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Lecture 12a Soil Chemistry / Soil pH

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1 Lecture 12a Soil Chemistry / Soil pH
Soil pH is the single most important property of soil chemistry as soil texture is to 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.

2 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. How do we measure the concentration of an acid or base? H3O+ depends on the Strength of acid and Initial concentration Of acid

3 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 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-]),

4 pH Neutral Solution Slightly Acidic Solution
Hydronium ions Hydroxide ions Water 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

5 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

6 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.

7 Cation 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+ soil Ca++ +2H+ + Ca++ soil solution soil colloid solution colloid

8 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

9 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

10 Milliequivalent (MEQ)
1 meq wt. of CEC has 6.02 x adsorption sites MEQ of Common Cations Element Na K+ Ca++ Mg++ Valence Eq. Wt /1= /1= /2= /2 = 12 MEQ wt

11 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

12 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+ + H > Al(OH) H+ Al(OH) H2O ---> Al(OH) H+ Al(OH) H20 ---> Al(OH)3 + H+

13 Sources of acidity in Soil
1. Nitrification: Ammonium to Nitrate (oxidation of NH4+) NH O2 ---> NO3- + H2O + 2 H+ 2. O.M. decomposition organic acids ionized : R-COOH---> R-COO- + H+ respiration: CO2 + H2O ----> H2CO3 = H+ HCO3-

14 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.

15 Acid Rain in USA 3. Acid Rain NO2 + OH --> HNO3--> NO3- + H+
SO2 +OH --> H2SO4 --> SO H+ NO2 + OH --> HNO3--> NO3- + H+ Hydrogen Ion Concentration in Rain – 2006 >5.3 <4.3

16 Changes in Acid Rain 1983 to 1997 From: Driscoll (2001).

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

20 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.

21 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 H20 ---> Ca(OH)2 + 2H+ -----> Ca OH-

22 pH values for Midwestern Soils
NRCS Data

23 Soil Acidity and Plant Growth
Soil acidity is a major environmental stress limiting the growth of many crops. Acid soils comprise 40% of the arable land in the world. Acid rain also accelerates soil acidification. Aluminum (Al) ion is solubilized from soils at low pH. it is a major toxic factor for plant growth for low pH soils. Acid soil (Yunnan Province, People's Republic of China).

24 Soil 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.

25 Acid 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
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 Toxic amounts of dissolved iron can then be washed into waterways.
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

28 Soil pH of Europe The End


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