What factors contribute to lower and higher cation exchange?

1.    What factors contribute to lower and higher cation exchange?

(a)    Types of clay.

 A sandy soil with little organic matter will have a very low CEC while a clay soil with a lot of organic matter (as humus) will have a high CEC.

(b)   Amount of clay

As the clay increases the cation exchange will also increase. Higher quantities of clay and organic matter be get higher cation exchange. Amounts of negative and positive charges are both expressed in milliequivalents. One milliequivalent of negative charge on a clay particle is neutralized by one milliequivalent of cation. A milliequivalent takes into account both the weight and the charge of the cation.

(c)    pH value

Many soil cation exchange is dependent upon the pH of the soil. This is due mostly to the lyotrophic series, which describes the relative strength of various cation absorption, and is generally as follows:Al⁺³ > H⁺ > Ca⁺² > Mg⁺² > K⁺ = NH₄⁺ > Na⁺.As soil acidity increases (pH decreases), more H⁺ ions are included in the cation exchange removing other cations. Inversely, when soils become more basic (pH increases), the available cation exchange increases because there are fewer H⁺ ions in the soil.

(d)   Hydrous oxides of Fe and Al

Many heavily weathered or oxidized soils, especially in the wet tropics, have a high concentration of Al⁺³. Since aluminum is toxic in high quantities for most plants, there are certain advantages and disadvantages to this. For one, due to the relatively high absorption rate of aluminum, it is taken out of the soil and the plant cannot use it. On the other hand, because it has 3 positive charges, it takes up a large amount of charge in a CEC. For example, Al⁺³ fills the same space as three NH₄⁺ ions. This makes many aluminum heavy soils relatively infertile. There is no easy way to remove Al ions from the CEC.

References:

1-      http://classes.css.wsu.edu/soils201/Presentations/lab%205%20CEC.pdf

2-      http://www.soilminerals.com/Cation_Exchange_Simplified.htm

3-      http://courses.soil.ncsu.edu/ssc012/Lecture/topic14.htm

What is different between low and high cation exchange?

http://www.spectrumanalytic.com/support/library/ff/CEC_BpH_and_percent_sat.htm

Low cation exchange		High cation exchange
Soil with a low CEC value is a good indication that a soil is sandy with little or no organic matter that cannot hold many cations. Sandy soil		A high CEC value is a good indicator that a soil has a high clay and/organic matter content and can hold a lot of cations. Clay soil
˂5	CEC value	˃25
Lower	Present of clay and organic matter	Higher
Lower	Water holding capacity	Greater
Potassium and magnesium  (and other cation) deficiencies. A large one-time addition of cations (e.g. potassium) can lead to large leaching losses as it is soil unable to hold on to the excess K.	Deficiencies of cations	Less susceptible to leaching losses of these cations
the faster the soil pH will decrease with time.	pH vs time	the slower the soil pH will decrease with time.

References:

1-      http://nmsp.cals.cornell.edu/publications/factsheets/factsheet22.pdf

2-      http://www.spectrumanalytic.com/support/library/ff/CEC_BpH_and_percent_sat.htm

What is cation exchange?

1.      What is cation exchange?

A process in which cations in solution are exchanged with cations held on the exchange sites of mineral and organic matter, particularly on the surfaces of colloids of clay and humus.

In clay minerals, a certain amount of ionic substitution occurs. For example some Si⁴⁺ ions in the silicate layers can be replaced by Al³⁺ ions as these two ions are of similar size. Substitution of Al³⁺ ions in the aluminium hydroxide layer by Mg²⁺ ions also take place. As these substitutions involve the replacement of one cation by another of lower valency the clay structure becomes negatively charged. This charge is compensated by positive ions, e.g. K⁺, Na⁺, Mg²⁺, Ca²⁺ and H⁺, absorbed at the edges of the clay minerals or between the layers. These ions are loosely held in the structure and as they can readily be replaced, one by another, are known as “cation exchange”.

            

References:

1-      http://www.stadiumturf.com/cation_exchange.htm

2-      http://www.waters.com/waters/nav.htm?cid=10083845&locale=en_US

3-      http://scholar.lib.vt.edu/ejournals/JARS/v30n4/v30n4-deul.htm

4-      Paul Rado, F.I. Ceram, An Introduction To The Technology Of Pottery; PERGAMON PRESS.