SWS3022 Exam Questions and Answers All Correct
What happens to phosphorus when the soil is basic (high pH)? - Answer-- insoluble solids are formed (Calcium phosphates)
- Ca + HPO4 = CaHPO4
- CaHPO4 will crystallize overtime and become highly insoluble
What pH is preferred for plant availab...
SWS3022 Exam Questions
and Answers All Correct
What happens to phosphorus when the soil is basic (high pH)? - Answer-- insoluble
solids are formed (Calcium phosphates)
- Ca + HPO4 = CaHPO4
- CaHPO4 will crystallize overtime and become highly insoluble
What pH is preferred for plant availability/solubility? - Answer-6.5
What are the four types of phosphorus immobilization? - Answer-- low pH (Fe and Al
phosphate solids)
- high pH (Ca phosphate)
- fixation in crystalline Fe and Al oxides
- anion exchange
How can you neutralize an acid? - Answer-- bases react with hydrogen and remove it
from the soil
OH- + H+ -> H2O
CO3^2- + H+ -> HCO3-
What is a common base used to increase the pH of the soil? - Answer-Decrease the H+
concentration in solution
- prevents Al toxicity
CaCO3
This is called liming
CaCO3 -> Ca2+ + CO3^2-
CO3^2- + H+ -> HCO3-
What is Al toxicity? - Answer-- restricts cell wall expansion an damages membrane sites
where Ca comes in
- more common and hurtful at pH below 5.2
What is finite immobilization? - Answer-- only a certain amount of Phosphorus that soil
can handle
- can exceed phosphorus absorption capacity
- if this is exceeded, phosphorus become more mobile in the soil so it becomes more
plant available, but greatly impacts aquatic systems (eutrophication)
,What is an acid? - Answer-any substance which increases the hydrogen ion
concentration in solution
pH of 0-7
high H+ concentration
What are sources of acids? - Answer-- atmosphere: respiration from microorganisms
- acid functional groups on organic matter
ammonium oxidation: nitrification and fertilizers
T/F charge balance must be maintained when plants uptake cations - Answer-true
Which side of the US has acidic vs basic rain? - Answer-- eastern US has acidic rain
- western US has basic rain
What is a base? - Answer-any substance that decreases the hydrogen ion
concentration in solution
OH- and CO3 ^2-
pH of 4-14
low H+ concentration
What are the two ions considered acidic in soils? - Answer-H+ and Al
- both are cationic
- Al participates in hydrolysis reactions with water
What is the hydrolysis of water? - Answer-- self-ionization of water
- H2O -> OH- + H+
H+ is the acid
OH- is the base
- in pure water, H+ and OH- amounts are equal
What is hydrolysis by aluminum? - Answer-- Al splits water and reacts with resulting
OH-
H2O -> OH- + H+
Al3+ + H2O = Al(OH)2+ + H+
Al(OH)2+ + H2O = Al(OH)2+ + H+
What is active acidity? - Answer-- acidity associated with soil solution
- this is what is measured when you take the pH of the soil (measure with a pH meter)
- cations held in solution
What is mineral formation? - Answer-When aluminosilicates dervive charge from
substitution of lower-charged cations for higher charged cations in the crystal matrix
What charge results from mineral formation? - Answer-permanent negative charge
What substitution happens during mineral formation? - Answer-- Al3+ for Si4+
, - Mg2+ for Al3+
What do charges on silicate crystal edges and on Al/Fe oxides depend on? - Answer-
protonation and deprotonation (results in variable, pH-dependent charge
What do organic colloids derive their charge from? - Answer-protonation/deprotonation
of hydrogen ions from acidic functional groups on humic organic materials
How do you measure active acidity? - Answer-- 1:1 or 2:1 extract (10g soil and 10 mL
water or 10g soil and 20 mL water)
What is the pH formula concerning log? - Answer-pH = -log(H+)
-pH = log(H+)
10^-pH = 10^log(H+)
10^-pH = (H+)mol/L
What is exchangeable acidity? - Answer-- H+, Al3+Al(OH)2+, and Al(OH)2+ are all
cations
- acidity associated with cation exchange sites on mineral or organic colloids
- aka salt-replaceable acidity
- 100-1,000x more than in active acidity
- allows it to contribute to the pH of soil
- cations held near minerals and materials
What is residual acidity? - Answer-tightly bound to clay minerals and humic organic
materials
What is the formula for total acidity? - Answer-total acidity = active + exchangeable +
residual
T/F Acids increase the H+ concentrations in solution - Answer-true
Acidity associated with the soil solution is called: - Answer-active acidity
Identify one source of H+ ions to soil - Answer-atmosphere (respiration from
microorganisms)
T/F Rainfall is naturally acidic - Answer-true
The two ions considered acids in soils are: - Answer-H+ and Al
Is CEC a good indication of fertility? - Answer-no
- CEC suggests ability of soil to store plant nutrients
Describe an example of ion saturation with a CEC of 20 cmolc/kg - Answer-Ca = 10
cmolc/kg (50%)
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