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SFPN Lab 3
Lab 3
32
Agriculture
Undergraduate 4
09/07/2016

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Term
Soils contain these colloids with predominantly negatively charged sites.
Definition
clay and organic
Term
Soils contain clay and organic colloids with these sites.
Definition
predominantly negatively charged
Term
possible sources of negatively charged sites
Definition
-permanent charge
-pH-dependent charges
Term
Permanent charge results from...
Definition
isomorphous substitution in the tetrahedral or octahedral sheet of clay minerals.
Term
pH-dependent charges result from...
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.
Term
how negatively charged sites and cations interact
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.
Term
cations are held on...
Definition
colloidal surfaces
Term
some ways cations get removed from the system
Definition
-plant uptake
-leaching
-precipitation
Term
As the cations in solution are removed from the system, they are replaced by...
Definition
cations from the exchange sites.
Term
cation exchange capacity (CEC)
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).
Term
SI unit for CEC
Definition
cmol(+)/kg
Term
the common acidic cations
Definition
-H+ -Al3+
Term
common basic cations
Definition
-Ca2+ -Mg2+ -K+ -Na+
Term
% base saturation (%BS)
Definition
The proportion of basic cations on the CEC
Term
Recommended liming and fertilization practices will vary for soils based on...
Definition
CEC
Term
relationship between CEC, buffering capacity (BC), and how fast the pH changes
Definition
The pH of soils with high CEC and high buffering capacity changes much more slowly than that of low-CEC soils.
Term
relationship between CEC and how frequently a soil needs to be limed
Definition
high-CEC soils generally do not need to be limed as frequently as low-CEC soils
Term
when high-CEC soils become acid, they require...
Definition
higher lime rates than low-CEC soils to reach optimum pH
Term
other than lime requirement (LR), CEC can also influence...
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.
Term
low CEC soils
Definition
soils w/ CEC less than 2meq/100g
Term
the objective of this CEC lab
Definition
to determine the CEC of a soil sample
with buffered and unbuffered solutions
Term
the 2 methods of measuring CEC
Definition
-direct method (we're using this in this lab)
-sum of exchangeable cations method
Term
the steps of the direct method
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

Term
the exchange reactions for the direct method
Definition
8 NH4++ [(SOIL)-(Al3+ + K+ + Ca2+ + Mg2+)] → [(SOIL)-8NH4+] + Al3++ K+ + Ca2+ + Mg2+ 8 KCl + [(SOIL)-8NH4+] → [(SOIL)-8K+] + 8 NH4+ + 8 Cl-
Term
Sum of Exchangeable Cations Method
Definition
summing up the exchangeable cations replaced by ammonium
Term
how each exchangeable cation can be measured
Definition
quantitatively by Atomic Absorption Spectrophotometry to yield a concentration in the extracting solution
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
Definition
the ratio of soil to extracting solution
Term
the method(s) of measuring CEC that can be used with buffered and unbuffered solutions
Definition
both
Term
A typical buffered solution used for measuring CEC
Definition
ammonium acetate (NH4OAc) adjusted to pH 7, which measures the CEC at pH 7
Term
A typical unbuffered solution used when measuring CEC
Definition
ammonium chloride (NH4Cl), which measures the CEC at the original pH of the soil
Term
When a buffered solution at pH 7 is used to measure the CEC of an acidic soil (pH < 7),...
Definition
the measured CEC is commonly greater than the CEC measured with an unbuffered
solution.
Term
why the measured CEC using a buffered solution is commonly greater than the CEC measured with an unbuffered solution
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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