Soil Chemistry

Soil is formed when crystalline structures release minerals during the weathering process. Both the minerals released and organic matter undergo chemical processes that reduce them to even smaller particles no longer visible by the naked eye.

What are Soil Colloids and Ions?

The very smallest of soil particles produced by weathering and subsequent chemical processes are called colloids.  Colloids are negatively charged soil particles which can attract and hold onto positively charged particles like a magnet.

There are two types of colloids:  organic and inorganic.  Organic colloids are created when organic matter decomposes extensively until it can no longer degrade, producing humus.  Inorganic colloids are almost exclusively clay minerals. 

Both types of colloids typically contribute equally to the chemical and physical properties of the soil.  While organic or humus colloids have greater nutrient-holding and water-holding capacities by weight when compared to that of inorganic colloids that make up clay particles, clay particles typically exist in larger quantities in most soils.

Mineral nutrients exist in the soil as elements or compounds with their own net negative or net positive charge.  These charged particles are called ions.  Mineral nutrient ions with negative charges like nitrate (NO3-), phosphate (PO43-), and sulfate (SO42-) are called anions.  Mineral nutrients with positive charges like ammonium (NH4+), potassium (K+), calcium (Ca2+), magnesium (Mg2+), and hydrogen (H+) are called cations.

Because clay and humus colloids are negatively charged soil particles, their surface attracts and holds onto positively charged mineral nutrients or cations through a process called adsorption, like a magnet would attract and hold onto iron filings.  Remember the old saying: opposites attract.

These same clay and humus colloidal soil particles, however, repel negatively charged mineral nutrients or anions.  Instead of these anions sticking to the surface of clay or humus soil particles, they remain free ions in the soil solution in the pores between soil particles.  Anions like nitrate, phosphate, and sulfate are more easily leached from the soil because they are in solution within its pores rather than attached to the surface of soil particles.

Plant roots pull water and nutrients from the soil solution located in pores and into the plant through a process called absorption.  Because anions will not attach to the surface of soil particles and instead exist in the soil solution, they are easily absorbed by plants.  However, for positively charged nutrient cations to be absorbed by plants, they must first be freed from the surface of the clay or humus soil particles and exist as free ions in the soil solutions located in the soil pores.

What is Cation Exchange Capacity?

Cation exchange capacity (CEC) measures the ability of soil particles to hold exchangeable positively charged ions or cations.  It is an inherent characteristic of soil and is difficult to alter significantly.  This is an important soil property as it influences soil structure, nutrient availability, soil pH, and how the soil reacts to fertilizers.

As mentioned previously, soil particles like sand, clay, and humus have negatively charged sites on their surface which adsorb and hold positively charged ions (cations) using electrostatic force. This is crucial to the supply of nutrients to plants because many nutrients like calcium, magnesium, potassium, and sodium exist as cations.  Sand has little surface area compared to clay and humus as the latter are made of much smaller soil particles than sand.

CEC is measured in milligram equivalents per 100 grams of soil (meq/100 g).  Pure sand has a very low CEC of less than 2 meq/100 g.  Clay soils have a much higher CEC than sand, typically in the range of 3 to 150 meq/100 g.  Organic matter has a very high CEC, typically in the range of 100 to 300 meq/100 g but can go as high as 400 meq/100 g.  Because the CEC of sand is negligible, CEC of soil depends primarily on the types and amounts of clay and organic matter present.