Soil pH Flashcards
what does pH stand for?
Hydrogen power (pouvoir hydrogène) it's a logarithmic scale, so each pH measure multiplies the acidity/alkalinity by 10
What is pH?
A measure of a soils acidity or alkalinity by indicating the concentration of H+ on a scale
ex.
pH 5 is acidic
pH 8 is basic
What is a buffering capacity?
A soil’s ability to maintain its pH when changes are being made to the soil
T or F: High CEC = high buffering capacity? why/why not?
TRUE bc of the multiple exchange sites
What is a typical soil pH?
between 4 and 10
What pH do most crops prefer?
5.5-8.5? but really I think it’s 6.5-7.5 (neutral) double check
Which plant nutrient’s availability is the most pH dependent?
pHospHorous!
T or F: in very acidic soils, all nutrients are deficient except Fe, Mn, and Z and they can be toxic in their concentration
TRUE
T or F: some nutrients like Fe, Al, and Mn can become toxic to some plants in high concentrations
TRUE
T or F: fungi tend to be less tolerant to acidity
FALSE, fungi tend to be MORE tolerant to acidic soils (think about coniferous forests = very acidic)
what are Acidophiles? and can be found where? What do they do in these places?
organisms that can live at low pHs
can be found in sulphur pools, geysers, and mine drainage
they oxidize reduced sulphur into sulphuric acid
What are alkaliphiles? where do they live? what do they do?
organisms that can live at very high pHs - very few
found in highly basic conditions like soda lakes and high carbonate soils
can be distinguished into 2 groups - alkaliphiles and haloalkaliphiles (salt)
T or F: bacteria and actinomycetes are more tolerant of acidic conditions than fungi?
FALSE - fungi is more tolerant of acidity than bacteria and actinomycetes
T or F: fungi can grow at pH 3?
TRUE
What are calcareous soils? What happens to them when ___ gets into the soil particles?
Do they have a high or low pH?
Sodium rich soils
When sodium gets into the soil particles, they explode and can create cut outs and deep gorges
High pH
What are the acidic cations (2) and which are the basic (5) cations?
Acidic:
Hydrogen + Aluminum
Basic: Calcium Magnesium Sodium Potassium Ammonium
When do soils become acidic?
When cation exchange occurs and H+ is added to the solution and exchanges with non-acid ions (Ca, Mg, K, Na) held onto humus and clay colloids and then the non-acids can be easily leached with the soil solution, leaving behind an H+ dominated soil.
What are the 3 pools of acidity? Describe them
- Residual/reserve Acidity:
- the H+ and Al3+ ions that are BOUND (non-exchangeable) on soil particles - Exchangeable/ Adsorbed Acidity:
- H+ and Al3+ ions that are EASILY exchanged by other cations in the soil solution
- pH will change - Active Acidity:
- the H+ and Al3+ ions IN the soil SOLUTION
To measure pH, which pools of acidity do we look at?
soil solution NOT the reserve
Why can it be difficult to alter the pH of a clay soil? Which pools of acidity must be considered to change the pH?
Because the reserve acidity pool will continue to send out H+ or Al3+ ions to the rest of the soil to maintain the acidic pH - must apply enough lime to counter that and then actually neutralize the pH in the other pools = BUFFERING CAPACITY
both the residual and exchangeable pools must be considered
If you wanted to change the pH of a sandy soil, which pool of acid would not be as important to consider?
The reserve pool, you would just need to change the acidity of the solution - active!
LOW BUFFERING CAPACITY
What is the difference between the reserve and active acidity pools?
Active: the H+ ions IN THE SOLUTION
Reserve: is the H and Al ions that are absorbed
What are the different methods of measuring pH? which is the best and why?
- Field probe (what we used in class) = the best because it is the most accurate; relatively inexpensive; can take it right out into the field; very clear
- Colour strips: great for a backyard test to know whether soil is acidic or basic, but doesn’t tell you the actual pH - inaccurate; could be confused/dirtied/or lost in the field
What are the 5 inherent factors affecting pH?
ClORPT!
- Climate: rainfall, temperature, weathering - effect leaching rates, erosion/weathering
- Parent material: mineral content effects soil texture
- organisms: vegetation
- relief: slope, etc.
- time: the longer the action, the more time there is to change pH
Why do warm, humid, and high rainfall climates have lower pH soils?
Increased acidification is due to the high leaching rates from the high rainfall
Why does leaching increase pH?
it removes the basic ions that are replaced
How does a dry environment effect pH? Why?
where weathering and leaching are less intense, soils can have a high pH or be neutral because base cations are able to accumulate in the soil rather than be leached out or eroded and OH- ions can offset H+ ions
T or F: Temperate coastal climates contribute to lower pHs? why?
