Soil Science

Soil Sampling and Testing

Soil sampling and testing are required for physical, geotechnical, chemical, nutrient, water (moisture), and contaminant analysis.

Sampling can be for analysis at the site or in the lab and can be for composite "disturbed" samples or "undisturbed" samples where soil layer integrity for geotechnical, moisture or environmental contaminant analysis is important.

Soil chemistry changes with time, as biological and chemical processes break down or combine compounds over time. As a result, soil sample storage, handling and moving can affect chemical composition accuracy and can change after the sample is removed from its natural ecosystem (temperature, moisture, solar).

Soil testing can be done in place with real time logging and data transmissions (in-situ) of physical parameters, or samples can be collected and transported to a lab for analysis. Proper sample preservation and collection procedures are essential for accurate analysis and depend significantly on the parameters being tested for. Typical tests involve parameters such as: shear, penetration, permeability, compaction, specific gravity, moisture content, grain size and Attenberg (liquid, plastic, shrinkage) limits.

Soil samplers are used to obtain soil or sediments in the vadose (shallow) zone, which is also called the unsaturated zone.

The unsaturated zone is the portion of the subsurface above the groundwater table. The soil and rock in this zone contains air as well as water in its pores. The vadose zone extends from the top of the ground surface to the water table. Water in the vadose has a pressure head less than atmospheric pressure, and is retained by a combination of adhesion and capillary action. The water in the vadose zone is termed soil moisture. In fine grained soils, capillary action can cause the pores of the soil to be fully saturated above the water table at a pressure less than atmospheric. The movement of water within the vadose zone is studied within soil physics and hydrology, and particularly hydrogeology.

The unsaturated (vadose) zone is of great importance in providing water and nutrients that are vital to agriculture, groundwater aquifer recharge and flood control, as well as significantly affecting contaminant transport and the soil's physical and geotechnical characteristics.

Hydrologically, the unsaturated zone is often the main factor controlling water movement from the land surface to the aquifer. Thus it strongly affects the rate of aquifer recharge, critical for the use and management of groundwater. It is often regarded as a filter that removes undesirable substances.

Soil core sampler types include screw-type augers, barrel augers and tube-type samplers. They can be hand or power operated, mechanically or hydrologically driven or drilled. Power augers can be hand held and driven by a small, lawn mower type engine or electric drill type motor. As the auger rotates into the soil, cuttings advance up the flights and up to the surface. The stem of the flighted auger can be solid or hollow stem. Samples can be collected from the bottom of the borehole by other methods or though the center of the hollow stem flighted auger. Power driven flighted augers can drill to depths of 50 ft (15 m) to 150 ft (45 m) easily. Often a combination of sampler types can be used together to obtain the sample required such as drilling with a rotating barrel-type auger to just above the required depth and then using a tube-type auger inserted to the bottom of the borehole without disturbing the sides of the borehole.

Screw-Type Augers

Screw-type, also called spiral and flighted or continuous flight, augers are hand or power operated spiral augers that can be used with extensions and a handle to rotate the auger to drill into the soil. They also include powered flighted and hollowstem augers that can be then used with open tube samplers for undisturbed soil samples. These screw-type augers are advanced to the full depth required and then removed with the soil from the deepest penetration being retained on the auger flights. These augers operate better in wet, cohesive soils than in dry samples. Very dry soils will typically not be retained on the auger flights as the unit is withdrawn.

Tube-Type

Tube-type samplers generally have smaller diameter and longer body lengths than barrel-type augers, sampling with these units requires driving the sampler (pushing) into the soil. Tube-type samplers are not rotated to take a sample like augers but are pushed straight down and when the tube is filled at each depth it is pulled back to the surface and the sample extracted. Both mechanical (drop hammers) and hydraulic (leverage) pushing units are available and vibratory heads both mechanical and sonic. Tube-type samplers do not work well in compacted, gravel or sandy (uncohesive) soils but do allow for more relatively undisturbed samples than the rotating augers do. The tube-type samplers do experience some compaction and so shorter tubes provide less disturbed samples than longer tubes. Tube-type samplers include soil sampling tubes, Veihmeyer or King tubes, thin-walled (Shelby) tubes, split barrel, ring lined barrel sampler, piston samplers and zero contamination samplers. They can be open sided or closed tubes. Tube samplers allow soil layer characterization (lithology), less exposure to air for better VOC sampling, especially when used with liners. When combined with an auger, manually driven tube samplers can be extracted from depths up to 20 feet in unconsolidated sediments. Power driven units can retrieve samples much deeper.

