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 physical 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.

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
• Phosphorus - information in this manual on phosphorus in stormwater