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Minnesota Stormwater Manual

Soil chemical properties and processes
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Overview of soil physical properties and associated activities affecting soil physical properties and processes. Click on links to go to a specific section.
Property Effects Desired value Management strategies
Phosphorus
Nitrogen
Major cations
Sulfur
Trace metals
Cation exchange capacity
Electrical conductivity
Organic matter (carbon)
Base saturation
Enzymes
Salinity
Sodium adsorption ratio
pH

Soil chemical properties include concentrations of specific chemicals (e.g. phosphorus, nitrogen, carbon, major cations (calcium, magnesium, sodium, potassium), sulfur, trace metals and elements), pH, cation exchange capacity, base saturation, salinity, sodium adsorption ratio, enzymes, and electrical conductivity. These properties affect processes such as nutrient cycling, biologic activity, soil formation, pollutant fate, and erosion.

This page provides an overview of soil chemical properties, processes they affect, effects of human activities, discussion of stormwater applications, and links to related topics, including information on sampling, testing, and soil health assessments.

Soil chemical properties

Soil chemical properties discussed below include phosphorus, nitrogen, major cations, trace metals, cation exchange capacity, electrical conductivity, enzymes, organic matter and carbon, base saturation, salinity, sodium adsorption ration, and pH.

Phosphorus

Phosphorus constitutes about 0.2 percent of a plant’s dry weight, where it is primarily a component of tissue molecules such as nucleic acids, phospholipids, and adenosine triphosphate (ATP). Along with nitrogen, phosphorus is often a limiting nutrient in soil. Soils limited in phosphorus reduce plant growth and development, while excess phosphorus can be exported from soil and enter freshwater bodies.

Approximately 30 to 65 percent of total soil phosphorus is in organic forms and the remaining 35 to 70 percent in inorganic forms. Soil microorganisms play a key role in processing and transforming organic forms into plant available forms. Inorganic phosphorus forms include the following:

  • plant-available phosphorus, comprised of inorganic phosphorus dissolved in soil water;
  • phosphorus attached (sorbed) to clay surfaces, iron (Fe), aluminum (Al), and calcium (Ca) oxides in soil, which can be released slowly for plant uptake; and
  • mineral phosphorus (e.g. apatite), which is very slowly released.

Phosphorus is typically measured in a laboratory with one of the following methods.

Phosphorus can also be determined in the field with appropriate equipment or with test strips. Test strips are less accurate but may be suitable for identifying phosphorus deficiencies.

The soil phosphorus cycle is somewhat complicated since phosphorus is affected by soil mineralogy and chemistry and by soil biotic processes.

  • Organic matter: Phosphorus availability generally increases with increasing soil organic matter since phosphorus is released through mineralization of organic matter. Fresh (non-composted) organic sources have greater amounts of available phosphorus.
  • Clay: Soils with high clay content have high phosphorus retention capacity.
  • Soil Mineralogy: Soils with high concentrations of aluminum, iron, and calcium have high phosphorus retention capacity.
  • Soil pH: Optimum soil pH between 6 and 7 will result in maximum phosphorus availability. At low pH (acidic soils), soils have greater amounts of aluminum and iron, which form very strong bonds with phosphate. At high pH phosphate tends to precipitate with calcium.
  • Temperature, moisture, and soil aeration can affect the rate of P mineralization from organic matter decomposition.

Stormwater application Stormwater runoff typically contains about 0.2-0.6 mg/L of total phosphorus, though this varies with land use and season. Particulate phosphorus accounts for about 50-75% of total phosphorus. Dissolved forms are more bioavailable. Phosphorus is not limiting in most stormwater applications. In many applications where engineered media are employed, phosphorus represents a risk for surface receiving waters. Link here for more information.

Additional reading

Nitrogen

Phosphorus constitutes about 1 percent of a plant’s dry weight, where it plays a role in the growth of tissues and cells found within the plant, as well as the formation of chlorophyll. Along with phosphorus, nitrogen is often a limiting nutrient in soil. Soils limited in nitrogen reduce plant growth and development, while excess nitrogen can be exported from soil and enter freshwater bodies, including groundwater.

Nitrogen occurs in both organic and inorganic forms in soil, with ammonium (NH4+) and nitrate (NO3-) being the dominant inorganic forms.

Measurement of nitrogen in soils depends on the form being measured.

The nitrogen cycle in soil is somewhat complex, with the following important processes (Plant and Soil Sciences e-library).

