Clay and humus particles are known as soil colloids and represent the chemically-active portion of soils. With the exception of soils with very low pH, most soils carry a net negative charge . These negative charges are located on the surface of soil colloidal particles.
Figure 1 below, and figures 9.9 and 9.13 in Plaster, (1992, pages 203 and 206 respectively) illustrate the surface of soil colloids . Note the many negatively-charged sites. These negatively-charged sites attract positively charged ions, cations , in the soil solution. Cations from solution become adsorbed to the colloid surface until all the negative charges are satisfied.
Adsorbed cations may include the following:
Ca2+, Mg2+, K+, NH4+, H+, Na+, Cu2+, Zn2+, Mn2+.
Many soil cations are important to plant nutrition.
Fig. 1. Conceptualization of a soil colloid particle.
Adsorbed cations can exchange position with other cations in the soil solution and become available for uptake by plant roots. The cations that can exchange places with cations in solution are known as exchangeable cations . The quantity of cations that a soil can adsorb (the quantity of negative charges) is called the soil's cation exchange capacity (CEC). Two chemical equations illustrating this reaction are:
NH4+ + H-soil colloid -------> NH4-soil colloid + H+
Ca2+ + H2-soil colloid -------> Ca-soil colloid + 2H+
Different soils have different quantities of adsorption sites and have correspondingly different CECs. A soil with a high CEC can adsorb more exchangeable cations than a soil with a low CEC. Soils with high CEC are generally considered more fertile since they can store more plant nutrients. They may also require greater amounts of chemical amendments, such as lime or gypsum, to reach a desired condition.
Determining soil pH requires measuring the amount of H+ in solution (H+ activity). The most accurate method for this uses a pH meter and electrodes. The difference in [H+] between the soil and solution in the electrode causes an electrometric potential difference, which is read on the pH meter.
Dyes are also used to determine pH. They are not quite as accurate as the electrode, but are good enough for most uses and easier to use in the field than most electrode methods. The dyes used in this method change color at different pH. Mixing a few drops of dye with soil and allowing a few minutes for reaction results in the color change which is then compared to a color chart to provide a good field estimate of the soil pH.
Soil pH does not predict the amount of lime necessary to neutralize an acid soil. That requires a determination of a soil's reserve acidity (lime requirement) which is a function of the soil's CEC as well as its acidity. To measure reserve acidity, the soil's cations must be extracted with a buffer solution. The adsorbed soil cations exchange with cations in the buffer solution; then they are collected by filtering. The quantity of acidic cations determines the amount of lime necessary to neutralize the acidity. A soil with a high CEC will require more lime than one with a low CEC.
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1. What are soil colloids and why are they important? 2. Why are soils with a high CEC considered to be more fertile than soils with a low CEC? 3. Can the lime requirement of an acid soil be determined from the soil pH? Explain your answer. 4. If all the soil you tested for CEC had equal pH values, which would require the most lime to neutralize exchangeable acidity? 5.What method might you use to remove the dye ions adsorbed on the colloids? 6. Looking at your data for both CEC and pH, which soil do you think will have the highest lime requirement to neutralize exchangeable acidity? Justify your answer. 7. Given your CEC and pH data, whcih soil would be best for general plant growth: a) if the soil was not treated, and b) if corrected for pH? Justify your answer. 8. Draw a diagram (refer to page 59 in study guide and illustratin above on this page) of a soil colloid surface and cations (adsorbed and in solution) that compares a relatively neutral soil to an acid one. 9. Define isomorphous substitution and use an example to illustrate this process. 10. The rust colored and adjacent gray colors as mottles or streaks in a soil are caused by what chemical reaction? 11. A purple cationic dye is filtered through 3 different soils. The filtrate for each appears as follows:
Has the greatest net negative charge? Most likely is a clay or clay loam? Is likely to have the greatest percentage of sand? Study Questions Answers1. Soil colloids are clay and humus particles that have net negative charges and represent the chemically active portion of soils. They affect many soil properties such as CEC, pH and therby affect soil fertility and water quality.2. High CEC soils can adsorb more exchangeable cations, and therefore can store more plant nutrients than soils with a low CEC. 3. No. Soil pH determines the active (exchangeable) acidity. The reserve acidity, a function of CEC as well as acidity, can be calculated from a buffer extract pH From this, the lime requirement of the soil is determined. 4. At equal pH, the soil with the highest CEC would require the most lime to neutralize the exchangeable acidity. 5. To remove the dye ions from the soil samples used in the Cation Exchange procedure, you might add another solution with large amounts of ex hangeable cations to the soil and filter it. 6. This isn't quantitative, but generally the soil with the highest CEC will reuire the most lime. A very acid, sandy soil will not require very much lime to neutralize the acidity; a moderately acid, clayey soil may require quite a bit. 7. The correct answer will include consideration for pH, CEC and texture. 8. A diagram comparing colloid surface and cations of acid and neutral soils should indicate relatively more acidic cations in the acid system, more basic cations in the neutral system. 9. According to Plaster (1997), this is defined as the "replacement of one atom by another of similar size in a crystal lattice. May result in a charge if the atom has a different charge than the one replaced." Remeber, this occurs within the clay lattice and results in a permanent charge to the colloid. Examples of isomorphous substitution are: replacement in the silica tetrahedra of a Si4+ by an Al3+ replacement in the alumana octahedra of an Al3+ by an Fe2+or a Mg2+ 10. These "mottles" are the result of oxidation and reduction of iron. They occur when a soil is frequently saturated, has a seasonal high water table. The more appropriate term for these colored splotches and streaks is "redoxomorphic features." 11. The soil with the filtrate that is almost clear would be expected to have the greatest net negative charge. That same soil would most likely have the greatest percentage of clay relative to the other two soils, so could be clay or clay loam The soil with the darkest filtrate could be expected to be high in sand (little charge, small surface area by volume of soil)
Exam Questions from LabsExample Exam questions from Lab1. Which of the following chemical equations represents the cation exchange process in soils?a. NH4+ + H-soil ----> NH4-soil + H+ b. 6CO2 + 12 H2O ----> C6H12O6 + 6O2 + 6H2O c. organic matter + O2 ----> CO2 + H2O + humus d. KNO3 ----> K+ + NO3- e. H-soil <----> H+ + soil- 2. Suppose soil A has a CEC of 26 cmolc/kg and a pH of 6.0; and soil B has a CEC of 9 cmolc/kg and a pH of 6.0. Which soil would have the greatest lime requirement to bring the pH to 7.0? a. soil A b. soil B c. both soils have the same requirement 3. Which of the following soils would be best for general plant growth i no fertilizer or lime additions are made? a. CEC=5.4; pH=5.4 b. CEC=17; pH=4.1 c. CEC=15; pH=8.7 d. CEC=22; pH=6.7 e. CEC=25; ph=7.8 Click here for the Answers to these questions. |
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