Agriculture and Industry

1. Agriculture and Industry

2. AI1 What do we want from agriculture? • The world population is growing • The population doubled from 1961 to 1999 • It is expected to reach 9 billion by 2042 • Our planet cannot produce an unlimited supply of food • The ultimate goal must be to feed everyone, without harming the environment • This involves; • Producing the right kind of food • In the right quantities • In the right place • At the right time • Without encroaching on the world’s forests and wildernesses

3. We need to make more efficient use of agricultural land; • Improve crop varieties • Better planting techniques • Better use of added plant nutrients (fertiliser and manure) • More effective pesticides • Learn from past mistakes • We need to develop sustainable systems for agriculture • This would enable it to go on indefinitely without degrading the environment • We know plants take CO2 from the air and H2O from the soil but they need other nutrients as well • These are absorbed from the soil via their roots…

4. The most important nutrient elements include; • Nitrogen • Phosphorus • Potassium • Calcium • Magnesium • Sulphur • Iron • Harvesting prevents decay from occurring • This disturbs the natural nutrient cycle • We therefore need to replenish these nutrients

5. AI2 The organic revolution • Remember… • Organic chemistry • Study of all carbon-based compounds except CO, CO2, CO32- and HCO3- • Organic farming • Farming using fertilisers and pesticides of only plant or animal origin • UK organic food market is growing annually by 25% • This will reduce our use of herbicides and pesticides • It is considered safer and more nutritious • The FSA have found that the balance of evidence does not support this view • In 2003 a £12 million, four-year investigation was begun. • Led by Newcastle University it grew organic and non-organic fruit and vegetables at sites across Europe • Early evidence from this suggests organic foods do have a greater nutritional value !

6. Soil health • In looking at soil we are interested in the 1-2m thick layer on top of the Earth’s crust • The fertility of a soil depends on complex interactions between biological, physical and chemical processes • We need to start with the structure of this soil; • The ratio of air : water is important for plant growth • Fine clay particles can bind positive ions to their surfaces, this keeps essential ions in the soil

7. Soil organic matter • The organic matter is made from plant and animal remains and excreta • Mineralisation converts elements in the organic compounds into ions such as NH4+, NO3-, NO2- , SO42- and PO43- • Microorganisms act on organic matter and convert it into humus • This contains lots of carboxylate and phenoxide groups which can then hold cations in the same way as clay

8. Nutrient cycling • Elements are continuously being cycled between the organic store, the inorganic store and living systems • Healthy crops depend on the ability of the soil to supply nutrients • The rates of these processes is therefore very important • Crop yields will be reduced if there is a shortage of even one nutrient

9. The Nitrogen Cycle • Almost all nitrogen in soil is present as complex organic compounds and so isn’t readily available to plants • Various processes convert these, and N2, into soluble NH4+ and NO3- ions which plants can use • CI11.3 - Group 5 Chemistry • Additions to soil nitrogen • Biological fixation • by bacteria in soil and root nodules of peas and beans N2(g)+ 8H+(aq) + 6e- 2NH4+ (aq) • Lightning and natural fires N2 + O2 NO

10. (II) Transformations in the soil • Mineralisation • micro organisms break down organic N compounds into simpler molecules and ions… Organic N compounds  NH4+ (aq) • NH4+ ions are held by clay minerals but are converted into NO3- • It is best in well drained soils and at high temperatures • Nitrification • NH4+ is oxidised to NO3- by aerobic bacteria in two steps… NH4+(aq)+1½ O2 (g) NO2-(aq) + 2H+(aq) +H2O (l) NO2- (aq) + ½O2 (g) NO3- (aq) • This stops in dry conditions but also in waterlogged soils as the oxygen content is low

11. (III) Losses of nitrogen from the soil • Denitrificationby anaerobic bacteria NO3- (aq)  NO2-(aq)  NO(g)  N2O(g)  N2(g) • Occurs most where oxygen content is low such as flooded soils such as paddy fields. • Leaching - NO3- is washed out of the soils • Loss of NH3(g) - made from NH4+ in alkaline soils NH4+ (aq) + OH- (aq)  NH3(g) + H2O(l) • Plant uptake • coniferous forest: 25 – 78 kg N per hectare per year • high yield crop: up to 500 kg N per hectare per year • Ass 3 • CI5.8 – Bonding, Structure and Properties • AI2.1, 2.2, 2.3

14. Organic farming • Foods grown ‘organically’ must adhere to strict guidelines • The Soil Association does not allow the use of; • Sewage sludge • Manure that doesn’t meet their standards • Chemically synthesised fertilisers • To maintain soil fertility and nitrogen content farmers can; • Grow nutrient-adding (nitrogen fixing) crops such as peas and beans as part of a crop rotation • Growing crops that are ploughed back into the soil such as clover • Adding farmyard manure • Adding permitted supplementary nutrients • Ass4

15. AI3 The Fertiliser Story • Between 1750 and 1825 the population of Britain doubled and the number of people living in towns increased rapidly… • …Britain’s farmers need to grow much more food. • This was helped by; • The introduction of clover as a crop • The growth of the iron industry • Importing of nitrogenous fertilisers (sodium nitrate and guano) from South America • By 1900 other industries also had demands for nitrogenous compounds and supplies were dwindling • Alternative sources of ammonium and nitrate compounds and ammonia had to be found

