Title: Lecture 12a Soil Chemistry Soil pH
1Lecture 12aSoil Chemistry / Soil pH
- Soil pH is the single most important chemical
property of the soil (like soil texture is to the
physical properties) - Knowing the pH of the soil will quickly allow you
to determine if the soil is suitable for plant
growth and what nutrients will be most limiting. - Strictly speaking, hydrogen ions are protons and
do not exist in the naked state in fluids
instead they react with water (H20) to form
hydronium ions, such as H3O - For most purposes H can be used to represent
these hydrated protons.
2Acid Base Chemistry
- Acid-base chemistry is an important part of
everyday life. The excess hydronium (H3O) ions
in acids give them interesting properties. - Acids can react with metals and other materials.
The strong acid HCl is produced in your stomach
to help digest food. In dilute concentrations,
acids are responsible for the sour taste of
lemons, limes, vinegar and other substances. - Bases are also very reactive. The strong base
NaOH is used in many household cleaning agents
such as oven cleaner and drain clog-remover. - How do we measure the concentration of an acid or
base?
H3O depends on the Strength of acid and Initial
concentration Of acid
3Measuring Acidity
- The acidity (or basicity) of a solution is
measured using the pH scale. (this scale is used
because of the very small concentrations that are
being measured) - The pH scale corresponds to the concentration of
hydronium ions in a solution. - If you take the exponent of the H3O
concentration and remove the negative sign, you
have the pH of a solution. - For example, in pure water the concentration of
hydronium ions is 1 x 10-7 M. - Thus, the pH of a solution of pure water is 7.
- The pH scale ranges from 0 to 14, where 7 is
considered neutral (H3O OH-),
4pH
Hydronium ions Hydroxide ions Water
- Acid solutions are when pH is lt 7.0
- Alkaline solutions are when pH gt 7.0
- An acid can be defined as a proton donor, a
chemical that increases the concentration of
hydronium ions in solution. - Conversely, a base is a proton acceptor, a
chemical that reduces the concentration of
hydronium ions in solution (and increases the
concentration of hydroxide ions).
Neutral Solution
Slightly Acidic Solution
5Soil pH- Measure H in the Soil Solution
- pH - the negative log of the hydrogen ion(H)
concentration in the soil water solution. - pH - log H
- the pH scale is how we measure acidity and
alkalinity of solutions -----at neutral (pH 7)
the number of H OH- - Remember
- at pH of 6 there are 10x more H ions than at a
pH 7 - and there are 100x more H ions between pH 7
5 - NEED TO LOOK AT Cation Exchange before further
discussion of soil pH
6Soil Cation Exchange
- Cation Exchange - the ability of the soil to hold
onto nutrients and prevent them from leaching
beyond the roots. - Cations are charged ions Ca, Mg, K,
NH4, - The more cation exchange a soil has the more
likely the soil will have a higher fertility
level.
7Cation Exchange
- The interchange between a cation in solution and
another cation on the surface of any negatively
charged material such as clay or organic matter
H H
soil
Ca 2H
Ca
soil solution
soil
colloid solution
colloid
8Cation exchange influenced by
- 1) Strength of adsorptionAl3 gt Ca2 gt Mg2 gt
K NH4 gt Na gtH - held tight --------------------------gt easily
replaced - 2) the relative concentration of the cations in
the Soil Solution
9Cation Exchange Capacity
- 1) the number of cation adsorption sites per unit
weight of soil or - 2) the sum total of exchangeable cations that a
soil can adsorb. - CEC is expressed in milliequivalents (meq) per
100 g of oven dry soil. - Equivalent weight molecular or atomic wt (g)
- valence or
charges per formula
10Milliequivalent (MEQ)
- 1 meq wt. of CEC has 6.02 x 10 20 adsorption
sites - MEQ of Common Cations
- Element Na K Ca Mg
- Valence 1 1 2 2
- Eq. Wt 23/123 39/139 40/220 24/2
12 - MEQ wt .023 .039 .02 .012
11Back to Soil pH
- Active Acidity - due to the H ion activity in
the soil solution at any given time - Reserve Acidity (on sites)- represented by the H
and Al3 that are easily exchanged by other
cations (positively charged ion) - H H H H H H
- H Ca H
- Mg Mg H
- Ca
Ca H H - H H H Na
soil
Reserve Acidity Active
Acidity -Soil solution
12Sources of acidity in Soil
- Hydrogen and Aluminum cations are responsible
for soil acidity - Exchangeable Hydrogen is the main source of H
at pH 6 and above. Below pH 6 Aluminum is the
main source of H due to dissociation of Al from
clay minerals. Aluminum becomes more soluble at
lower pHs - Al3 H20 ----gt Al(OH) H
- Al(OH) H2O ---gt Al(OH)2 H
- Al(OH)2 H20 ---gt Al(OH)3 H
13Sources of acidity in Soil
- 1. Nitrification Ammonium to Nitrate (oxidation
of NH4) - NH4 2O2 ---gt NO3- H2O 2 H
- 2. O.M. decomposition
- organic acids ionized
- R-COOH---gt R-COO- H
- respiration CO2 H2O ----gt
H2CO3 H HCO3-
143. Acid rain
- Acid rain is caused by the burning of fossil
fuels. - Burning oil, gas and coal in power stations
releases Sulfuric Dioxide (SO2) into the
atmosphere. - Burning oil and gasoline in motor vehicles puts
nitrogen oxides (NOX) into the atmosphere. - These gases mix with water droplets in the
atmosphere creating weak solutions of nitric and
sulfuric acids. - When precipitation occurs these solutions fall as
acid rain.
