4.3 Soil

• Plants need the nitrogen to be in a water-soluble form to be taken up by the roots, providing the nutrients needed to grow. In the atmosphere, N2 is a triple strong covalent bond which requires high energy and temperature to break, in which plants cannot physically do
• Soils naturally contain nutrients such as nitrogen, phosphorus, calcium, and potassium which allow plants to grow. When soil nutrients are in short supply, plants suffer from nutrient deficiency and stop growing. This is why fertilisers are required, which most commonly use the Haber process

  N2 + 3H2 -iron catalyst -> (equi) 2NH3 

  Increasing the pressure and concentration will drive the forward reaction. 
• Eutrophication is the process in which water bodies become enriched in plant nutrients from overuse of fertilisers. Excess fertilisers provide high conc. of nutrients to algae, increasing their population, covering the waterway surface. This prevents sunlight from reaching the underwater plants, preventing photosynthesis and they die. Since photosynthesis is not being conducted, oxygen levels decrease, killing organisms. Once dead, organisms decompose aerobically, depleting further oxygen levels. Anaerobic decomposition occurs produces toxic gases and substances making the water undrinkable.  
• Soil silicate structures have a covalent network bonding with high hardness and boiling point. When rocks weather to become soils, the silicon in the structure is replaced with aluminium. Since silicon has a 4+ charge whereas aluminium has a 3+ charge, there is a net charge of -1. To balance this, cations are held by the silicate, where more cations means the more fertile (nutrient dense) soil. Thus, the surface of soil silicates and aluminosilicates become available to plants via a cation exchange between the soil surface and soil water.
• Nitrogen can be fixed naturally through high energy events of lightning

  1. N2 + O2 —> 2NO 
  2. 2NO + O2 → 2NO2
  3. H2O (from rain/soil water) + NO2 → HNO3 (nitric acid) + HNO2 (nitrous acid)  
  4. HNO3 + H2O → (equil) H3O+ + NO3- (nitrate) 

  Nitrogen Fixing Bacteria 

  1. N2 -> NH3
  2. If in acidified soil N2 → NH4+ (already soluble)

  Then, Nitrifying bacteria…

  1. NH3 → NO2- (nitrite)
  2. NO2- → NO3- (nitrate)