Cation Exchange Capacity (CEC)

Technical Note DOER-C2

May 1999

material because of its low cost and growth-enhancing qualities. Liming can reduce the bioavail-

ability and toxicity often present in acidic soil when aluminum (Al), manganese (Mn), and other

metals (zinc (Zn), copper (Cu), or nickel (Ni)) are present at elevated concentrations.

Cation Exchange Capacity (CEC). Cation exchange reactions in a soil are important because

they alter soil physical properties, cause/correct acidity and basicity, affect soil fertility, and can

purify or alter percolating waters. Electrostatic charges are inherent on soil particles. Some charges

are permanent, while others are pH dependent. Exchangeable cations (positively charged ions) are

attracted to the negatively charged surfaces and replace the cations on the particle surfaces. As the

CEC increases, the amount of adsorbed cations increases. The CEC is pH dependent and directly

proportional to the clay concentration, organic matter content, and particle-size distribution.

Salinity. Salinity is a measure of the concentration of soluble salts. Cations (sodium, potassium,

calcium, and magnesium) and anions (sulfates and chlorides) are the predominant solutes that

contribute to salinity. Salt accumulations in soil can adversely affect its structure (decreases the

cohesiveness of particles), inhibit water and air movement, increase the osmotic potential, decrease

the available nutrient content, induce toxicity to specific ions, and prevent the growth of many plants

(except halophytes). Salinity is conventionally measured on aqueous extracts of a saturated paste.

A saturated paste is prepared by adding just enough water to saturate the soil sample until it glistens

and flows slightly when the container is tipped. The recommended ratio of soil:water in the paste

is the smallest amount that can be easily removed with vacuum or pressure since this amount readily

correlates to water content under field conditions. Other extraction ratios (1:1, 1:5, etc.) can be used but

are not readily correlated to water content under field conditions. The method utilized should be based

on the specific conditions and needs of the project. Vegetative responses to salt-affected soil are also

influenced by the ratio of calcium to the other ions in solution. Yields are generally reduced when the

ratio of calcium to other ions is less than 0.10 (the critical calcium ratio).

Sodium Adsorption Ratio (SAR). The SAR indicates the tendency for sodium to adsorb to

the cation exchange sites at greater concentration than calcium or magnesium and is an index of

-1

the relative sodium content of the soil solution expressed in mmol l . More specifically, it is the

2+ 0.5

ratio of sodium ions to the sum of the calcium and magnesium ions (SAR = (Na )/(Ca + Mg ) ).

Dredged materials with SAR values in the range of 10-13 or higher are generally considered sodic.

The concentration of exchangeable sodium in a sodic soil is so high that the soil becomes dispersed

and impermeable to water as the pores become clogged with dispersed or dislodged clay particles.

Plant growth is adversely affected when sodium occupies a high proportion of the exchange sites

because pH can become basic (8.5 to 10.5), and the soil aggregates required for plant growth

disintegrate and disperse.

Electrical Conductivity. Electrical conductivity is another measure of the soluble salts (ionic

-1

strength) present in the dredged material/soil and is reported in decisiemens per meter (dS m ). It

increases as the concentration of dissolved salts increases (electrical conductivity (dS m-1) 10 =

-1

mmol L of soluble salts (total cations or anions)). The electrical conductivity is usually measured

on a saturated paste extract of the dredged material using electrodes, and the value obtained can be

related to the actual soluble salt concentration in the dredged material. Generally, plants respond

in the following ways to electrical conductivity: <2, negligible; 2-4, slight reduction in yield of

sensitive plants; 4-8, reduced yield in most plants; 8-16, satisfactory yield only in salt-tolerant