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Minnesota Stormwater Manual

Soil sampling and tests
Revision as of 14:27, 11 January 2023 by Mtrojan (talk | contribs) (→‎Tests for soil physical and hydrologic properties)

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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

image of sampling rig
Example of direct push technology used to collect a soil sample.

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.

Documents

Videos of sample collection for lab analysis

Chemical tests

Laboratory tests

Below is a list of recommended laboratory tests.

Recommended holding times and preservation

Nutrients

For more information link here.

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.

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.

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.

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.

Exchange capacity

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

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.

Field methods

Field methods for soil chemical testing are generally not recommended, though they can be useful in providing general information. Typically field tests involve the use of test strips or probes. Portable laboratories can be used to conduct some of the analyses described above. Although these tests are conducted in the field, they utilize laboratory methods and are therefore more appropriately considered lab tests.

Here are some links to information on soil chemical field tests.

Tests for soil physical and hydrologic properties

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

Soil water (moisture) content

photo of soil tensiometer
Example of a tensiometer's vacuum gauge. Credit: Tina Dispenza, University of Florida/IFAS

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 100–110oC (105oC 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.

Available water capacity

Available water capacity is the maximum amount of plant available water a soil can provide, often calculated as water available between field capacity and the wilting point.

This link provides a discussion of methods for determining available water capacity.

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.

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

Infiltration rate

image of MPD infiltrometer
The modified Philip-Dunne infiltrometer, as well as other direct infiltration measurements such as double ring infiltrometers, are preferred methods for measuring infiltration rates, compared to soil borings and pits. Image: University of Minnesota, St. Anthony Falls Laboratory.

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

Videos illustrating measurement of infiltration rates.

Compaction (penetration resistance)

photo of cone penetrometer
Photo of cone penetrometer

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. Reference.
  • 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. These videos ([5], [6], [7]) demonstrate how to use penetrometers 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 [8], [9], and [10].
  • 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 [11], [12], and [13].

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

Videos

Texture

image of soil sieves
Soil sieves used to determine soil texture. Creative Commons Attribution-Share Alike 4.0 International

Soil texture is determined with one of the following methods.

  • Mechanical sieving, if particle size > 0.05 mm (Reference)
  • 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.

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

Soil structure

Measurement of soil structure typically focuses on determining soil stability and texture. See the methods above for measuring these properties. Additional references and links are provided below.

Surface crusting

Structural soil crusts are relatively thin, dense, somewhat continuous layers of non-aggregated soil particles on the surface of tilled and exposed soils. A surface crust indicates poor infiltration, a problematical seedbed, and reduced air exchange between the soil and atmosphere. It can also indicate that a soil has a high sodium content that increases soil dispersion when it is wetted by rainfall or irrigation. Tests typically are based on applying pressure to determine the strength of a crust, or methods to determine stability of the surface crust.

Below are resources for measuring soil crusting and strength of crusts.

Macroporosity

In soils with significant macroporosity, a high percentage of annual water and solute movement can occur within the macropores. This can lead to rapid movement of solutes, including pollutants of concern, deep into the soil profile and eventually into groundwater. Soil macroporosity can be measured directly in the field by physically excavating soil, typically in layers, and quantifying soil macropores. Colored or fluorescent dyes are often used, as well as imaging equipment. Examples of this methodology are provided at the following links.

Macroporosity can also be estimated using indirect methods, such as those based on bulk density, water content, particle size distribution, and infiltration. Examples are provided at the following links.

Biotic diversity

There are currently two types of tests that can be performed to determine soil diversity: Tests that analyze and classify parts of the microbial material itself (‘taxonomic tests’), and tests that look at the metabolic products of microbes (‘functional tests’). Below are links to websites and videos that provide information on testing for biotic diversity, including testing methods.

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