Soil sampling and tests

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There are hundreds of soil tests that can be conducted, both in the field or laboratory. This page provides an overview of more common soil tests, links to information on sampling, and links to test methods.

Information: Soil sampling should be conducted by trained and, where appropriate, certified professionals, such as licensed soil scientists and geoscientists

Information: Laboratory tests should be done by certified laboratories. The Minnesota Department of Health Environmental Laboratory Accreditation Program develops procedures and requirements to ensure accredited laboratories produce accurate and precise test results. Search for an accredited lab.

Information: Reference or links to any specific commercial product, process, or service by trade name, trademark, service mark, manufacturer, or otherwise does not constitute or imply endorsement, recommendation, or favoring by the Minnesota Pollution Control Agency.

Sample collection

Soil sample collection methods vary and covering all acceptable methods is beyond the scope of this page. Below are links to sampling methods, including videos.

Sampling for chemical tests

Note that these references provide information on soil sample collection. Except where noted, they do not include field procedures associated with specific tests and most do not include information on quality assurance and quality control (QA/QC). Use professional, certified/licensed individuals or firms to ensure appropriate QA/QC procedures are followed.

Documents

• US EPA - focus on VOCs and PFAS
• Testing Your Soil Why and How to Take a Soil-Test Sample - University of Hawaii
• Sampling Soils for Nutrient Management - USDA-NRCS; focus on phosphorus
• Procedure for taking soil samples - Stephen F. Austin State University
• Soil Sampling Testing - USDA-NRCS
• Soil Sampling Guidelines - Purdue University
• Composite Soil Sampling - MCSE - Kellogg Biological Station
• Soil sample collection and analysis procedures - MPCA Petroleum Remediation Program
• Sampling and Analysis Plan Midland Plant and Salzburg Landfill
• Soil Sampling Quality Assurance User's Guide - US EPA

Videos of sample collection for lab analysis

• How to Collect a Soil Sample for Lab Analysis
• Collecting Soil Samples: Part 1 - Tools, Part 2 - Sample collection
• How to Take a Soil Sample

Chemical tests

Laboratory tests

Below is a list of recommended laboratory tests.

Recommended holding times and preservation

• [1]
• [2]
• SW-846 inorganics
• SW-846 organics
• [3]

Nutrients

Soil macronutrients include phosphorus, nitrogen, potassium, sulfur, calcium, and magnesium. Phosphorus is an important pollutant of concern in surface water, particularly lakes. Though there are several forms of phosphorus, they can roughly be divided into dissolved phosphorus (includes orthophosphate and soluble phosphorus) and particulate phosphorus, with dissolved phosphorus being much more bioavailable than particulate forms. Dissolved phosphorus is typically identified as phosphorus passing through a 0.45 micron filter. For a detailed discussion of phosphorus, link here.

Nitrogen is also an important nutrient in both surface water and groundwater. Nitrogen concentrations in stormwater are typically below levels of concern for receiving waters.

Potassium, sulfur, calcium, and magnesium are typically not pollutants of concern in stormwater runoff, but they may be deficient in some soils and therefore potentially impact vegetation.

• Phosphorus
  • Bray Method: use on acidic and neutral soils
  • Olsen Method: preferred on high pH soils
  • Mehlich-3 Method: provides a better indicator of plant-available phosphorus
• Nitrogen
  • Soil Nitrate And Extractable Ammonium By Flow Injection Analyzer Method (cadmium reduction method)
  • Total Kjeldahl nitrogen (wet oxidation method)
  • Total Nitrogen And Carbon - Combustion Method (Dumas Method)]
• Potassium, Sodium, Calcium, Magnesium, Boron and sulfate-Sulfur
  • Inductively Coupled Plasma Emission Spectrometry (ICP-AES) - recommended due to lower detection limits, but more expensive
  • Atomic absorption spectroscopy

Metals

The primary sources of metals in stormwater runoff are associated with automobiles, both from fluids and wear of parts, including tires. Concentrations of metals in stormwater runoff are generally below aquatic life and drinking water criteria, though concentrations may exceed criteria for sensitive species and in specific land uses, such as high traffic transportation areas. Metals of greatest concern include copper, zinc, nickel, cadmium, and lead.

Samples are typically collected for total metals, meaning samples are not filtered. For dissolved metal concentrations, samples are filtered using a 0.45 micron filter. From an environmental perspective, dissolved metal concentrations more accurately reflect potential risk to receptors, since most metal bound to particles is retained in stormwater bmps. Lab methods include the following.

• Inductively Coupled Plasma Emission Spectrometry (ICP-AES) - recommended due to lower detection limits, but more expensive
• Atomic absorption spectroscopy

pH

Soil pH typically ranges from 6 to 8. Soils with elevated organic matter concentrations may have lower pH. Soil pH affects biologic activity and chemical reactions, particularly of some metals. Soil pH is generally not a concern, though some amendments, such as lime (increases pH), may lead to soil pH values that adversely affect soil biology, vegetation, mobilize metals, or bind up nutrients. Recommended lab methods include the following.

