AA - ASPECTS OF AGRICULTURE
The Soil
p-Block Elements
Fertilisers, Pesticides and Herbicides 
Artificial Fertilisers
Pesticides
Herbicides
Artificial Fertilisers
Common fertilisers used are, ammonium nitrate (V), ammonium sulphate, ammonium phosphate, urea, CO(NH2)2, and potassium nitrate (V). The compounds are used individually or mixed to produce different N:P:K ratios.
Ammonia is the basis of most fertilisers. Ammonia is manufactured by the Haber Process. Nitrogen and hydrogen are passed under pressure over a catalyst. Hydrogen is obtained by reacting water with natural gas. Nitrogen is obtained from air.
2NH3(g) 
H = -92 kJ mol-1The reaction is reversible with the equilibrium well over to the left. To increase the yield it is necessary to use a high pressure and a low temperature. High pressures are expensive, low temperatures mean a slow reaction. In practice a compromise of about 150 atmospheres pressure and about 450oC are used with an iron catalyst.
What's related: The Effect of Temperature on Reaction Rate
Pesticides
Crops can be devastated by insects such as locusts or by diseases. Pesticides kill insects which eat crops, and moulds which rot plants. Unfortunately, pesticides can cause other problems. Some pesticides remain in the soil for a long time and also get in the food chain. They have caused the death of many Birds of Prey. Pests can also build up a resistance to chemicals. There is a continual search for new pesticides.
Pyrethroids are pesticides which are able to penetrate rapidly the membranes of nerve cells. They cause massive disruption of the nervous system of insects. A key factor in their activity is that they are much more soluble in fats than in water. Hence they can pass readily from the aqueous solution used for spraying into the fatty tissues of the insect. The difference in solubility can be measured as a partition coefficient. Partition coefficients are equilibrium constants which measure the ratio of concentrations of a substance dissolved in two immiscible solvents in contact with each other.
For biological systems, the most common solvents are octan-1-ol and water.
Kow for pyrethroids are very high, 106 to 107 so log10 Kow is usually quoted (returning values between 6 and 7. The concentrations of pyrethroids needed for insecticidal activity can be very low.
Pyrethroids are rapidly broken down by oxidation or hydrolysis; the products do not build up in the environment.
Herbicides
Herbicides can increase crop yields by destroying weeds which compete with crops. There are two main types: total herbicides and selective herbicides. Total herbicides destroy all green plant material. An example is paraquat, a highly toxic but rapidly inactivated when brought in contact with the soil, due to ion exchange.
Some herbicides have been developed which can kill broad-leaved plants without damaging grasses. Others kill grasses but not broad-leaved crops like soya beans and sugar beet. Their selectivity depends on differences in metabolism between plants.
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