Soil science
Prof. Blaskó Lajos (2008)
Debreceni Egyetem a TÁMOP 4.1.2 pályázat keretein belül
Sources of negative charge:

Sources of negative charge:

Figure 17 Permanent charge due to isomorphous substitution (http://www.slideworld.com/slideshows.aspx/Visual-comparison-of-common-silicate-clays-ppt-891467

Figure 18 Negative charges of clay minerals (http://www.slideworld.com/slideshows.aspx/Visual-comparison-of-common-silicate-clays-ppt-891467)

Figure 19 Negative charges of clay minerals balacced by cations (http://www.slideworld.com/slideshows.aspx/Visual-comparison-of-common-silicate-clays-ppt-891467)

The main source of charge on clay minerals is isomorphous substitution which confers permanent charge on the surface of most layer silicates.

Ionization of hydroxyl groups on the surface of other soil colloids and organic matter can result in what is describes as pH dependent charges-mainly due to the dependent on the pH of the soil environment. Unlike permanent charges developed by isomorphous substitution, pH-dependent charges are variable and increase with increasing pH.

Figure 20 Ionization of hydroxyl groups on the surface of other soil colloids and organic matter (http://www.slideworld.com/slideshows.aspx/Visual-comparison-of-common-silicate-clays-ppt-891467

Figure 21 Cation retention on organic matter (http://www.slideworld.com/slideshows.aspx/Visual-comparison-of-common-silicate-clays-ppt-891467)

Presence of surface and broken - edge -OH groups gives the kaolinite clay particles their electronegativity and their capacity to absorb cations. In most soils there is a combination of constant and variable charge. Cation-a positively charged ion There are two types of cations, acidic or acid-forming cations, and basic, or alkaline-forming cations. The Hydrogen cation H+ and the Aluminum cation Al+++ are acid-forming.

Figure 22 The most important exchageable cations (http://www.soilminerals.com/Cation_Exchange_Simplified.htm)

The positively charged nutrients that we are mainly concerned with here are Calcium, Magnesium, Potassium and Sodium. These are all alkaline cations, also called basic cations or bases. Both types of cations 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 the garden or get a good rainstorm. Clay particles almost always have a negative (-) charge, so they attract and hold positively (+) charged nutrients and non-nutrients. Soil organic matter (SOM) has both positive and negative charges, so it can hold on to both cations and anions.( http://www.soilminerals.com/Cation_Exchange_Simplified.htm)

Table 2. Normal Range of CEC Values for Common Color/Texture Soil Groups (http://www.extension.purdue.edu/extmedia/AY/AY-238.html)

Anion-a negatively charged ion (NO3-, PO42-, SO42-, etc...)

Soil particles and organic matter have negative charges on their surfaces. Mineral cations can adsorb to the negative surface charges or the inorganic and organic soil particles. Once adsorbed, these minerals are not easily lost when the soil is leached by water and they also provide a nutrient reserve available to plant roots.

These minerals can then be replaced or exchanged by other cations (i.e., cation exchange)

vThe exchage processes (Figure 23) are REVERSIBLE (unless something precipitates, volatilizes, or is strongly adsorbed).

Figure 23 Cation exchange

CEC is highly dependent upon soil texture and organic matter content Table 3, 4.). In general, the more clay and organic matter in the soil, the higher the CEC. Clay content is important because these small particles have a high ration of surface area to volume. Different types of clays also vary in CEC. Smectites have the highest CEC (80-100 millequivalents 100 g-1), followed by illites (15-40 meq 100 g-1) and kaolinites (3-15 meq 100 g-1).

Table 3. Examples of CEC values for different soil textures are as follows:

Table 4. Cation Exchange Capacities of Clay Minerals Adapted From Sparks 1995. Envornmental Chemistry of Soils. Academic Press. (http://jan.ucc.nau.edu/~doetqp-p/courses/env320/lec13/Lec13.html)