Why the Cation Exchange Capacity (CEC) is given much more emphasis in studying soil fertility management than the Anion Exchange Capacity (AEC) considering that major plants nutrients such as: nitrates, nitrites, phosphate, sulphate ions are all negatively charged ions?
Good question. Most of the soils are negatively charged, and therefore CEC is important for soil fertility management. However, Not all soil surfaces are negatively charged. For example, in volcanic soil, Fe and Al hydroxides/oxides form at the surface of clay minerals. These Fe/Al oxides positively charged and specifically adsorb negatively charged anions. Allophanic soils suffer P deficiency due to high AEC (or Anion Specificities Capacity or P retention), it is a major research area in literature especially for allophanic/andisols. While PO4- is strongly adsorbed, SO4- is lesser extent in specifically adsorption, and NO3- is not adsorbed at all, easily leached from soils.
Cation exchange capacity (CEC) is more popular because it is a measure of the soil's ability to hold positively charged ions. It is a very important soil property influencing soil structure stability, nutrient availability, soil pH and the soil's reaction to fertilisers and other ameliorants.
Because cation exchange capacity represents the potential reservoir of cationic nutrients that are bound to the surface of the soil colloids. While anion exchange toxicity has a greater influence on soil fertility under the following conditions:
If the colloid is in a medium where pH > PCZ of colloid: negative charge.
- If the colloid is in a medium where the pH < PCZ of colloid: positive charge.
- If the colloid is in a medium where the pH = PCZ of colloid: zero charge.
It is worth noting that under conditions where predomonin anion exchange capacity there may be greater availability of plant toxic nutrients, eg Al3 +. Also, in this condition, some macro and micronutrients become unavailable.
Thank you all for your answers!
Dear Adijailton, as I am not a soil scientist, could you please spell out PCZ? What is it? Also, I do not understand.... predomonin anion exchange capacity . Could you please explain? Thank You.
PCZ is the point of zero charge, i.e. the pH at wich the charge density of a surface is =0.
If a given particle is in an environment with a pH below or above its PCZ its surface will be charged positively or negatively charged. Not all particles singnificatly change surface charge with pH. Soil particles that change surface charge depending on pH include organic matter and Fe- or MnOOH. Varying surface charge with pH is mostly a question of (de-)protonation of OH groups.
In most soil almost all anion exchange sites are pH dependent. Whereas (at lest some) cation exchange sites are pH independent if they come from isomorphic substitutions in octahedral layers of clay minerals (i.e. sunbstitution of Al by Mg in Motmorillonite). Note that there are also pH dependend cation exchange sites in soils. That means that CEC is more of a "soil property" whereas AEC is more dependent on evironmental parameters that can change for example in the rhizosphere, explaining the more frequent use of CEC.
Thanks a bunch Simon! Your explanation helped a lot and I appreciate it very much.
I can provide a protocol PCZ determination. However, the material is written in Portuguese.
Zero charge point (PCZ) is the pH value at which the adsorption of potential determinant ions (H + and OH-) is equal. One way to determine it is based on the potentiometric titration of the soil in different ionic forces of the medium. The PCZ would then be the crossing point between these different electrolytic curves. Performed in this way, the PCZ would also be recognized as Null Saline Effect Point (PESN). Since this attribute is a function of surface reactions that occur in soil variable load colloids, it is assumed that the magnitude of the PCZ reflects the nature of these colloids. In addition, the signal and magnitude of the soil surface charge will be a function of the difference between soil pH and PCZ: if the soil pH is higher than PCZ, the soil will be negatively charged, thus showing exchange capacity. cationic (CTC). If the pH is lower than PCZ, the soil will be positively charged and will have anion exchange capacity (CTA).
Below is a document that is used in Brazil as a standard standard for soil analysis. In this there is a good description for determining the zero charge point.