The diffuse double layer + +

Transcription

1 Soil chemistry

2 The diffuse double layer Diffuse doppio strato double diffuso layer soluzione Soil solution del terreno

3 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 soil colloid Ca 2Na Na Ca Na soil solution soil colloid solution

4 The cation exchange capacity of a soil is simply a measure of the quantity of sites on soil surfaces that can retain positively charged ions by electrostatic forces. Cations retained electrostatically are easily exchangeable with other cations in the soil solution and are thus readily available for plant uptake. Thus, CEC is important for maintaining adequate quantities of plant available calcium (Ca 2 ), magnesium (Mg 2 ) and potassium (K) in soils. Other cations include Al 3 ( when ph < 5.5), Na, and H.

5 Cation exchange influenced by: 1) Strength of adsorption: depends on charge and hydration Al 3 > Ca 2 > Mg 2 > K = NH 4 > Na >H held tight > easily replaced 2) the relative concentration of the cations in the Soil Solution

6 Main excheangeable cations in soil Ca2, Mg2 Al3/H Na,K, NH4 acid soil neutral soil sodic soil Exchange reactions are reversible and every change in the soil solution composition is reflected by a change in the composition of the exchange complex

7 On the edge of a kaolinite crystal the oxygen bound to the silicon ion has a charge of 1 that can be neutralized by a proton. The Si-OH group has a pka = 9,5. The oxygen linked to both the silicon and the aluminium (who respectively contribute with 1 e 1 / 2 to the neutralization of the negative charge) has a charge = 1 /2. By binding to a proton the charge becomes 1 / 2. The pka of (OH) ½ is 7. The oxygen linked to aluminium has a charge = 3 / 2 it can bind two protons and get a positive charge= 1 / 2. The pka of (OH 2 ) 1/2 is 5,5.

8 CATION EXCHANGE CAPACITY rw Soil 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 Cation Exchange Capacity (CEC) is the ability of the soil to hold onto nutrients and prevent them from leaching beyond the roots. The more cation exchange capacity a soil has, the more likely the soil will have a higher fertility level. When combined with other measures of soil fertility, CEC is a good indicator of soil quality and productivity.

9 Cation exchange is influenced by: 1) strength of adsorption: Strong adsorption» Al3 > Ca2 > Mg2 > K=NH 4 > Na >H»Weak adsorption 2) the relative concentration of the cations in the soil solution. At any time the quantity of ions on the exchange compared to what is in the soil solution is determined by the kind of ions present and the quantity in the soil.

10 La CSC dipende dal - contenuto in argilla - contenuto in sostanze umiche (humus) La CSC viene espressa in milliequivalenti/100g suolo oppure in centimoli di carica/kg di suolo I cationi assorbiti possono ritornare in soluzione mediante reazioni di scambio ionico.

11 Characteristics of cation exchange in soil: 1. Rapidity 2. Stechiometry only equivalents amounts of charge are exchanged soil K K Ca 2 soil K Ca 2 K

12 3. Reversibility (all cations can be released by exchange) 4. The exchange obeys the mass action law (a change in concentration in the soil solution causes a shift in the equilibrium)

13 Charge units in soil Every negative charge unit on a clay crystal equals one e- And can be balanced with a positive charge unit (e) from a cation. The CEC of a soil is measured in centimolesmoles of charge per kg of soil cmol C /kg (cmol C ) = 6.02 * carge units - for a monovalent ion 1 mole = 1 mole C - for a divalent ion 1 mole = 2 moles C - for a trivalent ion 1 mole = 3 moles C

14 KAOLINITE CEC C moles/kg Specific surface m 2 /g 10 MICHE VERMICULITE SMECTITI CLORITI ALLOFANE

15 PERCHE E IMPORTANTE CONOSCERE LA CSC? LA CSC ESPRIME IL POTERE CHE HA IL TERRENO DI TRATTENERE ELEMENTI CATIONICI INORGANICI (POTASSIO, MAGNESIO, CALCIO ) E ORGANICI (ERBICIDI CON CARICA POSITIVA ) EVITANDONE LA PERDITA PER LISCIVIAZIONE MA PERMETTENDO GLI SCAMBI CON LA SOLUZIONE. I CATIONI SCAMBIABILI COSTITUISCONO UNA RISERVA PRONTAMENTE DISPONIBILE DI ELEMENTI NUTRITIVI LA CUI CONSISTENZA E IL PRINCIPALE FATTORE DI FERTILITA DEL TERRENO.

16 Predicting CEC CEC estimates can be made based on the texture and amount of organic matter of the soil. 1) Estimation based on texture: CEC & Soil Texture Texture Sand LS to SL Loam Clay Loam Clay CEC cmoles c /kg >30 2) Calculation of CEC with % clay and % OM. Assume avg. CEC for % OM=200 cmoles c /kg and the avg. CEC for % clay=50 cmoles c /kg. CEC=(% OM x 200) (% Clay x 50) soil with 2% (.02)OM and 10%(.1) Clay CEC=(200 x.02 (50 x.1)=(4 5)==9 cmoles c /kg

17 Soils can have:. Less than 10 meqc/100g ---- LOW CEC meqc/100g --- good CEC. More than meqc/100g ---- HIGH CEC

18 HOW CAN WE MEASURE CEC? THERE ARE MANY DIFFERENT METHODS. The most precise is the following: FIRST ALL CATIONS IN THE SOIL SAMPLE ARE EXCHANGED WITH BARIUM by shaking the soil with a buffered (triethanolamine ph9.5) barium cloride solution BaCl 2 soil x y soil Ba 2 x y 2Cl -

19 The surnatant solution is separated by centrifugation and stored for analysis of cations The sediment is washed with distilled H 2 O To eliminate x, y, 2Cl - soil Ba 2

20 The sediment is then shaken with a solution containing A KNOWN EXCESS AMOUNT OF MgSO 4 soil Ba 2 MgSO 4 soil Mg 2 BaSO 4 precipitates and Shifts the equilibrium! ALL THE SOIL IS NOW SATURATED WITH Mg 2

21 INITIAL AMOUNT OF Mg - FINAL AMOUNT OF Mg = Mg RETAINED BY THE SOIL soil Mg 2 Sorbed by soil particles Mg 2 IN SOLUTION

22 TRITRATION OF the MgSO 4 that remained in the solution with: HOOC-CH 3 HOOC-CH 3 N-CH 2 -CH 2 -N CH 3 -COOH CH 3 -COOH EDTA = ethylendiaminotetracetic acid

23 Base saturation The percent base saturation of a soil is the composition of the exchange complex with respect to the content in excheangeable bases that is all excheangeable ions except H and Al 3 % base saturation = CEC (Al 3 H ) exchangeable CEC The percent base saturation of a soilchanges according to ph it gives an indication of the availability of nutrients in that soil.

24 SOIL CHEMISTRY CSC laboratory experiments