Cation Exchange Capacity & Buffering Capacity
In this blog we understand, what is cation exchange capacity, Percent saturation and buffering capacity and the relationship between them?
Cation & Anion
In order to absorb nutrients by the plants, the nutrients must be dissolved. When nutrients are dissolved, they are in a form called “ions”. This simply means that they have electrical charges.
For example, salt is sodium chloride (NaCl), when it dissolves it forms two ions; one of sodium (Na+) and one of chloride (Cl–).
Here, the sodium has a plus charge and is called a “cation”. The chloride has a negative charge is called an “anion”.
As, “opposites attract” and “likes repel”, nutrients in the ionic form can be attracted to any opposite charges present in soil.
Some important elements with a positive electrical charge in their plant-available form include potassium (K+), ammonium (NH4+), magnesium ( Mg++), calcium (Ca++), zinc (Zn+), manganese (Mn++), iron (Fe++), copper (Cu+) and hydrogen (H+). While hydrogen is not a nutrient, it affects the degree of acidity (pH) of the soil, so it is also important.
Nutrients have a negative electrical charge in their plant-available form include nitrate (NO3–), phosphate (H2PO4– and HPO4—), sulfate (SO4–), borate (BO3–), and molybdate (MoO4—). Phosphates are unique among the negatively charged anions, in that they are not mobile in the soil.
See… MINERAL NUTRITION- AN OVERVIEW
Cation Exchange Capacity
- CEC is an estimate of the soils ability to attract, retain, and exchange cation elements. It is expressed in millequivalents per 100 grams of soil (meq/100g).
- The clay and in some extent organic matter components of soil have a large number of negative charges on their surface which attract cation elements and contribute to a higher CEC. At the same time, they also repel anion nutrients (“like” charges). Sandy soil has lower CEC while clay has higher CEC.
- Larger CEC values indicate that a soil has a greater capacity to hold cations. Therefore, it requires higher rates of fertilizer or lime to change a high CEC soil.
- Soil with high CEC requires a higher soil cation level, to provide adequate crop nutrition.
- Low CEC soils hold fewer nutrients, and will likely be subject to leaching of mobile “anion” nutrients. These soils may benefit from split applications of several nutrients.
- The particular CEC of a soil is neither good nor bad, but knowing it is a valuable management tool.
Percent Saturation
- Both Percent Nutrient Saturation and Percent Base Saturation refer to a measurement of the percent of the soil CEC that is occupied by a particular nutrient (nutrient saturation), or the sum of a group of nutrients (base saturation).
- It is useful in predicting the soils ability to provide adequate crop nutrients, and indicate needed changes in fertilizer or lime applications.
Soil pH
- Soil pH is a measure of the soil acidity or alkalinity and is sometimes called the soil “water” pH. This is because it is a measure of the pH of the soil solution, which is considered the active pH that affects plant growth.
- The total range of the pH scale is from 0 to 14. Values (pH<7.0) are acidic and (pH >7.0) are alkaline. A soil pH of 7.0 is considered to be neutral.
- The ideal pH for most plants is 6.5 to 7.5. The small changes in a soil pH can have large effects on nutrient availability and plant growth.
See… Acidic, Saline and Alkaline Soil
Buffer pH (BpH)
- Basically, the BpH is the resulting sample pH after the laboratory has added a liming material. So BpH value is generated in the laboratory for lime recommendation. It is not an existing feature of the soil nor any other practical value.
- Simply, if after adding buffer solution pH changes mean that low lime is required and if a little changed then large amount of lime is required.
- The reasons that a soil may require differing amounts of lime to change the soil pH relates to the soil CEC and the “reserve” acidity that is contained by the soil.
- Soil acidity is controlled by the amount of hydrogen (H+) and aluminum (Al+++) that is either contained in, or generated by the soil and its components.
- Soils with a high CEC have a greater capacity to contain or generate these sources of acidity. Therefore, at a given soil pH, a soil with a higher CEC (thus a lower buffer pH) will normally require more lime to reach a given target pH than a soil with a lower CEC.
Soil pH buffering
- The buffering capacity of a soil indicates the capacity of the soil to resist pH change.
- Hydrogen ions in soil are present both in the soil solution and adsorbed onto the soil surfaces. pH measures the concentration of hydrogen ions in the soil solution.
- Soils with large numbers of sites able to hold hydrogen ions thus able to resist change in the concentration of hydrogen ions in the soil solution and therefore have a high buffering capacity.
- Clay soil has higher buffering capacity while sandy soil has lower. Therefore, Sandy soils acidify quicker because of the lower buffering capacity but the pH can be recovered faster with the application of less lime compared to clay soils.
- Soils with a high proportion of clay or organic matter have a larger number of surface sites able to hold hydrogen ions and are able to resist a decrease in pH.
Read also…
ROLE OF ESSENTIAL PLANT NUTRIENTS
ESSENTIAL NUTRIENTS-DEFICIENCY SYMPTOMS
SOIL FERTILITY AND PRODUCTIVITY
FORMATION OF SOIL-PROCESS & FACTOR OF SOIL FORMATION