Base Saturation (B.S.) refers to the number of basic cations that are held on the soil exchange (CEC sites) in comparison to the total number of sites.
CEC or soil exchange sites are the negative sites on the clay particles that are created due to the unsatisfied charges of broken sites at the edge of minerals or internal ionic exchange. The negative sites attract and hold positive ions (cations).
If we have an acid soil, a neutral soil and an alkaline soil, one logical question is: what conditions produce equal concentrations of H+ and OH- ions in the soil solution and on the exchange complex?
Base saturation can be defined as the amount of basic cations that occupy the cation exchange sites, divided by the total cation exchange capacity (CEC).
Bases ÷ CEC × 100=Base Saturation %
Hydrogen saturation is the amount of hydrogen cations that occupy exchange sites, divided by the CEC. Together, they must equal 100%.
The unit of measurement for CEC is the milliequivalent; that is, the milliequivalents of cations adsorbed per 100 grams of oven dry soil (meq/100g). The notation of CEC for the international SI units is centimoles of positive charge per kilogram [cmol(+)/kg]. A soil with 10 meq/100g will also have 10 [cmol+/kg].
From the diagram (which is applicable to midwestern soils), when base saturation is 0, the lowest pH possible is 3.5. When base saturation is 100%, the highest pH is near 7.6. Therefore, in order to have a neutral pH, the base saturation has to be higher than 50%. In fact, it would be about 80%.
The reason that the base saturation is higher than 50% is because most basic cations are divalent (2+) and are, therefore, held to the exchange sites tighter than the monovalent H+ ions. Thus, it takes more basic cations on the exchange to produce the same amount of OH- and H+ in the soil solution.
When the OH- and H+ are equal on the exchange sites, the pH will be about 5.5. You will need to know the relationships in this diagram so that when given pH values you can approximately select the percent base saturation in midwestern soils.
SAMPLE SOIL TEST REPORT DATA
| CEC | Acidity | % Base | Saturation | |||
|---|---|---|---|---|---|---|
| meq/100g | Ca | Mg | K | Na | Total | |
| 5 | 1 | 50 | 20 | 5 | 5 | 80 |
The percent base saturation tells what percent of the exchange sites are occupied by the basic cations. If calcium has a base saturation value of 50% and magnesium has a base saturation value of 20% as shown above, then calcium occupies half of the total exchange sites (CEC) and magnesium occupies one-fifth of the total exchange sites.
In this example where the soil has a CEC of 5 meq/100g, 2.5 meq/100g of the CEC is occupied by calcium and 1 meq/100g of the CEC is occupied by magnesium. If all the exchangeable bases (Ca, Mg, K and Na) total 100%, then there is no exchangeable acidity.
The acidity on the report is the amount of the total CEC occupied by the acidic cations (H+ and Al+3). The acidity, like the CEC, is expressed as meq/100g of soil. If the CEC is 5 meq/100g of soil and the acidity is 1 meq/100g of soil, then one-fifth of the exchange sites in the soil are occupied by acidic hydrogen and aluminum ions. The remaining 4 meq/100g of soil (or 80% of the CEC) will be occupied by the basic cations. The more acidic a soil is and the lower the soil pH value, the closer the acidity number will be to the CEC number.
Chapter 4 Calcareous, Saline, & Sodic Soils
Chapters for Soil pH
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