Question

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3. The cations on an exchangeable site of a 40 g soil were displaced by repeatedly washing by a salt and found to be 40 mg Ca+2, 11.5 mg Na+1, 39 mg K+1, 12 mg Mg+2, and 9 mg Al+3. The pH of the soil was 5.0. Calculate the CEC (cmolc/kg), percentage base saturation (PBS) and percent acidic saturation (PAC) per kg of the soil.

4. A give soil has a CEC of 12.50 cmol of charge per kg of soil. If 70%, 15%, 10% and 5% of the CECis occupied by Ca+2, K+1, Mg and Na+1 , respectively, calculate the weight (grams) of Ca, K, Mg and Na in the soil.

5. A given soil contains the following colloids: 1.0% humus (CEC=200), 30% kaolinite (CEC=5), 5%smectite (montmorillonite) (CEC=80), 10% illite (CEC=20), 5% mica (CEC=70) and 49% sesquioxides (CEC=2). Calculate the CEC of the soil? Which colloids contributed the most and which contributed the least? (All CEC are in molc/kg soil).

6. Calculate the amount of pure CaCO3that could theoretically neutralize the H+ in one-year acid rain if a 1-hectare (ha) site received 200 mm of rain per year and the average pH of the rain was 5.0.

7. Calculate the pH and pOH of a soil with the following H+ concentrations: (a) 0.0000001M (b) 0.00001M (c) 0.005M? Which soil is relatively most acidic? Most basic?

8. Determine the calcium carbonate equivalent (CCE) of the following compounds: (amount that has the same neutralizing value as 100 g pure CaCO3) (a) KOH (b) Mg(OH)2 (c) and CaMg(CO3)2.

9. How many grams of limestone with a CaCO3 equivalent of 100% would you need to apply to an acidic soil with 85% exchangeable Al saturation to reduce it to 10%? The CEC of the soil is 12.5cmolc/kg.

10. How many grams of gypsum (CaSO4.2H2O) would you need to reclaim a sodic soil with an exchangeable sodium percentage (ESP) of 80% to reduce to it to 50%? The CEC of the soil is 12.5cmolc/kg.

Answer 1

3. Cation-exchange capacity is defined as the amount of positive charge that can be exchanged per mass of soil, usually measured in cmolc/kg. CEC is typically higher near the soil surface, where organic matter content is highest, and declines with depth.

The Percent Base Saturation is then calculated from these values by dividing the milliequivalents of each by the CEC as shown below-

The Percent Base Saturation could be defined as the relative availability of each of these cations.