  • Fixation is the process of converting dinitrogen gas (N2) to chemically reactive forms. Bacteria can convert nitrogen to organic forms through fixation. Fixation can occur either in free-living organisms or symbiotically in association with legumes.
  • Once nitrogen is fixed, microorganisms can convert organic forms to inorganic forms through mineralization. This often is a two-step process where proteins are converted to simple compounds and then to ammonium.
  • Nitrification is a two-step process in which microbes convert ammonium to nitrite and then nitrate. Nitrite is toxic to plants, but it is typically rapidly converted to nitrate, which is a plant available form.
  • Denitrification is a process in which microbes convert nitrate to nitrogen gas. This occurs in the absence of oxygen.
  • Volatilization involves the conversion of urea to ammonia, which is lost as a gas. This typically occurs under moist and warm conditions.
  • Nitrate not taken up by plants is readily leached in aerated soils, eventually reaching groundwater, surface waters through baseflow, or being denitrified in the absence of oxygen.
  • Nitrogen can temporarily become unavailable through conversion to organic forms or assimilation in plant or animal tissue.

Stormwater application Stormwater runoff typically contains about 2-3 mg/L of total nitrogen, with roughly equal proportions of nitrate and reduced forms (ammonia and organic nitrogen). Nitrogen is not limiting in most stormwater applications and represents a relatively low risk for receiving waters in most situation. Link here for more information.

Additional reading

Major cations

Cations are positively charged elements. Major cations in soil include calcium (Ca2+), magnesium (Mg2+), sodium (Na+), and potassium (K+). These elements are utilized in smaller quantities than phosphorus or nitrogen, but calcium, magnesium, and potassium are essential plant nutrients since they are involved in a variety of plant functions and metabolic processes, while all four can have adverse effects on soil or plants.

These elements can be determined as part of a metal scan using Inductively Coupled Plasma Emission Spectrometry (ICP-AES), but less expensive methods are suitable if sampling is just for these major cations. These methods include ion chromatography, atomic absorption spectroscopy, and flame photometry.

While these cations are generally not limiting in soil and they rarely occur at toxic levels, they can compete with each other and affect the fate of other elements in soil. Examples include but are not limited to the following.

  • Excess calcium may affect the availability of phosphorus, potassium, magnesium, boron, copper, iron, or zinc.
  • Excess sodium can be toxic to plants and disperse clays, resulting in reduced infiltration and soil surface crusting.
  • Excess potassium can affect plant uptake of nutrients.
  • Excess magnesium can form soil crusting and negatively affect soil structure.

These elements are often considered in the context of soil cation exchange capacity (CEC), an important property affecting the fate of other chemicals in soil, including pollutants. For information on soil CEC, link here.

Stormwater application These elements are generally not a concern in stormwater applications and represent a low risk to receiving waters. An exception is sodium in stormwater management systems affected by road salt applications.

Additional reading

Sulfur

Sulfur is an essential nutrient and plays a major role in the formation of the proteins. It is taken up by plants in similar amounts as phosphorus. More then 90 percent of the sulfur in soil occurs in organic forms. Soil microbes mineralize this sulfur, converting it to sulfate, the form taken up by plants. Other sources of sulfur include minerals, atmospheric deposition, and in some cases fertilizer.

Sulfur concentrations in soil are determined in the laboratory. Several reliable methods exist, such as high-performance liquid chromatography and inductively coupled plasma atomic emission spectroscopy. Analysis methods, including digestion procedures, depend on what is being tested (e.g. elemental sulfur, organic sulfur, sulfate).

Stormwater application Sulfur is not a concern in most stormwater applications. Sulfur deficiencies may occur in well-drained sandy soils.

Additional reading

Trace metals

Trace metals include cadmium (Cd), chromium (Cr), copper (Cu), lead (Pb), manganese (Mn), mercury (Hg), molybdenum (Mo), nickel (Ni), selenium (Se), and zinc (Zn). Arsenic (As), boron, and selenium are metalloids but are usually included in discussions of metals in soil. Heavy metals refers to metals with a density of more than 7 g/cm3 (Pb, Cd, Ni, Hg, Cr). Minerals (rocks) are the primary source of most trace metals in soil.

Some trace metals are essential micronutrients (e.g. boron, zinc, manganese, copper, molybdenum). In higher concentrations, trace metals are environmental pollutants and may be toxic to plants and soil biota.

Soil 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.

The behavior of trace metals in soil varies widely with soil properties. Processes affecting metal behavior include dissolution/solubilization, precipitation, hydrolysis/pH, exchange/adsorption, oxidation-reduction (redox), and chelation. Below is a brief summary of primary factors affecting trace metals in soil.

  • Arsenic: iron (Fe) and aluminum (Al) contents, pH, redox potential, and competing anions (e.g. phosphorus)([1])
  • Boron: pH, organic matter, aluminum and iron ([2])
  • Cadmium: iron and manganese oxides, pH ([3])
  • Chromium: Iron and aluminum oxides, pH ([4])
  • Copper: organic matter ([5])
  • Lead: pH and complexation with organic matter ([6])
  • Manganese: redox, pH, organic matter ([7])
  • Mercury: pH and redox potential ([8])
  • Molybdenum: pH, organic matter ([9])
  • Nickel: iron and manganese oxides, organic matter ([10])
  • Selenium: iron oxides, pH, redox ([11])
  • Zinc: iron and manganese oxides, pH ([12])

Stormwater application Trace metals in stormwater runoff may exceed aquatic life standards for receiving waters, particularly in heavy transportation areas, industrial areas, and areas affected by contaminated soils. Metals of greatest concern include lead, arsenic, cadmium, and copper. Except for sandy soils with limited organic matter, most soils and engineered stormwater media effectively retain metals. There have been concerns about buildup of metals in soil and stormwater media receiving high inputs from runoff.

