Soil Chemistry

Colloids Clay minerals and humus and complexes The most chemically active part of soil Large surface area Electrical charge (usually net negative) Some +, some – Nutrient cations (+ions) and anions (- ions) are held on colloid surfaces, in reserve for plants

Why are colloids negative? 1.Clay: Oxygen ions along edges of micelles

2. humus: H+ ions tend to migrate away from organic compounds in humus, to soil solution, leaving net negative charge (OH-)

Ion Exchange What is the exchange? exchange of ions on soil surfaces with ions in soil solution. Cations and anions are involved

Where does exchange take place? Organic colloids (humus) and inorganic micelles (clays) and complexes of both Where do ions in soil come from? Release from organic matter Rain Weathering of parent material

Leaching ions (on soil surfaces) cannot be removed by leaching. ions (in solution) can be removed by leaching.

When soil is dried… …the ions on soil surfaces STAY ON adsorption sites …the soluble ions (in soil solution) precipitate or crystallize (come out of solution) as salts.

Examples of soluble cations precipitating

Ion exchange Exchangeable ions on soil surface trading places with ions in solution.

On soil surfaces, there are exchangeable and nonexchangeable ions Exchangeable: weakly held, in contact with soil solution, ready for quick replacement, available for plants “outer sphere complex”

adsorbed by strong bonds or held in inaccessible places Nonexchangeable: adsorbed by strong bonds or held in inaccessible places (e.g., the K+ between layers of illite) “inner sphere complex” not part of ion exchange !

Cation exchange capacity (CEC) Sum total of exchangeable cations that a soil can adsorb. If a soil has a high CEC, it prevents nutrients from being leached away from roots

CEC Expressed in: milliequivalents per 100 g (meq/100g)

Dynamic equilibrium Strive for equivalent proportions of solution and exchangeable ions. Upset equilibrium by: removal by plants leaching fertilization weathering Initiate ion exchange

Ion exchange example Add K fertilizer Ca+2 Ca+2 K+ Ca+2 + K+ + K+ K+ Ca+2 K+ K+ K+ exchangeable exchangeable solution solution

Rules of ion exchange Process is Reversible Charge by charge basis Ratio Law: ratio of exchangeable cations will be same as ratio of solution cations

K+ Ca+2 Ca+2 K+ Ca+2 + K+ + K+ K+ Ca+2 K+ K+ K+ 1 Ca : 2 K 1 Ca : 2 K Same ratio

Energy of adsorption The more strongly a cation is attracted to the exchange surface, the greater the chance of adsorption. Depends on: 1. charge 2. hydrated radius

Al+3 > Ca+2 > Mg+2 > [K+ = NH4+ ] > Na+ > H+ Strong --------------------------------------Weak Al+3 > Ca+2 > Mg+2 > [K+ = NH4+ ] > Na+ > H+ Radius Unit Na+ K+ Mg2+ Ca2+ Al3+ Non-hydrated nm 0.095 0.133 0.066 0.099 0.050 Hydrated 0.360 0.330 0.430 0.410 0.480

The less tightly held (lower energy of adsorption) ions are the ones furthest from the soil surfaces and can be leached more easily and are further down the soil profile. The strongly held ones are closer to the soil particle surfaces and tend to move more slowly down profile

How do plants get nutrients? Nutrients on the colloids are kept within root zone of plants. H+ from roots exchange with cations on a charge-by-charge basis

Two videos: CEC CEC

Al+3 > Ca+2 > Mg+2 > [K+ = NH4+ ] > Na+ > H+ Treating a sodic soil Sodic: too much sodium (Na) Add gypsum (CaSO4) : increases calcium concentration; Ca is adsorbed at expense of sodium Al+3 > Ca+2 > Mg+2 > [K+ = NH4+ ] > Na+ > H+

Base saturation % of exchange sites occupied by basic cations (cations other than H+ and Al+3) Base saturation + H+/Al ion saturation should equal 100%

Base saturation and pH relationship (for midwest US soils) Notice neutral pH (7.0) requires a base sat of 80%.

Soil pH

Soil pH importance Determines solubility of nutrients Before plants can get nutrients, they must be dissolved in soil solution Microbial activity also depends on pH

pH negative log of the hydrogen ion concentration (also a measure of OH- concentration) If H+ concentration > OH- : acidic If OH- > H+ : basic Soil pH is pH of solution, NOT exchange complex

General soil pH conditions: “Slightly acid” 6.0 – 6.6 “Moderately acid” 5.0 – 6.0 “Strongly acid” < 5.0 “Slightly basic” 7.4 – 8.0 “Moderately basic” 8.0 – 9.0 “Strongly basic” > 9.0

In soil, both H+ and Al+3 ions produce acidity Al+3 produces H+ ions when it reacts with water. (when pH below 6: Al+3 is the cause of acidity)

Causes of soil basicity Hydrolysis of basic cations Hydrolysis of carbonates

1. Hydrolysis of basic cations: (especially Ca+2, Mg+2, K+, NH4+, Na+) (also called exchangeable bases) Extent to which exchangeable bases will hydrolyze depends on ability to compete with H+ ions for exchange sites. Na Na Na Na + H2O + OH- Na H Na + Na Na Na Na

K+ and Na+ are weakly held compared to Ca+2 and Mg+2. Recall energy of adsorption So, K+ and Na+ are hydrolyzed easily and yield higher pHs .

2. Hydrolysis of carbonates (especially CaCO3, MgCO3, Na2CO3) As long as there are carbonates in the soil, carbonate hydrolysis controls pH. Calcareous soils remain alkaline because H+ ions combine with OH- to form H2O. For those soils to become acid, all carbonates must be leached. Basic cations replaced by Al+3 and H+ CaCO3 + H2O Ca+2 + HCO3- + OH- Na2CO3 + H2O Na + HCO3- + OH- (higher pH because Na more soluble)

Causes of soil acidity Accumulation of soluble acids Exchangeable acids (Al+3, H+)

Accumulation of soluble acids at faster rate than they can be neutralized or removed Carbonic acid (respiration and atmospheric CO2) b. Mineralization of organic matter (produces organic, nitric, sulfuric acids) Precipitation increases both a and b

2. Exchangeable acids Dissociation of exchangeable H+ or Al+3 Al+3 ties up OH- from water, releases an equivalent amount of H+ ions. Al+3 + H2O AlOH+2 + H+

CEC and pH Only 2:1 silicate clays do not have pH-dependent CECs. Others are pH-dependent: 1:1 kaolinite: low pH: low CEC high pH: high CEC Oxidic clays pH dependence of humus