4. Urea at a rate of 160 kg/ha is mixed with a layer of soil 1 cm thick. The ure

Question

4. Urea at a rate of 160 kg/ha is mixed with a layer of soil 1 cm thick. The urea dissolves and hydrolyzes in that layer according to the reaction shown below (Initial pH-6.4; bulk density 1.4 g/cm3; pH buffering capacity 15 mmol OH/kg soil/pH unit; mol. wt. urea- 60.06, atomic weight of N-14.00; 1 ha = 10,000 m, 1 cm = 0.01 m). CO(NH2)H202H>CO2+2 NH This reaction will raise soil pH because it consumes H (which is equivalent to generating OH ions). Calculate the soil pH after all the urea is hydrolyzed. Hints a. How many H' equivalents/ha are consumed by the urea hydrolysis? b. What is the weight of a 1-cm slice of 1 ha of soil? c. How many H equivalents are consumed per kg of soil in that 1-cm slice of 1 ha? d. What would be the pH of the soil in that layer after complete urea hydrolysis? e. Would ammonia volatilization be a significant problem in this case? Why or why not?

Explanation / Answer

WKT, 1 mole of Urea is equal to 60.05526 grams, which in turn, means that 1g of urea =0.01665133079mol of Urea.

Therefore, 160 Kg of Urea = 2664.21 mol Urea

Let us assume that the total area = 1 ha = 10000m2

Since Urea is mixed with a layer of 1cm thick soil, the total volume of soil that underwent the treatment,

V = 10000m2 * 0.01 m = 100 m3

The bulk density of the soil is given as 1.4g/cm3 = 1.4 * 103 Kg/ m3

Since bulk density is equal to the weight of the soil per volume of soil,

The weight of soil = 1.4 * 103 Kg/ m3 * 100 m3 = 1.4 * 105 Kg

The total Urea applied was 160 Kg/ ha for 1cm thick = 2664.21 mol / 1.4 * 105 Kg = 19.03 mmol/Kg

As it can be seen in the given equation, one mole of Urea reduces 2 mol of [H+] ions, therefore, 19.03 mmol/Kg of Urea can reduce 38.06 mmol/Kg of [H+] ion which essentially means generation 38.06 mmol of OH- ions.

The pH buffering capacity of the soil is given as 15mmol OH/Kg soil/ pH unit.

Therefore, the increase in pH = 38.06 mmol of OH-/Kg /  15mmol OH/Kg soil/ pH unit = 2.54 Increase in pH

Since the initial pH of soil was 6.4, th pH after Urea is hydrolyzed is 6.4+2.54 = 8.94

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