Additional reading

Cation exchange capacity

Cation exchange capacity (CEC) is the total capacity of a soil to hold exchangeable cations. It affects the fate of other soil chemicals, including nutrients and pollutants, and provides a buffer against soil acidification. Clay soils have a higher CEC than sandy soils, but soil organic matter has the greatest effect on a soil's CEC. The main ions associated with CEC are calcium (Ca2+), magnesium (Mg2+), sodium (Na+) and potassium (K+). In acidic soils, aluminum (Al3+) and manganese (Mn2+) are important.

Units for CEC are millequivalents per mass of soil (meq/100-g soil). Typical values range from less than 5 for sands to more than 30 for clays. CEC varies with soil pH, which affects the cations involved in reactions. Adding organic matter will increase a soil's CEC since organic matter has a CEC about 4 times greater than that of clay.

CEC and exchangeable ions are determined in the laboratory.

Additional reading

Electrical conductivity

Electrical conductivity (EC) is the ability of a material to conduct (transmit) an electrical current. It is commonly expressed in units of milliSiemens per meter (mS/m) or deciSiemens per meter (dS/m). EC is primarily used to assess salt concentration in soil. Although EC does not provide a direct measurement of specific ions or salt compounds, it has been correlated to concentrations of nitrates, potassium, sodium, chloride, sulfate, and ammonia.

Values greater than 8 dS/m are considered to be moderately saline, while soils greater than 16 dS/m are considered strongly saline. Saline soils are most likely is soils impacted by a specific source, such as road salt, in arid climates, in irrigated soils, and in soils receiving large quantities of manue or biosolids.

EC can be measured in the field using a meter or electrodes. Meters should be properly calibrated.

Stormwater application EC is not widely used in stormwater applications, but is a potentially useful indicator of soils impacted by road salts or other high salt materials.

Additional reading

Organic matter (carbon)

Soil organic matter affects chemical, physical, and biological properties of and processes in soil. Chemically, soil organic matter affects the cation exchange capacity, provides nutrients, and affects the capacity for buffering changes in soil pH. These in turn affect the fate and transport of chemicals including nutrients and pollutants.

Recommended levels of organic matter are 2-8 percent by weight. Most Minnesota soils fall within this range. Engineered media mixes often have higher concentrations (15 percent or more).

The chemical composition and properties of organic matter vary with source and age. Sources such as bark, stems, and roots have low nutrient value compared to biosolids, manure, and fresh green organic materials. As organic materials age, nutrient levels decrease, particularly if the organic material is composted. Some sources, such as sphagnum peat, may lower soil pH.

Soil organic matter is measured in the field, usually with one of the following methods.

Stormwater application Organic matter is an important component of engineered media in stormwater practices. It is the most important factor affecting retention of metals and most organic pollutants from stormwater runoff. Conversely, phosphorus and nitrogen, as nitrate, may be released from organic matter and therefore represent a potential concern for receiving waters. It is also an important source of nutrients.

Additional reading

Base saturation

Base saturation is calculated as the percentage of CEC occupied by base cations (calcium, magnesium, potassium). As base saturation increases, pH increases. In soils where CEC is dominated by aluminum, base saturation is low and plant growth may be inhibited by the elevated aluminum concentration. Soils with high base saturation also have greater buffering capacity.

Base saturation does not provide an indication of exchangeable cations in soil. To determine these, cation exchange capacity and specific cation ratios must be determined. Normal ranges for exchangeable bases are 40-80% for calcium, 10-40% for magnesium (Mg), and 1-5% for potassium (K). The ratio of K to Mg should be between 0.2 to 0.3 for best uptake.

Stormwater applications Base saturation is not widely used in stormwater applications, but is recommended in acid soils where aluminum concentrations may be elevated. In soils amended with aluminum or iron products, base saturation may be also be recommended.

Additional reading

Enzymes

Salinity

Salinity is a measure of the salt concentration in a soil. Saline soils are uncommon in Minnesota but may occur if a specific salt source exists (e.g. road salt, biosolid or manure application), in irrigated soils, and in arid soils.

Electrical conductivity (EC) is a good indicator of soil salinity. EC values greater than 16 dS/m indicate saline soils. If EC suggests saline soils, it may be useful to take laboratory samples to identify specific chemicals of concern (e.g. sodium, potassium).

Stormwater application Salinity is not widely used in stormwater applications, but is a potentially useful indicator of soils impacted by road salts or other high salt materials.

Additional reading

Sodium adsorption ratio

pH

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