Question

1) You have been drip irrigating your soil for too many years. Now the soil is too saline and you want to treat this by adding gypsum (CaSO₄^. 2H₂O). Assume you have a 1 hectare field and 25% of the cation exchange capacity of the 15 cm thick A horizon is occupied by sodium (Na⁺). You want to remove 40% of the Na⁺, using CaSO₄^. 2H₂O. What is the minimum amount of CaSO₄^. 2H₂O needed to remove the Na⁺? Assume the bulk density of the A horizon is 1.25 Mg/m3 and the cation exchange capacity is 40 cmolc/kg of soil.

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Hints: How much sodium must be removed and thus how much calcium is needed per kg of soil? How many grams of CaSO₄^. 2H₂O must be used in 1 kg soil? What is the mass of soil in 1 ha, 15 cm thick, that has a bulk density of 1.25 Mg/m³? How much gypsum must be used on total mass of soil?

Answer 1

The mass of soil to be treated is 100m*100m*0.15m*1.25Mg/m3=1875Mg

A mg is 100kg. The soil to be treated has 40000cmolc/mg or 40000*1875=75*10⁶ cmolc

25% of this charge is Na or 18,750,000 cmolc that must be replaced with Ca

we need to supply a minimum of 18,750,000 cmolc of calcium and 1 cmolc Ca has a mass of 0.20 g

18,750,000 cmolc * 0.20 g/cmolc = 3,750,000 g of calcium or 3,750 Kg or 3.75 Mg calcium / hectare but CaSO₄ is in form of gypsum so we need to add more CaSO₄

40g Ca/172 g CaSO₄.2H₂O = 3,750,000 / X g CaSO₄ = 16,125,000g/ha or 16,125 kg/has or 16.125 Mg / ha of CaSO₄