Cooperative Extension Service
West Lafayette, IN 47907
Soils are composed of a mixture of sand, silt, clay and organic matter. Both the clay and organic matter particles have a net negative charge. Thus, these negatively-charged soil particles will attract and hold positively-charged particles, much like the opposite poles of a magnet attract each other. By the same token, they will repel other negatively-charged particles, as like poles of a magnet repel each other.
The most common soil cations (including their chemical symbol and charge) are: calcium (Ca++), magnesium (Mg++), potassium (K+), ammonium (NH4+), hydrogen (H+) and sodium (Na+). Notice that some cations have more than one positive charge.
Common soil anions (with their symbol and charge) include: chlorine (Cl-), nitrate (NO3-), sulfate (S04=) and phosphate (PO43-). Note also that anions can have more than one negative charge and may be combinations of elements with oxygen.
The total number of cations a soil can hold--or its total negative charge--is the soil's cation exchange capacity. The higher the CEC, the higher the negative charge and the more cations that can be held.
CEC is measured in millequivalents per 100 grams of soil (meq/100g). A meq is the number of ions which total a specific quantity of electrical charges. In the case of potassium (K+), for example, a meq of K ions is approximately 6 x 1020 positive charges. With calcium, on the other hand, a meq of Ca++ is also 6 x 1020 positive charges, but only ions because each Ca ion has two positive charges.
Following are the common soil nutrient cations and the amounts in
pounds per acre that equal 1 meq/100g:
Calcium (Ca++) - 400 lb./acre Magnesium (Mg++) - 240 lb./acre Potassium (K+) 780 lb./acre Ammonium (NH4+) - 360 lb./acre
CEC in Soil groups Examples meg/100g ----------------------------------------------- Light colored sands Plainfield 3-5 Bloomfield Dark colored sands Maumee 10-20 Gilford Light colored loams and Clermont-Miami 10-20 silt loams Miami Dark colored loams and Sidell 15-25 silt loams Gennesee Dark colored silty clay Pewamo 30-40 loams and silty clays Hoytville Organic soils Carlisle muck 50-100 -------------------------------------------------Cation exchange capacity is usually measured in soil testing labs by one of two methods. The direct method is to replace the normal mixture of cations on the exchange sites with a single cation such as ammonium (NH4+), to replace that exchangeable NH4+ with another cation, and then to measure the amount of NH4+ exchanged (which was how much the soil had held).
More commonly. the soil testing labs estimate CEC by summing the calcium, magnesium and potassium measured in the soil testing procedure with an estimate of exchangeable hydrogen obtained from the buffer pH. Generally, CEC values arrived at by this summation method will be slightly lower than those obtained by direct measures.
Cations can be classified as either acidic (acid- forming) or basic. The common acidic cations are hydrogen and aluminum; common basic ones are calcium, magnesium, potassium and sodium. The proportion of acids and bases on the CEC is called the percent base saturation and can be calculated as follows:
Total meq of bases on exchange sites Pct. base =(i.e., meq Ca++ meq Mg++ + meq K+) saturation ------------------------------- x 100 Cation exchange capacity
The concept of base saturation is important, because the relative proportion of acids and bases on the exchange sites determines a soil's pH. As the number of Ca++ and Mg++ions decreases and the number of H+ and Al+++ions increases, the pH drops. Adding limestone replaces acidic hydrogen and aluminum cations with basic calcium and magnesium cations, which increases the base saturation and raises the pH.
In the case of Midwestern soils, the actual mix of cations found on the exchange sites can vary markedly. On most, however, Ca++ and Mg++ are the dominant basic cations and are in greater concentrations than K+. Normally, very little sodium is found in Midwestern soils.
CEC can also influence when and how often nitrogen and potassium fertilizers can be applied. On low-CEC soils (less than 5 meg/20000g), for example, some leaching of cations can occur. Fall applications of ammonium N and potassium on these soils could result in some leaching below the root zone, particularly in the case of sandy soils with low-CEC subsoils. Thus, spring fertilizer application may mean improved production efficiency. Also, multi-year potash applications are not recommended on low-CEC soils.
Higher-CEC soils (greater than 10 meg/100g), on the other hand, experience little cation leaching, thus making fall application of N and K a realistic alternative. Applying potassium for two crops can also be done effectively on these soils. Thus, other factors such as drainage will have a greater effect on the fertility management practices used on high- CEC soils.
Cooperative Extension work in Agriculture and Home Economics, State of Indiana, Purdue University and U.S. Department of Agriculture cooperating: H.A. Wadsworth, Director, West Lafayette, IN. Issued in furtherance of the acts of May 8 and June 30, 1914. The Cooperative Extension Service of Purdue University is an equal opportunity/equal access institution.