• Saturated paste - recommended
• 1:1 and 2:1 water ratios

Organic matter and carbon

By itself, organic matter is not generally a pollutant of concern unless it contains bound pollutants at levels of concern (e.g. metals, organic pollutants such as oil and pesticides). Organic matter can create oxygen demand in receiving waters and, as mentioned above, transport attached chemicals that may become a concern in receiving waters, including nutrients, metals , and organic pollutants. Organic matter also provides a food source for bacteria and pathogens.

• organic carbon combustion
• organic matter Walkley-Black
• Organic matter loss on ignition - recommended due to ease of use and since organic matter is more commonly used compared to organic carbon

Exchange capacity

Exchange capacity affects the fate of other soil chemicals, including nutrients and pollutants, and provides a buffer against soil acidification.

• Cation exchange capacity - multiple methods are available for cation exchange capacity. For more information read here.
  • Barium chloride Compulsive Exchange Method - recommended but is time consuming and generates a hazardous waste
  • Ammonium Acetate Method - acceptable if soil pH is near 7.0
  • Agronomic Soil Tests - estimates CEC from test extractable Ca, K, and Mg and some rapid measure of exchangeable acidity (see next bullet)
• Exchangeable Potassium, Calcium, Magnesium, Sodium And Estimated Cation Exchange Capacity

Enzyme activity

Enzymes in soil mediate numerous chemical reactions involved in soil nutrient cycling, transformation of plant and microbial debris, mineralization and transformation of organic matter within the carbon cycle, and transformation and degradation of potentially hazardous pollutants.

• Fluorescence assays - synthetic C-, N-, or P-rich substrates bound with a fluorescent dye are added to soil samples. When intact, the labeled substrates do not fluoresce. Enzyme activity is measured as the increase in fluorescence as the fluorescent dyes are cleaved from their substrates, which allows them to fluoresce. Enzyme measurements can be expressed in units of molarity or activity. See this article and associated video for a description of the method.
• Spectrophotometric assays - the course of the enzymatic reaction is followed by measuring a change in how much light the assay solution absorbs
• Calorimetric assays - measure the heat released or absorbed by chemical reactions. These assays are very general.
• Radiometric assays - measure the incorporation of radioactivity into substrates or its release from substrates. These assays are both extremely sensitive and specific.
• Chromatographic assays - measure product formation by separating the reaction mixture into its components by chromatography. This is usually done by high-performance liquid chromatography (HPLC), but can also use thin layer chromatography.

Specific recommended procedures are not provided as there is a wide range of methods depending on objectives of the sampling. This video provides a discussion of enzymes and soil enzymes, including sample collection and measurement (starting at about the 39 minute mark). This website provides a discussion of soil enzymes including limitations of testing methods. Additional references include the following.

• Soil enzyme - Ward Laboratory
• Emerging tools for measuring and modeling the in situ activity of soil extracellular enzymes

Tests for soil physical properties

Most soil physical soil tests can be done in the field, but some require additional procedures performed in the laboratory.

Soil water (moisture) content

Laboratory analysis of soil water content is recommended for point-in-time measurements. Lab methods involve weighing a soil sample prior to drying, then drying to constant weight in oven at temperature between 100–110^oC (105^oC is typical). The difference in weight represents the mass of water in the sample. The water content is then expressed on a mass basis (g of water to g of dry soil), or if the bulk density is known, the volume of water to volume of soil. It is important that samples collected in the field be properly stored to avoid water loss prior to analysis. For further reading see [4].

For continuous measurements, field methods must be employed. Field methods are summarized below. The most common methods are electrical resistance (e.g. time domain reflectometry), tensiometric, and radioactive (e.g. neutron probe). This document and this document provide discussions of methods for measuring soil water content. This one hour video provides an overview of soil water measurement.

• Electrical-resistance
  • Video
  • Video Time Domain Reflectometry
  • Electrical Methods of Soil Moisture Measurement: A Review
• Heat-diffusion
• Absorption
• Tensiometric
  • Video
  • Soil Moisture Tensiometer Materials and Construction
• Penetration
• Radioactive
  • Video
  • Video neutron probe

Bulk density

Soil bulk density is an important measurement for determining soil infiltration and plant rooting properties. Measuring bulk density involves proper sample collection and laboratory analysis. Below are links to videos demonstrating methods for collecting bulk density samples.

• Bulk density and soil water (moisture) content - a 12 minute video illustrating how to collect the sample in field and measure in the lab using the core method
• Bulk density and soil water (moisture) content - a 10 minute video using the core method
• Bulk density and soil water (moisture) content - this 34 minute video describes the clod method, which is preferred but more time consuming and cannot be used for soils where intact clods cannot be collected

Methods for measuring bulk density are provided in the following documents.

• Bulk Density – Measurement
• Bulk Density - UC Davis

Infiltration rate

Infiltration rates should be measured in the field. This page provides information on measuring soil infiltration rates.

Videos illustrating measurement of infiltration rates.

