AY-238

Soils (Fertility)

Purdue University
Cooperative Extension Service
West Lafayette, IN 47907

Soils can be thought of as storehouses for plant nutrients. Many nutrients, such as calcium and magnesium, may be supplied to plants solely from reserves held in the soil. Others like potassium are added regularly to soils as fertilizer for the purpose of being withdrawn as needed by crops. The relative ability of soils to store one particular group of nutrients, the cations, is referred to as cation exchange capacity or CEC.

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.

Forms of Nutrient Elements in Soils

Elements having an electrical charge are called ions. Positively-charged ions are cations; negatively-charged ones are anions.

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 (PO₄^3-). Note also that anions can have more than one negative charge and may be combinations of elements with oxygen.

Defining Cation Exchange Capacity

Cations held on the clay and organic matter particles in soils can be replaced by other cations; thus, they are exchangeable. For instance, potassium can be replaced by cations such as calcium or hydrogen, and vice versa.

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 $3\; x\; 1020$ 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

Measuring Cation Exchange Capacity

Since a soil's CEC comes from the clay and organic matter present, it can be estimated from soil texture and color. Table 1 lists some soil groups based on color and texture, representative soil series in each group, and common CEC value measures on these soils.

Table 1. Normal Range of CEC Values for Common Color/Texture Soil Groups.

                                        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.

Buffer Capacity and Percent Base Saturation

Cations on the soil's exchange sites serve as a source of resupply for those in soil water which were removed by plant roots or lost through leaching. The higher the CEC, the more cations which can be supplied. This is called the soil's buffer capacity.

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 real 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.

Relationship Between CEC and Fertilization Practices

Recommended liming and fertilization practices will vary for soils with widely differing cation exchange capacities. For instance, soils having a high CEC and high buffer capacity change pH much more slowly under normal management than low-CEC soils. Therefore, high-CEC soils generally do not need to be limed as frequently as low-CEC soils; but when they do become acid and require liming, higher lime rates are needed to reach optimum pH.

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.

Summary

The cation exchange capacity of a soil determines the number of positively-charged ions cations-that the soil can hold. This, in turn, can have a significant effect on the fertility management of the soil.

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