Lab 6 Measuring CEC Use soils from last

Lab 6: Measuring CEC Use soils from last week to – > extract exchangeable cations > measure basic cations by AA spectroscopy > measure acid cations by titration > calculation CEC, % BS for your soil > convert from meq/100 g to lbs/acre

Procedure: 1. Weigh 3. 0 g soil into 50 m. L centrifuge tube 2. Pump in 35 m. L Na. Cl extracting solution 3. Shake for 20 minutes to displace exchangeable cations from colloid surface into solution…

Ion Exchange by Added Na soln … Mass Action Mg+2 K+ Ca+2 Al+3 Displaced cations

Measurement of exchangeable cations: 1. Centrifuge, filter to separate clear solution. 2. Fill glass vial about ½ full of solution for AA analysis. 3. Measure remaining solution using grad cylinder—record this number, this will be your “volume titrated”. Pour back into Erlenmeyer flask.

Titration of exchangeable acids: 1. To Erlenmeyer flask: add stir bar and 3 -4 drops phenolpthalein indicator , turn on stirrer to slow speed. 2. Add Na. OH from burette SLOWLY (drop by drop) until solution stays pink for 20 -30 seconds. Record volume base used from burette. At endpoint: p. H is near neutral, and AMT BASE ADDED = AMT ACID PRESENT NB x VB = N A x VA

Titration calculations: NB x VB = N A x VA NB: conc of base: 0. 005 N (eq/L = meq/m. L) VB: volume of base (m. L, burette) VA: volume of acid (“m. Ls titrated”) NA: conc of acid: UNKNOWN… meq acid/m. L

Conc (solution) Conc (soil) We used 35 m. L of Na. Cl solution to extract 3 g soil to get cations into solution…. Meq acid x 35 m. L soln x 100 = meq acid m. L soln 3 g soil 100 g soil Same for basic cations by AA: Meq Ca x 0. 035 L soln x 100 = meq Ca L soln 3 g soil 100 g soil

CEC of soil = cations (in meq/100 g): Ca + Mg + K + acidity (H+Al) Mg and K: written on board (meq/L)…. . % base saturation (BS) = bases (Ca+Mg+K)/CEC Convert meq/100 g into lbs/acre-furrow slice….

CEC calculations: meq/100 g to lbs/acre Assume a soil has 2 meq K+/100 g: how many lbs/acre? (“acre” = acre-furrow slice= 2 x 106 lbs) Note: Atomic mass of K=39 g/mole, and 1 mol=1 equiv (K+1) So-- Now: “mg/kg” is PPM = parts per million… and LBs/AFS = lbs/2 x 106 lbs, which is “parts per 2 million”…. SO: 780 ppm K x 2 = 1560 lbs K/acre