Barrel-Type Augers

Barrel augers are the most common for environmental samples and include Edelman Dutch-type (clay, mud, peat), regular (general purpose), sand, mud and clay type augers. The regular auger is best suited for loamy soil, while the sand auger works well in dry soils and the mud for wet clay type soils. Also, specialized riverside and stony soil augers, stone catching and planer augers are available for soft soil. Barrel augers consist of a bit with cutting faces welded to the end of a short tube. The Dutch-type consists of strap steel twisted and forged into an auger which also retains a sample which is then removed from the "barrel." Extension rods can be used to reach to the desired sampling depth. These augers can also be used with liners in a variety of materials and are available in stainless steel for environmental samples. Barrel augers are able to provide larger samples than screw-type augers and are made more efficient. The Dutch-type auger works best in clay and peat.

Soil Water Sampling and Monitoring

Monitoring soil moisture is an important component to measure or sample for geotechnical parameters such as permeability, hydraulic conductivity, erosion rates, leaching and drainage efficiencies, soil sorptivity, soil matrix potentials, and slope stability to name a few. Soil contamination determination also requires soil moisture monitoring and sampling to determine the concentration of contaminants as well as potential transports through the vadose zone as well as aquifer contamination potential. Agriculture and ecosystem moisture requirements as well as chemical composition, pH and other important parameters require sampling and monitoring.

Soil moisture sensors, lysimeters, and tensiometers (irrometers) are used to monitor soil moisture for use in hydrology studies. Monitoring soil moisture can assist in watershed runoff studies and determining optimum irrigation rates. Instruments are often left installed as a permanent station, but some are portable and can give readings within minutes at any location. These instruments can be categorized as either sampling or in-situ monitoring and the monitoring can be further divided into measuring soil moisture which is volumetric water content (VWC) by capacitance-type sensors or soil water tension and soil water potential which is measured in centibars (cb) and kilopascals (kPa).

Soil Moisture Sensors

Volumetric water content (VWC) sensors measure soil moisture content and use a capacitance type sensing element that measures the dielectric constant of the soil and operates with frequent domain reflectometry (FDR). VWC sensors minimize salinity and soil textural effects so that accurate data can be obtained in most all soils. Air and water occupy the soils pore space and VWC is the ration of the volume of water to the overall volume of soil.

Time domain reflectometry (TDR) sensors function similarly to the FDR sensors but send a pulse down into the soil which is terminated at the end of a probe using wave guides. The TDR sensor measures the water content based upon the time it takes the pulse to return along the probe. TDR probes require precise calibration and are sensitive to soil salinity.

Gypsum block sensors (resistive type) use two electrodes embedded in small blocks of gypsum to measure soil water tension. Soil moisture is determined by electrical resistance between the electrodes. The sensor is relatively inexpensive but has to be replaced periodically as the gypsum deteriorates and is sensitive to high salinity soil.

Soil Water Potential Sensors

A tensiometer soil water potential sensors or just tensiometers, are the only instruments that can directly measure "soil suction" - the force that determines moisture flow. Each instrument consists of a sealed water filled chamber equipped with a special vacuum gage and a porous tip that is installed in the ground at desired depths. In dry soils, water is drawn out of the instrument which creates a vacuum that registers on a gage. Irrigation reverses this action, as water is drawn back into the instrument. An irrometer is also a tensiometer but is a specific brand. A function of the tensiometer is designed to show how hard a plant is working to obtain moisture for growth and health.

Dielectric water potential sensors (DWP) measure a wide range of soil water potentials with minimal maintenance and user involvement unlike the skill and time required with tensiometers. The DWP sensor is placed in a hole, hooked up to a data logger and left to record data. Common uses are for water potential monitoring in the vadose zone, deficit irrigation monitoring and control and waste water drainage studies.

Soil Water Sampling

Soil water sampling is best accomplished with a lysimeter which is a device for collecting water from the pore spaces of soil and determining the soluble constituents in the pore water. Large pan lysimeters collect soil water as it percolates down through saturated soils by gravity. Most commonly the suction pressure lysimeter is used allowing samples to be extracted from unsaturated soils at most any depth required in a relatively short time. The porous (usually ceramic) interface material allows the transport of soil water without contamination or leaking. Ceramic cups are the visual sampling configuration with a tube attached for collection of the pore soil water.