TRUE bc of the higher rainfall levels
ppt = more evaporation and leaching = more H+ ions in soil and less base ions
What are 7 causes of acidity?
- Rain (carbonic acid)
- biological activity
- accumulation of OM
- nitrification
- Acid rain (industry)
- plant uptake of cations and release of H+ (especially when plants are harvested, base ions are removed from the soil instead of being put back in when the plant dies)
- Industrial fertilizers
What are 4 problems with acidic soils?
- Al, Mn, Fe can become toxic to plants
- reduced microbial activity
- deficiency of other essential plant nutrients (Mg, Ca, Molybdenum)
- slow OM decomposition - deficiency in N, P, S
What are 8 ways a pH level that is too high or too low can effect soil function?
- nutrient deficiencies from leaching
- decline in microbial activity
- decrease in crop yields
- deterioration of overall soil health
- inhabitation of the N cycle
- limited effectiveness of herbicide and insecticides
- limited solubility of heavy metals
- a lack of effectiveness of herbicides
What are 4 ways acidification be limited or corrected?
- adding lime to increase pH
- adding N or S in the correct amounts and at the right times can be beneficial for plants
- diversifying crop rotations
- OM application increases the buffering capacity for pH changes
What pH does an acid have?
less than 7
What pH does a base/alkaline have?
more than 7
What is an example of something that has neutral pH?
pure water, it is neither an acid or a base
T or F: a solution that contains many H+ ions will be acidic
TRUE because a basic substance will combine with the H+ ions in a soil to neutralize the acid
What happens when CO2 from the atmosphere dissolves?
Carbonic acid is formed (a weak acid). then some of the weak acid will break down and free H+ ions making it more acidic
Why is CO2 in water bad? what’s an example of this?
it mixes with the H+ ions in water and becomes more acidic - think ocean acidification
T or F: a solution with a high concentration of OH- will be basic?
TRUE
What does soil pH affect? (5) and what is it an indicator of?
pH is a major indicator of soil health!
- crop yields
- crop suitability (which crops can be grown in that pH)
- plant nutrient availability
- survival of soil microorganisms
- key processes in the soil!
Give 2 examples of crops that can be grown in very acidic soils (pH around 4.5-5)?
Blueberries and cranberries
Give 2 examples of crops that can be grown around neutral pH (6-6.5)?
corn and oats
Give 2 examples of crops that can be grown around/above a pH of 7?
alfalfa and barley
Which pH is best for most plant nutrients?
around 6.5-7
In acidic soils, which plant nutrients are NOT deficient? (3)
Iron, Manganese, and Zinc
Large amounts of which plant nutrients can become toxic to plants? (3)
Aluminum, Iron, Manganese
at a high pH which plant nutrients will be deficient? (5)
Iron, Zinc, Manganese, Boron, and Copper
What elements combine to make sulphuric acid? when does this occur?
Pyrite and sulphur when oxidized make sulphuric acid which can leach out a very strong acid.
This can occur when we take our marine deposits and dump them on the surface
What does calcareous mean? where is an example?
Sodium rich soil
- so much sodium that the soil particles explode when they are intruded by sodium = creates the deep gorges in soil
Ex. Australia
why do humid regions become acidic quicker?
more base ions are lost at a faster rate because of the rainfall; lots of H+ ions coming in and replacing basic ions that are then lost
why do arid regions become acidic?
there are lots of acids coming into the soil but not being removed from leaching because there is not enough rainfall, the build up of H+ allows more to replace any base ions
On a fundamental level, when do soils become basic?
When CE occurs and H+ is exchanged onto the soil particle
more H+=More acidic
what does adsorbed mean?
the ions are exchangeable
what does residual mean?
the ions are bound to a soil particle
Through which process does bound aluminium supply H+ ions into the soil solution?
Hydrolysis
Why is it difficult to alter the pH of a soil that is high in clay/OM/colloids?
because there are much more ions in the residual and exchangeable pools of acidic ions than there are in the active soil solution.
Al is bound tightly to the colloid surfaces and through hydrolysis releases H+ ions into the soil.
Neutralizing/liming will not be very effective because the ions can be replaced very quickly from ions in the residual or exchangeable pools of acid.
WHY CLAYS/COLLOIDS HAVE HIGH BUFFERING CAPACITY
Active acidity is…
the H+ ions in the soil solution
Adsorbed H+ and Al3+ ions are the….
reserved acidity pool
how does carbonic acid cause soil acidity?
CO2 from the atmosphere or soil air mixes the H+ ions in water and becomes very acidic
root respiration and soil decomposition can increase CO2 levels
ie. marine deposits being taken out and oxidized
How does biological activity cause soil acidity?
when microbes decompose OM they produce and release organic acids
How does the accumulation of OM cause soil acidity? what’s an example of soil that is very acidic because of this?
