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Minnesota Stormwater Manual

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.

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

Videos of sample collection for lab analysis

Laboratory tests

Below is a list of recommended laboratory tests.

Recommended holding times and preservation

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 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 potential 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

Exchange capacity

Field methods

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 100–110oC (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.

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

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

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].

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