Term
| Soils contain these colloids with predominantly negatively charged sites. |
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Definition
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Term
| Soils contain clay and organic colloids with these sites. |
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Definition
| predominantly negatively charged |
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Term
| possible sources of negatively charged sites |
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Definition
-permanent charge
-pH-dependent charges |
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Term
| Permanent charge results from... |
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Definition
| isomorphous substitution in the tetrahedral or octahedral sheet of clay minerals. |
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Term
| pH-dependent charges result from... |
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Definition
| broken bonds at crystal lattice edges, dissociation of hydroxyl groups from the surfaces of amorphous minerals and hydrous oxides, and dissociation of carboxyl groups and/or hydroxyl groups on organic matter. |
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Term
| how negatively charged sites and cations interact |
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Definition
| These negatively charged sites are able to attract positively charged ions termed cations, which are held on colloidal surfaces and are available for exchange. In general, these exchangeable cations are in equilibrium with cations in solution surrounding the colloidal surfaces. As the cations in solution are removed from the system through plant uptake, leaching, and precipitation, they are replaced by cations from the exchange sites. |
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Term
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Definition
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Term
| some ways cations get removed from the system |
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Definition
-plant uptake
-leaching
-precipitation |
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Term
| As the cations in solution are removed from the system, they are replaced by... |
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Definition
| cations from the exchange sites. |
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Term
| cation exchange capacity (CEC) |
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Definition
| a measure of the negatively charged sites (on the soil colloids) that are capable of adsorbing cations; it is usually expressed in milliequivalents per 100 grams of soil (meq/100 g) or in cmol (+)/kg (SI unit). |
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Term
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Definition
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Term
| the common acidic cations |
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Definition
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Term
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Definition
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Term
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Definition
| The proportion of basic cations on the CEC |
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Term
| Recommended liming and fertilization practices will vary for soils based on... |
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Definition
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Term
| relationship between CEC, buffering capacity (BC), and how fast the pH changes |
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Definition
| The pH of soils with high CEC and high buffering capacity changes much more slowly than that of low-CEC soils. |
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Term
| relationship between CEC and how frequently a soil needs to be limed |
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Definition
| high-CEC soils generally do not need to be limed as frequently as low-CEC soils |
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Term
| when high-CEC soils become acid, they require... |
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Definition
| higher lime rates than low-CEC soils to reach optimum pH |
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Term
| other than lime requirement (LR), CEC can also influence... |
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Definition
time and frequency of application of nitrogen and potassium fertilizers
example: On low-CEC soils (less than 2
meq/100 g), for example, some leaching of ammonium-N and potassium can occur with
fall applications. Thus, spring fertilizer application may mean improved production
efficiency in those soils. |
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Term
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Definition
| soils w/ CEC less than 2meq/100g |
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Term
| the objective of this CEC lab |
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Definition
to determine the CEC of a soil sample
with buffered and unbuffered solutions |
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Term
| the 2 methods of measuring CEC |
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Definition
-direct method (we're using this in this lab)
-sum of exchangeable cations method |
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Term
| the steps of the direct method |
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Definition
1) replacing the normal mixture of cations on the exchange sites with a single cation such as ammonium
2) replacing that exchangeable ammonium with another cation (such as potassium)
3) measuring the amount of ammonium displaced |
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Term
| the exchange reactions for the direct method |
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Definition
| 8 NH4++ [(SOIL)-(Al3+ + K+ + Ca2+ + Mg2+)] → [(SOIL)-8NH4+] + Al3++ K+ + Ca2+ + Mg2+
8 KCl + [(SOIL)-8NH4+] → [(SOIL)-8K+] + 8 NH4+ + 8 Cl- |
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Term
| Sum of Exchangeable Cations Method |
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Definition
| summing up the exchangeable cations replaced by ammonium |
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Term
| how each exchangeable cation can be measured |
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Definition
| quantitatively by Atomic Absorption Spectrophotometry to yield a concentration in the extracting solution |
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Term
something you need to know to calculate the CEC and concentration of exchangeable cations in the
soil using the Sum of Exchangeable Cations Method |
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Definition
| the ratio of soil to extracting solution |
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Term
| the method(s) of measuring CEC that can be used with buffered and unbuffered solutions |
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Definition
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Term
| A typical buffered solution used for measuring CEC |
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Definition
| ammonium acetate (NH4OAc) adjusted to pH 7, which measures the CEC at pH 7 |
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Term
| A typical unbuffered solution used when measuring CEC |
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Definition
| ammonium chloride (NH4Cl), which measures the CEC at the original pH of the soil |
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Term
| When a buffered solution at pH 7 is used to measure the CEC of an acidic soil (pH < 7),... |
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Definition
the measured CEC is commonly greater than the CEC measured with an unbuffered
solution. |
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Term
| why the measured CEC using a buffered solution is commonly greater than the CEC measured with an unbuffered solution |
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Definition
| because pH-dependent charges appear as the soil pH is increased from its original value to a pH of 7 |
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