Agriculture Reference
In-Depth Information
Chapter 2
Cation Exchange Capacity in Soils, Simplified
(Revised December 2013)
Adsorb vs Absorb
adsorb(ad sôrb, -zôrb), v.t. Physical Chem. to gather (a gas, liquid, or dissolved
substance) on a surface in a condensed layer: Charcoal will adsorb gases .
Please note the definition above, taken from the large hardbound version of the
Random House Second Edition Unabridged Dictionary. It's not absorb, it's
adsorb , with a "d". We all know that a sponge absorbs water, a cast iron pot
absorbs heat, a flat-black wall absorbs light. None of those gathers anything on
the surface in a condensed layer, they soak it right in, they absorb it.
Adsorb is different, because it means to gather on a surface in a condensed layer.
This is pretty much the same thing as static cling, like when you take a synthetic
fabric shirt out of the clothes dryer and it wants to stick to you. You don't absorb
the nylon blouse, you adsorb it. Everyone got that? Good. On to Cation Exchange
Capacity.
The Exchange Capacity of your soil is a measure of its ability to hold and release
various elements and compounds. We are mostly concerned with the soil's ability
to hold and release plant nutrients, obviously. Specifically here today, we are
concerned with the soil's ability to hold and release positively charged nutrients.
Something that has a positive (+) charge is called a cation, pronounced
cat-eye-on. If it has a negative charge (-) it is called an anion, pronounced
ann-eye-on. (Both words are accented on the first syllable.) The word "ion" simply
means a charged particle; a positive charge is attracted to a negative charge and
vice-versa.
Positively charged particles are known as cations. There are two types of cations,
acidic or acid-forming cations, and basic, or alkaline-forming cations. The
Hydrogen cation H+ and theAluminum cationAl+++ are acid-forming. Neither are
plant nutrients.Asoil with high levels of H+ orAl+++ is an acid soil, with a low pH.
The positively charged nutrients that we will be discussing here are Calcium,
Magnesium, Potassium and Sodium. These are all alkaline cations, also called
basic cations or bases. Both types of cations (alkaline or acidic) may be adsorbed
onto either a clay particle or soil organic matter (SOM).All of the nutrients in the
soil need to be held there somehow, or they will just wash away when you water
