assess changes in soil properties, soil health, and soil use over time. These data are usually collected as part of a special project, such as a soil change assessment, a wetland assessment, soil formation and morphology studies, and studies of program effects on soil quality and soil health. Note that dynamic soil properties are determined by using the concept of “substituting space for time,” thereby allowing the determination of the impacts of a change agent, such as land management, without monitoring over extended time periods.

Site selection for data collection depends primarily on the objectives, which include:

  • evaluation and documentation of the progress toward management goals,
  • detection of changes that may be an early warning of future degradation, and
  • determination of the trend for areas in desired condition, at risk, or with potential for recovery.

If the objective is to determine progress or trend, the sites that are representative of a management unit should be selected. If the objective is to provide an opportunity to modify management before degradation occurs, the sites that are most vulnerable should be selected. The changes must be measurable and must occur rapidly enough for land managers to correct problems before undesired and perhaps irreversible loss of soil quality occurs. The long-term data collection plan should include the proper measurement frequency, which either limits or captures seasonal variability, as dictated by the objectives.

Other sections of this handbook may provide additional information on a specific area of interest.

Monitoring water tables and anaerobic conditions (630.04)

One of the most important characteristics assessed using soil morphology is a seasonal high water table. This information is frequently used in the design of farm practices, septic systems, and buildings and in the creation and restoration of wetlands. It is also important in the accurate identification of hydric soils for the Farm Bill and Clean Water Act. A technology note on the installation of monitoring wells should be followed when instrumentation for evaluating water table depth and movement is installed. The hydric soils technical standard provides information on the evaluation of anaerobic conditions in soil. See also TSSH Part 617 (“Water Table Determinations”) and Part 618 (“Determinations and Delineations of Hydric Soils”).

Monitoring soil moisture (630.05)

Plants, in most cases, depend upon soil water for the majority of their moisture requirements. Soil water is a major limiting factor for plant distribution, plant growth, and ecosystem productivity in arid and semiarid ecosystems. Soil water storage is dependent upon the amount of precipitation, water intake rates, and storage capacity of soils. The soil receives moisture from onsite rainfall and snowmelt; minor amounts of moisture are supplied by dew and fog.

Rainfall and snowmelt are natural sources of soil water and are normally greatly reduced during periods of drought. Slope shape, slope gradient, and surface roughness can affect soil water content: surface or subsurface run-on from adjacent upslope sites can increase soil moisture, and surface runoff can remove water from a site. Evaporation, plant transpiration, and deep percolation beyond rooting depth are other factors that deplete soil moisture.

The best plant growth occurs when the soil water content remains in the upper half of the plant-available soil moisture range. All plants are under stress when soil moisture is at wilting point or is drier. Rangeland plants use various methods to survive when soil moisture drops below the wilting point; for example, some plants go dormant or utilize water stored in plant tissues. The amount of total soil water content at any soil water tension value varies by soil type, depending on soil texture, organic matter content, and structure. Some soil moisture measurement techniques and instruments measure total soil water content and must be converted to plant-available water content to determine rangeland plant stress.

Visual estimation of soil moisture can be used as a rough initial approximation. Information on estimating soil moisture is available in the Soil Survey Manual, chapter 3.2. Soil moisture measurements should be taken at a minimum of three sample locations at each monitoring site. Sample locations should have similar soil types, vegetation, aspect, slope, and elevation and should not be adjacent to a road or trail. Sampling or measurements within a watershed should be taken on the same day to be comparable. If a large difference in moisture content occurs between samples, it would be advisable to sample an extra location or two. Avoid sampling small areas of low spots or ridges that do not represent the majority of the area of concern. A soil map may be valuable in determining a major change in soil characteristics, such as texture, which will affect soil moisture measurements.

Measurements should be taken or samples collected at a depth of 10 to 12 inches below the soil surface. Use a soil auger to excavate down to the desired depth and collect the soil sample, or insert the soil moisture probe into the soil at that depth. A second measurement can be taken at a depth of 2 feet, and even a third measurement could be taken at a depth of 3 feet if it is appropriate to measure soil moisture at a deeper rooting depth. The majority of roots of most rangeland plant species are within the top 12 to 24 inches of the soil. Taproots of some rangeland plant species extend down more than 10 feet, but it is impractical to measure soil moisture below a depth of 3 feet on a widespread basis. Also, plants typically extract about 40 percent of their water needs from the top quarter of the root zone, 30 percent from the next quarter, 20 percent from the third quarter, and only 10 percent from the deepest quarter. Measurements to determine potential seeding success should be taken at a depth of 0 to 3 inches. Soil samples for the gravimetric method need to be placed immediately in resealable plastic bags or other sealed, waterproof containers to avoid changes in sample moisture.

When the data are reported, it is important to note the approximate percentage of rock fragments in the layer from which the sample or moisture reading was collected. Rock fragments can make a large difference in the amount of water available to plants.