• Modified Philip-Dunne permeameter
• Guelph permeameter
• Double ring infiltrometer
• Double ring infiltraometer

Compaction (penetration resistance)

Soil compaction results from repeated traffic, generally from machinery, or repeated tillage at the same depth, which results in a compacted layer at the tillage depth. Compaction inhibits infiltration, gas and water movement, may impede root growth, disrupts habitat for soil biota, and affects nutrient cycling. There are several field methods for determining soil compaction or penetration resistance.

• Sand Cone Method: The sand cone test is a relatively simple test that doesn't take long to perform, but does require drying soil samples. It is not suitable for saturated and soft soils. The method consists of digging a hole in a soil, determining the volume of the hole by filling the whole with sand of known dry density, determining the dry density of the soil removed from the whole, and computing percent compaction. The procedure is described in this video. This worksheet can be used to make the calculations. Calculations and procedures are also shown on this website.
• Rubber balloon method: Rubber Balloon Density test are similar to the sand cone method. A balloon density apparatus is positioned over the hole, and instead of using sand to measure volume, the calibrated water vessel is pressurized, forcing a rubber membrane into the excavation. Graduations on the vessel are read to determine the amount of water displaced so the whole volume can be calculated. The test method is described in ASTM D2167 / AASHTO T 205 (withdrawn). The tests simpler to perform than the sand cone and can be repeated quickly since the water is retained in the vessel.
• Penetrometers: Penetrometers measure the force needed to push a metal rod of known diameter into a growing medium. They may be hand operated or machine driven. The cone penetrometer, the most commonly used penetrometer, simulates a root growing through the soil. A hand operated unit is pushed into a soil and a gauge on the device measures the amount of force needed to penetrate the soil. This video demonstrates how to use a portable penetrometer in the field. While easy to use, there are some limitations. Measurements should be made when the soil is near field capacity. The device may have limitations in granular soils, clay soils, and soils with sharp boundary layers. For more information, including procedure descriptions and equipment needs, see [5], [6], and [7].
• Nuclear test: Nuclear density gauges determine soil density by measuring gamma radiation transmission between a probe containing a radioactive Cesium 137 (or other) source and detection sensors in the base of the gauge. Dense soils allow fewer gamma particles to be detected in a given time period. Soil moisture is measured at the same time. Nuclear density gauges are efficient on large projects requiring rapid results and multiple tests but are subject to regulatory requirements and require advanced training and radiation dosage monitoring of personnel. Test methods are described in ASTM D6938 / AASHTO T 310.

For additional information on measuring soil compaction, see [8], [9], and [10].

Aggregate stability

Non-granular soils (e.g. clays) form aggregates that are important in maintaining soil physical, chemical, and biologic processes. Methods for assessing aggregate stability are somewhat qualitative and different methods do not correlate well. The method selected should simulate field processes likely to affect aggregate stability (e.g. rainfall impact, ponded (flooded) conditions, tillage).

• Sieve or strainer method: The most commonly used method for testing aggregate stability involves placing a soil sample on a nest of soil sieves with screen sizes typically ranging from 1 mm to 45 microns and then moving this nest of sieves up and down in a bucket of water. The more stable aggregates will stay on the top sieve, while less stable aggregates will move through the larger sieves to the finer sieves. Soil stability is assessed by measuring the mass of soil remaining, as a percent of initial soil mass, on the sieve after a specified number of dipping cycles (e.g. 5 cycles). Soils with high aggregate stability will typically retain 50 percent or more of the initial soil mass. This method may represent stability under rainfall.
• Slake test: Dried aggregates are placed into a container filled with water. Assess the aggregates after specified times (e.g. 20 minutes, 2 hours, 20 hours). This method may represent stability under flooded (water immersed) conditions.
• Vibration methods: An ultrasonic probe immersed in water containing soil aggregates vibrates at different vibration amplitudes. Soil stability is assessed after specific time intervals. This method may represent stability under tillage conditions.

Further reading

• Field soil aggregate stability kit for soil quality and rangeland health assessments
• How is soil aggregate stability tested?
• Assessing soil aggregate stability
• Evaluation of methods for determining soil aggregate stability
• Characterization of soil aggregate stability using low intensity ultrasonic vibrations
• Soil aggregate stability

Videos

• [11]
• [12]
• [13]

Texture

Soil texture is determined with one of the following methods.

• Mechanical sieving, if particle size > 0.05 mm
• Sedimentation if size < 0.05 mm. Sedimentation measures the settling rate of particles in liquid medium and relates this rate to the particle mass by use of the Stoke's Law. Forces acting on soil particle are gravitation, buoyancy and drag forces, all of which depend on particle size. Larger particles settle first. The particle mass is determined by density and particle size. Soils must be dispersed prior to measurement. Two methods are commonly used.
  • Hydrometer method (see here for description)
  • Pipette method (see this video for a description)

Other methods, which employ qualitative approaches, include the feel method, ball and ribbon methods, and ball throwing method. These are described here.