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Download - Lecture 12a Soil Chemistry / Soil pH Soil pH is the single most important chemical property of the soil (like soil texture is to the physical properties)

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Page 1: Lecture 12a Soil Chemistry / Soil pH Soil pH is the single most important chemical property of the soil (like soil texture is to the physical properties)

Lecture 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.

Hydrogen ions are protons - in water they react (H20) to form hydronium ions, such as H3O+

For most purposes H+ can be used to represent these hydrated protons (H3O+)

Page 2: Lecture 12a Soil Chemistry / Soil pH Soil pH is the single most important chemical property of the soil (like soil texture is to the physical properties)

Acid 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.

H3O+ depends on the

Strength of acid and

Initial concentration

Of acid

Page 3: Lecture 12a Soil Chemistry / Soil pH Soil pH is the single most important chemical property of the soil (like soil texture is to the physical properties)

Measuring 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 hydrogen ions in a solution.

If you take the exponent of the H+ concentration and remove the negative sign, you have the pH of a solution.

For example, in pure water the concentration of hydrogen 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-]),

Page 4: Lecture 12a Soil Chemistry / Soil pH Soil pH is the single most important chemical property of the soil (like soil texture is to the physical properties)

pH Acid solutions are when pH is <

7.0 Alkaline solutions are when pH >

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

Hydronium ionsHydroxide ionsWater

Page 5: Lecture 12a Soil Chemistry / Soil pH Soil pH is the single most important chemical property of the soil (like soil texture is to the physical properties)

Soil 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

Page 6: Lecture 12a Soil Chemistry / Soil pH Soil pH is the single most important chemical property of the soil (like soil texture is to the physical properties)

Soil 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.

Page 7: Lecture 12a Soil Chemistry / Soil pH Soil pH is the single most important chemical property of the soil (like soil texture is to the physical properties)

Cation ExchangeCation 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

soil

colloid

Ca++ +2H+

H+

H+

+ Ca++

soil solution soil colloid solution

Page 8: Lecture 12a Soil Chemistry / Soil pH Soil pH is the single most important chemical property of the soil (like soil texture is to the physical properties)

Cation exchange influenced by:

1) Strength of adsorption:Al+3 > Ca2+ > Mg2+ > K+ =NH4+ > Na+ >H+

held tight --------------------------> easily replaced 2) the relative concentration of the cations in

the Soil Solution K+ K+ Mg++ Mg++ K | K+ K+ K+ Ca++ H+ -- K | Soil

Particle

Page 9: Lecture 12a Soil Chemistry / Soil pH Soil pH is the single most important chemical property of the soil (like soil texture is to the physical properties)

Cation 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

Page 10: Lecture 12a Soil Chemistry / Soil pH Soil pH is the single most important chemical property of the soil (like soil texture is to the physical properties)

Milliequivalent (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 23/1=23 39/1=39 40/2=20 24/2 = 12

MEQ wt .023 .039 .02 .012

Page 11: Lecture 12a Soil Chemistry / Soil pH Soil pH is the single most important chemical property of the soil (like soil texture is to the physical properties)

Back 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

Page 12: Lecture 12a Soil Chemistry / Soil pH Soil pH is the single most important chemical property of the soil (like soil texture is to the physical properties)

Sources 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 pH’s

Al3+ + H20 ----> Al(OH)++ + H+ Al(OH)++ + H2O ---> Al(OH)2

+ + H+ Al(OH)2

+ + H20 ---> Al(OH)3 + H+

Page 13: Lecture 12a Soil Chemistry / Soil pH Soil pH is the single most important chemical property of the soil (like soil texture is to the physical properties)

Sources of acidity in Soil

1. Nitrification: Anhydrous ammonia placed in soil Ammonium to Nitrate (oxidation of NH4+)

NH4+ + 2O2 ---> NO3- + H2O + 2 H+ 2. O.M. decomposition organic acids ionized : R-COOH---> R-COO- + H+ respiration: CO2 + H2O ---->

H2CO3 = H+ HCO3-

Ammonia gas

applying anhydrous ammonia

Page 14: Lecture 12a Soil Chemistry / Soil pH Soil pH is the single most important chemical property of the soil (like soil texture is to the physical properties)

3. 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.

Page 15: Lecture 12a Soil Chemistry / Soil pH Soil pH is the single most important chemical property of the soil (like soil texture is to the physical properties)

Acid Rain in USA 3. Acid Rain SO2 +OH --> H2SO4 --> SO4- + 2 H+ NO2 + OH --> HNO3--> NO3- + H+

Hydrogen Ion Concentration in Rain – 2006

>5.3

<4.3

Page 16: Lecture 12a Soil Chemistry / Soil pH Soil pH is the single most important chemical property of the soil (like soil texture is to the physical properties)

Changes in Acid Rain 1983 to 1997

From: Driscoll (2001).

Page 17: Lecture 12a Soil Chemistry / Soil pH Soil pH is the single most important chemical property of the soil (like soil texture is to the physical properties)
Page 18: Lecture 12a Soil Chemistry / Soil pH Soil pH is the single most important chemical property of the soil (like soil texture is to the physical properties)
Page 19: Lecture 12a Soil Chemistry / Soil pH Soil pH is the single most important chemical property of the soil (like soil texture is to the physical properties)

Dyad

Have you experienced acid rain? If so where. If not, where might you experience the results of this environmental problem.

Page 20: Lecture 12a Soil Chemistry / Soil pH Soil pH is the single most important chemical property of the soil (like soil texture is to the physical properties)

Sources 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.

Page 21: Lecture 12a Soil Chemistry / Soil pH Soil pH is the single most important chemical property of the soil (like soil texture is to the physical properties)

Leaching 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 ---> Ca(OH)2 + 2H+

-----> Ca++ + 2OH-

Page 22: Lecture 12a Soil Chemistry / Soil pH Soil pH is the single most important chemical property of the soil (like soil texture is to the physical properties)

NRCS Data

pH values for Midwestern Soils

Page 23: Lecture 12a Soil Chemistry / Soil pH Soil pH is the single most important chemical property of the soil (like soil texture is to the physical properties)

Soil 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 Landscapes – Central Wales

Page 24: Lecture 12a Soil Chemistry / Soil pH Soil pH is the single most important chemical property of the soil (like soil texture is to the physical properties)

Soil Acidity in the United KingdomSoil acidification is a natural process that is part of all landscape evolution.

Land clearing of native forest vegetation and introduction of pasture grasses and row crops accelerated acidification over the past 3000 years in the UK

Location of soils on nextslide

Location of soilsprevious slide

Page 25: Lecture 12a Soil Chemistry / Soil pH Soil pH is the single most important chemical property of the soil (like soil texture is to the physical properties)

Soil Acidity in the United Kingdom

High rainfall levels and naturally low base status soils has resulted in many soils with pH values less than 4.0

Management of soil pH is an ongoing process on most soils in the western and northern areas of the country.

Page 26: Lecture 12a Soil Chemistry / Soil pH Soil pH is the single most important chemical property of the soil (like soil texture is to the physical properties)

Soil pH of Europe


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