Lecture 12a Soil Chemistry / Soil pH

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+)

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

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-]),

pH 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

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 • 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.

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+ H+ soil Ca++ +2H+ + Ca++ soil solution soil colloid solution colloid

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

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 • MEQ of Common Cations • Element Na+ K+ Ca++ Mg++ • Valence 1 1 2 2 • Eq. Wt23/1=23 39/1=39 40/2=20 24/2 = 12 • MEQ wt .023 .039 .02 .012

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

Sources of acidity in Soil • * Hydrogen and Aluminumcations 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+

Sources of acidity in Soil applying anhydrous ammonia • 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

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.

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

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

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

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.

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-

pH values for Midwestern Soils NRCS Data

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

Soil Acidity in the United Kingdom Soil 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 next slide Location of soils previous slide

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.

Soil pH of Europe The End

Lecture 12a Soil Chemistry / Soil pH