Soil chemical properties and processes

This site is currently undergoing revision. For more information, open this link.
This page is in development

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.

• 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

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

• Phosphorus Basics: Understanding Phosphorus Forms and Their Cycling in the Soil
• Understanding phosphorus in Minnesota soils
• Soil phosphorus - USDA-NRCS
• Phosphorus
• How to Effectively Manage Phosphorus Levels in the Soil

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.

• • 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)]

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

• Plant and Soil Sciences e-library - Forms of Nitrogen in the Soil
• Understanding Nitrogen in Soils - University of Minnesota Extension
• Soil nitrogen - USDA-NRCS
• What Happens to Nitrogen in Soils?
• Nitrogen Basics – The Nitrogen Cycle - Cornell University

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

• Calcium in plants and soil
• Magnesium in plants and soil
• Potassium in plants and soil
• Calcium in the soil
• Sodium affected soils
• Soil potassium
• High magnesium soils

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.

Additional reading

• Cations and Cation Exchange Capacity