15Acid Rain in USA
- 3. Acid Rain
- SO2 OH --gt H2SO4 --gt SO4- 2 H
- NO2 OH --gt HNO3--gt NO3- H
Hydrogen Ion Concentration in Rain 2006
gt5.3
lt4.3
16Changes in Acid Rain 1983 to 1997
From Driscoll (2001).
17(No Transcript)
18(No Transcript)
19Dyad
- Have you experienced acid rain? If so where.
- If not, where might you experience the results of
this environmental problem.
20Sources of acidity in Soil (cont.)
- 4. Uptake of basic cations by plants. Basic
cations are sources of OH- to the soil solution. - Ca, Mg, K,
- Basic cations that are taken up by plants no
longer contribute OH- to the soil solution. - H ions are released to the soil solution.
21Leaching
- 5. Leaching of basic cations -
as basic
cations are removed from the soil solution by
leaching they no longer contribute the OH- ions
to neutralize the ever increasing amounts of H - Ca 2 H20 ---gt Ca(OH)2 2H
- -----gt Ca 2OH-
22pH values for Midwestern Soils
NRCS Data
23Soil Acidity and Plant Growth
- Soil acidity is a major environmental stress
factor which limits the growth of most crops. - Acid soils are widely dispersed and comprise
approximately 40 of the arable land in the
world. - In addition, acid rain also accelerates the
acidification of soils. - Aluminum (Al) ion is solubilized from soils at
low pH. This is a major toxic factor for plant
growth for low pH soils.
Acid soil (Yunnan Province, People's Republic of
China).
24Soil Acidity in Australia
- Soil acidification is a natural process that is
part of all landscapes. - Land clearing of native vegetation and their
replacement with productive crops and pastures
has accelerated acidification over the past 200
years. - In the past 50 years in Australia significant
lowering in soil pH due to greater use of
fertilizers, increased production, a greater use
of legumes and an increase in irrigated
agriculture. - An effective option in some cases is a matter of
applying lime as long as the land use is matched
to the capability of the land and soil, available
rainfall, and rate of acidification.
25Acid Sulfate Soils
- Acid sulfate soils form when pyritic (mineral of
FeS2) estuarine sediments in the subsoil are
exposed to air, oxidizing to form sulfuric acid. - A variety of soil minerals react with the acid
and release free aluminum, toxic to crops and
marine life.
The red color of the water is caused by oxidized
iron
Pyrite is the most common iron disulphide
mineral in rock. It is found most often in
metamorphic and sedimentary rocks where it occurs
as either a primary mineral or a fine, widespread
impregnation of subsequent origin. Pyrite is
frequently found in association with coal and
shale deposits.
26- Acid sulfate soils are extremely acidic (at times
less than 3.0) soil horizons resulting from the
aeration of soil materials that are rich in iron
sulfides, (FeS) - Acid sulfate soils are unique in that the impacts
can be so severe that they can affect engineering
works, agricultural productivity, and water
quality of estuarine systems
Acid soils have cost Australia 10 billion in
environmental damage as a result of coastal
development.
27- Iron staining is often a good indicator of
disturbed acid sulfate soils. - When acid sulfate soils are disturbed and undergo
oxidation, the sulfuric acid produced mobilizes
iron, aluminum and heavy metals present in the
soil. - Toxic amounts of dissolved iron can then be
washed into waterways. - This iron can precipitate when in contact with
less acid water, such as rainwater or seawater. - This results in a rust-colored iron oxide scum or
floc which can smother vegetation and stain
concrete and soil. - From 'QASSIT, Qld Department of Natural
Resources and Mines'
Green acid water leached from Acid Sulfate Soils
pours into a river
28Soil pH of Europe
The End