OM is a source of H+ ions and
cations are leached when soluble humic complexes combine with non-acid cations like Ca2+
ex. Peats have very low pH
How does nitrification cause soil acidity?
The process of Nitrification (nitrifying bacteria oxidizing ammonium ions (NH4+) into NO3-) results in H+ ions being released into the soil
How do acids in precipitation/industry emissions/acid rain cause soil acidity?
burning coal/petroleum generates nitric and sulphuric acids when oxidized with atmospheric oxygen and water. These acids then fall onto soil as precipitation and H+, NO3-, and SO4, ions cause acidification. This also mobilizes Al and increases leaching of Ca and Mg.
How does plant uptake of cations cause soil acidity? How does this relate to pH decline when crops are removed and/or soils are left fallow?
plant roots uptake cations and release H+ ions in exchange.
when crops are removed, they take with them important cations and leave behind H+ ions = increasing acidity
cover crops or leaving plant residue on the surface can ensure the nutrients continue to cycle
- How do fertilizers cause acidity?
- Which fertilizers have lower acidification potential? why?
- Which type of fertilizer can actually increase pH?
- An ammonium-based fertilizer will acidify soil as it generates 2 H+ ions for each ammonium molecule that is nitrified - releases a lot of H+ ions into the soil.
- Anhydrous ammonia and urea have lower acidification potential because 1 H+ ion is consumed in conversion to ammonia.
- Nitrate-based fertilizers can increase pH because one H+ ion is absorbed by the plant (or OH- is excreted) in the uptake of nitrate
What is a chemical concern for soils with a high pH? why?
HIGH LEACHING POTENTIAL FOR NITRATE IN WATER SUPPLY
if conditions are favourable for nitrification, this process will increase because the increased availability of Ca and Mg can outcompete uptake of other cations like NH4 and K and NO3 is very easily leached through water
what are 4 chemical concerns for soils with a low pH?
- ammonium can accumulate in the soil because microbes cannot oxidate NH4+
- high concentrations of Fe, Mn, Al can become toxic
- micronutrients become insoluble
- Mg, Ca, Mo are deficient and P is not available
What is Aluminum’s role in soil acidity?
through hydrolysis, Al binds with OH when added to water and releases H+ ions into the soil solution, increasing the acidity
What is the primary source of H+ ions in acid soils?
Al3+ ions and associated hydroxyl Al ions through hydrolysis reactions
What does aluminum toxicity in highly acidic soils do to plants?
stunts root system growth
Order from lowest to highest amount of OM in soils: 1. Grasslands; 2. Forests; 3. Agricultural land. why does the highest have so much OM?
LOWEST: agricultural land
Medium: Forests
HIGHEST: grasslands because the grasses have extensive underground root systems creating more stable structure and more fertile soil
How does liming work to neutralize/raise pH of acidic soils?
Lime is calcium carbonate
the carbonate ions neutralize acidic ions when added into the soil, NOT THE CALCIUM
What are the 2 common liming materials? which is best for sandy soils that lack Mg?
- ground calcitic limestone (CaCO3 - calcium carbonate): almost pure calcium carbonate
forms on the seafloor from deposits of calcium precipitate - Dolomitic limestone:
- high in Mg
- mix of CaCO3 and Mg carbonate - GREAT FOR SANDY SOILS
Which is better for maintaining a neutral pH: one large dose of lime or multiple smaller doses of lime? why?
multiple small doses because soil pH will gradually decline until there is a new application of lime
bad to do in one large go because it can be lost to erosion or leached with run off water
as pH lowers, what happens to the nutrition, yield, and growth of most crops?
they will all decrease
4 examples of management/dynamic factors that affect soil pH?
- liming can raise pH of an acidic soil
- cover crops increase OM and maintain the buffering capacity of a soil and also continue the nutrient cycling
- removal of crops also removes the important cations that plants uptake in exchange for the H+ ions they release into the soil
- application of fertilizers
How can gypsum help with highly acidic soils that have a toxic Al concentration?
It will not alter the pH, but the calcium can help alleviate the Al toxicity by replacing it
How does lime work? (2)
- the anion base (carbonate, hydroxide, and oxide) neutralizes the acidity by accepting H+ ions from the soil solution and makes them part of the water molecules
- Calcium (or Mg) can replace H+ and Al ions on exchange sites by MASS ACTION = frees them into the soil solution and allows them to be neutralized
How does burned or hydrated limestone compare to dolomite limestone in terms of effectiveness?
Burned/hydrated are fast acting vs. dolomite are slower and require more application steps
What 4 factors can indicate the amount of lime required?
- current pH
- desired pH
- buffering capacity of the soil
- the type of liming material to be used
