Option E: Environmental Chemistry: Soil (HL)

DONE BY LIAO WEI CHENG

OBJECTIVES


After this section, you should be able to:

1) State what is meant by the term cation-exchange capacity (CEC) and outline its importance.

2) Discuss the effects of soil pH on cation-exchange capacity and availability of nutrients.

3) Describe the chemical functions of soil organic matter.

© IBO 2007

14.1. CATION-EXCHANGE CAPACITY (CEC)
The CEC measures the soil's ability to hold the positively charged cations by electrical attraction. It is the capacity to exchange cations with the soil solution. Soil organic matter and clay particles consist of negatively charged particles called colloids, which will be able to attract and bond to the positively charged cations. The stronger the negative charge, the greater the capacity to hold and exchange with other cations.

Figure 14.1.1 A diagrammatic representation of the colloid. Its negative charges are along the edges of the colloid. The cations are attached to the colloid by electrical attraction between the positive and negative charges.
Figure 14.1.1 A diagrammatic representation of the colloid. Its negative charges are along the edges of the colloid. The cations are attached to the colloid by electrical attraction between the positive and negative charges.

The cations can be basic, e.g. calcium (Ca2+), magnesium (Mg2+), potassium (K+) and sodium (Na+); or acidic, e.g. aluminium (Al3+), iron (Fe2+), manganese (Mn2+) and hydrogen (H+). These cations are exchanged with other cations in the soil solution and are not removed --- only cations are added in.


Figure 14.1.2 Diagram showing how cation-exchange occurs.
Figure 14.1.2 Diagram showing how cation-exchange occurs.

In Figure 14.1.2, the cations from the organic matters, the humus particle and clay particle, are exchanged with the hydrogen ions on the root hairs of the carrot root. The exchange of cations provide mineral ions (nutrients) for the plants. This is especially important in agriculture farming in cultivating these plants.The CEC is therefore used as a measure of potential soil fertility. A higher CEC indicates a more fertile soil. The problem of soil with low CEC can be resolved with the addition of organic matter to the soil to increase the colloids for cation exchange, and thus, increasing the CEC. An example will be the addition of manure (Figure 14.1.3).

Figure 14.1.3 Manure can be added to soil of low CEC to increase the soil fertility.
Figure 14.1.3 Manure can be added to soil of low CEC to increase the soil fertility.

14.2. pH AND SOIL

Soil pH is especially important for CEC as it affects how efficiently a crop grows by affecting the availability of nutrients. By analyzing the soil, we can compare the total concentration of basic cations to that of the acidic cations. The more acidic the soil, the greater the number of acid cations such as aluminium ions and manganese ions. However, as can be seen from Figure 14.2.1, an acidic soil is not optimum for many plants and is not suitable for many plants. This is one of the result of acid rain as the soil's pH will be lowered and thus, increase in acidic ions which are toxic to plants. An optimum pH for plant growth is 6.0 to 8.0 as the aluminium ions will then be precipitated out of the soil solution. To overcome the problem of acidic soil, lime is sometimes added to increase the soil pH and the concentration of basic cations. As mentioned in 14.1, with reference to Figure 14.1.2, nutrients are available to the plants via the cation exchange that helps replace the hydrogen ions on the root hairs with nutrient cations, which are adsorbed by the roots and surrounding soil surfaces, which may later be diffused into the plant root cells.

Figure 14.2.1 The importance of pH on plant growth.
Figure 14.2.1 The importance of pH on plant growth.

14.3. CHEMICAL FUNCTIONS OF SOIL ORGANIC MATTER

Recall: Soil (SL) - Soil Organic Matter.

Soil organic matter is a source of pH buffering for the soil. It also binds both organic and inorganic compounds in the soil which reduces contamination that result in negative environmental effects, in addition to nutrient cations. Such inorganic matter includes heavy metal, pesticides and other pollutants. This chemical binding is similar to that of the ion exchange used in the tertiary treatment of waste water (Refer to Water Treatment), contributing to the overall CEC of the soil.

