Bài giảng Soil pH chemistry

Soil Chemistry / Soil pH n Soil pH is the single most important chemical property of the soil like soil texture is to the physical properties n Knowing the pH of the soil will quickl

Soil Chemistry / Soil pH

n   Soil pH is the single most important chemical property of the soil (like soil texture is to the physical properties)

n   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

n   Hydrogen ions are protons - in water they

react (H20) to form hydronium ions, such as

H3O+

n   For most purposes H+ can be used to

represent these hydrated protons (H3O + )

Acid Base Chemistry

n   Acid-base chemistry is an important

part of everyday life The excess

hydronium (H3O+) ions in acids give

them interesting properties

n   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

n   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

Measuring Acidity

n   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)

n   The pH scale corresponds to the concentration of

hydrogen ions in a solution

n   If you take the exponent of the H+ concentration and

remove the negative sign, you have the pH of a solution

n   For example, in pure water the concentration of hydrogen ions is 1 x 10-7 M

n   Thus, the pH of a solution of pure water is 7

n   The pH scale ranges from 0 to 14, where 7 is considered neutral ([H3O+ ] = [OH-]),

pH

Ø  Alkaline solutions are when pH > 7.0

Ø  An acid can be defined as a

proton donor, a chemical that

increases the concentration of

hydronium ions in solution

Ø  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 ions Hydroxide ions Water

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

Soil Cation Exchange

n   Cation Exchange - the

ability of the soil to

hold onto nutrients and

prevent them from

leaching beyond the

roots

n   Cations are + charged

ions = Ca++, Mg++, K+,

NH4+,

n   The more cation

exchange a soil has the

more likely the soil will

have a higher fertility

level

Cation Exchange

solution and another cation on the surface

of any negatively charged material such as

clay or organic matter

colloid solution

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

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

Milliequivalent (MEQ)

  1 meq wt of CEC has 6.02 x 10 20

adsorption sites

  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

Back to Soil pH

activity in the soil solution at any given

time

by the H+ and Al3+ that are easily exchanged

by other cations (positively charged ion)

Sources of acidity in Soil

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

Acid Rain in USA

  SO2 +OH > H2SO4 > SO4- + 2 H+

  NO2 + OH > HNO3 > NO3- + H+

Hydrogen Ion Concentration in Rain – 2006

>5.3

<4.3

Changes in Acid Rain 1983 to 1997

From: Driscoll (2001)

Dyad

n   Have you experienced acid rain? If so where

n   If not, where might you experience the results of this environmental problem

Sources of acidity in Soil (cont.)

the soil solution

to the soil solution

Leaching

cations -

as basic cations are

removed from the soil

solution by leaching they no

longer contribute the OH-

ions to neutralize the ever

  Ca++ + 2 H20 -> Ca(OH)2 + 2H

+

  -> Ca++ + 2OH-

NRCS Data

pH values for Midwestern Soils

Soil Acidity and Plant

Growth

n   Soil acidity is a major

environmental stress

factor which limits the

growth of most crops

n   Acid soils are widely

dispersed and comprise

approximately 40% of

the arable land in the

world

n   In addition, acid rain

also accelerates the

acidification of soils

n   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

Soil Acidity in the

Location of soils previous slide

Soil Acidity in the United Kingdom

Soil pH of Europe