AA - ASPECTS OF AGRICULTURE
The Soil
p-Block Elements 
Fertilisers, Pesticides and Herbicides
Group 4
Group 5
The Fertility of Soil
The Nitrogen Cycle
Group 4
The elements in Group 4 are:
| Element | Symbol | Electron configuration |
|---|---|---|
| Carbon | C | [He] 2s22p2 |
| Silicon | Si | [Ne] 3s23p2 |
| Germanium | Ge | [Ar] 3d104s24p2 |
| Tin | Sn | [Kr] 4d105s25p2 |
| Lead | Pb | [Xe] 4f145d106s26p2 |
The elements differ significantly from each other. Carbon is a non-metal, silicon and germanium are semi-conductors, tin and lead are metals. They all have outer electron configurations s2p2. They form compounds with oxidation state +4. Tin and lead form compounds with oxidation state +2. The stability of the +2 oxidation state relative to +4 increases down the group. Compounds with +4 oxidation state tend to be covalent, those with +2 tend to be ionic. In the +2 state oxides tend to be basic. In the +4 state, oxides are acidic at the top becoming more basic as one goes down the Group.
Carbon exists in three allotropic forms, diamond, graphite and fullerenes. Diamond and graphite have giant covalent structures. Carbon and silicon react with oxygen to form oxides. Carbon dioxide, CO22, is a high melting point solid with a giant covalent structure. SiO2 is an extended network of SiO4 units in which the central silicon is covalently bonded to each of four oxygen atoms.
Group 5
The elements in Group 5 are:
| Element | Symbol | Electron configuration |
|---|---|---|
| Nitrogen | N | [He] 2s22p3 |
| Phosphorus | P | [Ne] 3s23p3 |
| Arsenic | As | [Ar] 3d105s25p3 |
| Antimony | Sb | [Kr] 4d105s25p3 |
| Bismuth | Bi | [Xe] 4f146s26p3 |
At the top of the Group are non-metals, N and P; at the bottom are metalloids, Sb and Bi.
Atoms can form three covalent bonds. Oxidation states are +3 and +5. The lone pair of electrons enables these elements to form dative covalent bonds.
Nitrogen, N2, is a very unreactive gas because it has a triple bond holding the atoms together. The most important compounds of nitrogen are ammonia, nitrogen oxides and nitrates, all of which are involved in the nitrogen cycle.
Ammonia, NH3, has a lone pair of electrons on the nitrogen hence it can accept a H+ ion, i.e. it can act as a base. It can also act as a ligand with transition metal ions.
Nitrogen forms many oxides, all of which are gases. The most important are:
- nitrogen monoxide, NO which is a colourless gas. It readily reacts with oxygen to form NO2. It is formed in car engines, in thunderstorms and in the soil by denitrifying bacteria.
- nitrogen dioxide, NO2, is a brown toxic gas.
- dinitrogen oxide, N2O is a colourless gas formed in the soil by denitrifying bacteria.
There are two nitrate ions: nitrate (III), NO2- and nitrate (V), NO3-. Nitrate (III) is a reducing or oxidising agent. The two N-O bonds are the same length due to delocalisation of electrons. Nitrate (V) is an oxidising agent. Like the nitrate (III) ion, all three N-O bonds are the same length due to delocalisation of electrons.
The Fertility of Soil
Apart from CO2 from the air, plants get all their nutrients from the soil. These nutrients come from an inorganic store in the soil and an organic store, partly on top and partly in the soil. Nutrients are lost by leaching and by uptake of crops.
The Nitrogen Cycle
Various processes convert gaseous nitrogen and organic nitrogen compounds into ammonium and nitrate (V) ions which plants can use.
- Biological fixation. Certain bacteria in the soil of in the root nodules of legumes can convert nitrogen gas to ammonium ions:
N2(g) + 8H+(aq) + 6e- 2NH4+(aq)
- Lightning and burning of fossil fuels produces nitrogen oxides which are carried into the soil by rainwater.
- Mineralisation. Soil bacteria breaks down organic nitrogen compounds into simpler molecules and ions, e.g. ammonium ions.
- Nitrification. Ammonium ions are oxidised by certain aerobic bacteria in the soil to nitrate (V) ions.
Nitrogen can also be lost from the soil in a number of ways:
- Denitrification. Anærobic bacteria reduce nitrate (V) to eventually nitrogen gas.
- Leaching. Nitrates (V) ions, which are very soluble are easily lost by leaching. This depends on the type of soil and the rainfall.
- Loss of ammonia gas. Ammonium ions are converted into ammonia gas under alkaline conditions.
- Uptake by plants. Crops which are regularly harvested remove more nitrogen than is put back naturally.
In order to maintain or increase crop yields, nitrogen must be added to the soil in a form that plants can readily use. Nutrients are added in two ways:
- as manure from livestock,
- as artificial fertilisers.