SUMMARY

Cation Exchange Capacity (CEC)
Capacity to exchange cations with soil solution

Organic matter consists of negatively charged particles that attract cations and exchanges with other cations.

Cations can be acidic or basic.
Importance of CEC
Important for measure of potential soil fertility, especially in agricultural use.
Effects of Soil pH
6.0-8.0 is the optimum pH for healthy plant/crop growth

Too acidic soil will result in increase of acidic ions which are toxic to plants.

Basic soil will curb acidic problems and raise concentration of basic cations to facilitate nutrient cations exchange to the plants.
Availability of Nutrients
Nutrient cations will replace hydrogen cations on root hair of plants to be adsorbed by the root surface. Adding lime to the soil will increase soil pH and thus, increase of concentration of the nutrient cations.
Chemical Functions of Soil Organic Matter
1. pH buffer for soil.
2. Binds inorganic and organic compounds in soil, including pollutants. Thus, reduces negative environmental impacts.
3. Source of measure of CEC.
4. Source of nutrients (nutrient cations) for plants.

SECTION 14. EXERCISES

1. Define Cation Exchange Capacity.
2. Draw a labelled diagram (producing an example) outlining the process of CEC and outline its importance.
3. Describe the role of soil pH on the CEC and availability of nutrients in the soil.
4. List 2 chemical functions of Soil Organic Matter and describe them.

Discussion Question: Discuss the effectiveness of soil organic matter in reducing the negative environmental impacts by binding on to pollutants.
Discuss this question under the Discussion tab.



BIBLIOGRAPHY

NB: Due to the difficult nature of Wikispace's formatting, the indent of the bibliography below may be out of place.

Images:
Figure 14.1.1: http://www.dpi.nsw.gov.au/__data/assets/image/0008/166418/cec-diagram.gif
Figure 14.1.2: http://microsoil.com/images/CationExchange.gif
Figure 14.1.3: http://southcarolina1670.files.wordpress.com/2011/02/pile-of-manure.jpg
Figure 14.2.1: http://www.omafra.gov.on.ca/english/environment/soil/chemicalf1.jpg

Print Sources:
Neuss, G. (2007). IB DIPLOMA PROGRAMME Chemistry Course Companion. Oxford, OX:
Oxford University Press.

Electronic Sources:
Baldock, J. (2008, Nov 21). Why Soil Organic Matter matters. Retrieved April 7, 2011, from
CSIRO: http://www.csiro.edu.au/resources/soil-organic-matter.html
Camberato, J.J.. (2001). Cation Exchange Capacity – Everything You Want to Know and
Much More. Retrieved April 7, 2011, from: http://www.jbhs.ccs.k12.nc.us/facultyandstaff/withers/Supplemental%20Notes%20For%20APES/cation%20exchange%20capacity.pdf
Cation Exchange Capacity. (n.d.). Retrieved April 7, 2011, from Microsoil:
http://microsoil.com/CEC.htm
Cation exchange capacity. (2002, Aug 16). Retrieved April 7, 2011, from NSW Government:
http://www.dpi.nsw.gov.au/agriculture/resources/soils/structure/cec
Cation-Exchange Capacity. (2004, Jul 9). Retrieved April 7, 2011, from Washington State
University: http://soils.tfrec.wsu.edu/webnutritiongood/soilprops/04CEC.htm
Krull, E.S., Skjemstad, J.O., & Baldock, J.A.. (2004, Oct). Functions of soil organic matter
and the effect on soil properties. Retrieved April 7, 2011, from Grains Research and Development Corporation: http://users.datarealm.com/treepower//soils/organicmatter.html
Soil Organic Matter. (2010, May 19). Retrieved April 7, 2011, from Department of Primary
Industries, Parks, Water and Environment: http://www.dpiw.tas.gov.au/inter.nsf/WebPages/TPRY-5YW6YZ?open