If a crop of wheat removes 20 kg of nitrogen per hectare of soil, what mass in kg. of the fertilizer calcium nitrate would be required to replace the nitrogen in a 10 hectare field.

[N = 14 ; O = 16 ; Ca = 40]

Molecular weight of Ca(NO₃)₂
= 40 + 2[14 + 3(16)]
= 40 + 2[14 + 48]
= 40 + 2(62)
= 40 + 124 = 164 g

Total nitrogen that has to be replaced in 10 hectares of land = 20 x 10 = 200 kg

28 g of N is present in 164 kg of calcium nitrate

∴ 200 kg of N is present in = $\dfrac{164}{28}$ x 200 = 1171 kg

Hence, 1171 kg of fertilizer will be required