16. In 1908 Fritz Haber discovered that, with the right catalyst, he could form the equilibrium… N2 (g) + 3H2 (g) 2NH3 (g) • By 1909 he had managed to make 100g of ammonia • About 6500 experiments were then carried out to find the most effective catalyst! • This was then scaled up by Carl Bosch (a chemical engineer) • In 1913 the first industrial ammonia plant produced 30 tonnes/day • Both men were awarded Nobel Prizes in Chemistry • Modern plants use the same principles but make 1500 tonnes/day

17. The modern Haber Process • H2 is made by reacting water with natural gas • N2 is extracted from the air • The heated gases are passed over a catalyst of finely divided iron • The reaction is exothermic but is also reversible and, if left, will form an equilibrium… N2 (g) + 3H2 (g) 2NH3 (g) • CI7.1 Chemical equilibria (revision) • CI7.2 Equilibria and concentrations • AI3.1 • AI3.2 • AI3.3

18. The yield depends on the position of equilibrium • This depends on temperature and pressure (Le Chatelier) • However, there are other considerations as well as equilibrium… • Rate – we need to make our ammonia as quickly as possible • Cost – set up costs and running costs • Safety – we must reduce the risk of workers being injured • For maximum yield (to shift eqm to RHS)… • Low temp and high pressure • For maximum rate • High temp and high pressure • For minimum cost… • Low temp and low pressure • For maximum safety… • Low temp and low pressure

19. A compromise is needed… • Common conditions are 450°Cand100 atm pressure • Ammonia is separated from the N2 and H2 before eqm is reached and the unreacted gases are recycled • AI3.4 • CI10.2 + 10.3 (revision) • Ass 5, 6 • Fertilisers made include; • Ammonium nitrate(V) • Ammonium sulphate • Calcium phosphate • Urea • Potassium chloride • “NPK” fertilisers mixthese to give farmers the N:P:K ratio they need

20. Fertilisers need to be added in the right amounts and at the right times of year • If not, it wastes money and NO3- gets leached into rivers and drinking water… • January; NO3- levels are low • Spring; mineralisation increases; NO3- rises • Crop growth; NO3- falls • Harvesting; NO3- rises • Ploughing; introduces air and encourages microbial activity • Autumn; warm, moist soils encourage mineralisation; NO3- rises • Winter; waterlogged soils encourage denitrification; NO3- falls • Winter; leaching NO3- falls • Ass7

21. Controlling soil acidity • Clay soils are sheet structures with –ve charges on their surfaces • These can hold +ve ions (cations) • H+ ions from rain displace other ions • This makes the soil more acidic and reduces the amounts of other ‘exchangeable cations’ in the soil • Bases can be added; e.g. lime (Ca(OH)2) and limestone (CaCO3) • As some of the H+ ions are removed from solution by the base ... • … some of those bound to the soil will replace them • This means the soil’s pH will not change as much as expected • The soil “resists changes in pH despite the addition of small amounts of base” – it is acting as a buffer • How much base is needed depends on the soil’s ability to resist neutralisation – the soil’s buffering capacity • e.g clay soil needs more lime that sandy soil to cause the same pH change

22. AI4 Competition for food • Up to 50% of the global wheat crop is lost to pests and up to 80% of the global cotton crop • Weeds are the biggest cause, then animal pests and then diseases • There are 3 types of pesticide; • Insecticides • kill insects that eat crops • Herbicides • kill weeds that compete with crops • Fungicides • kill moulds that rot plants

23. However, there are problems: • Some pesticides can damage human health • Some can adversely affect the environment • Organisms other than the target one can be killed (perhaps predators that eat the pests) • Some older pesticides build up in the food chain (DDT) • Some may get into water supplies • Pests may build up resistance • The challenge for agricultural chemists is to find substances that … • are specific to target organisms; • will kill at low dosages so only small quantities are required • will not persist in the environment or get into water supplies

24. The search for a new pesticide • Important factors to consider when producing a successful compound: - Ease of manufacture - Specificity - Persistence in soil - Cost - Marketability - Leaching losses into water - Toxicity to humans - Patents – profits! - Comparison with known compounds The pyrethroid story • Dried chrysanthemum flower heads had been used to ward off insects for centuries. • The structures of these natural insecticides were determined in the 1930s •  One of them was pyrethrin 1… • Ass9

25. Pyrethrins were powerful insecticides but harmless to mammals • Unfortunately, they are unstable to light   • Synthetic, stable variants were developed called pyrethroids • The first to be widely used was called permethrin (a mixture of stereoisomers including biopermethrin) • This was then modified to make the more active biocypermethrin • In mammals the pyrethroids are oxidised or hydrolysed into polar products • As they are polar they stay in solution and are excreted • They are similarly hydrolysed in the soil and so don’t build up there either

26. This is a good example of the way chemists develop new active chemicals • Not only agrochemicals but also pharmaceuticals; • Find a substance in nature with special properties • Isolate the substance which gives it those properties • Work out how to synthesise it • Test its effectiveness • Use this molecule as a starting point (a lead chemical) • Develop and test modified versions of the lead chemical to develop molecules which are more effective, have fewer side effects, etc… • Trial their effectiveness, toxicity, etc… • Scale up from lab to industrial scale • AI4 • Ass 10

27. Herbicides • Work by destroying weeds • 2 types; total or selective • Paraquat is a total herbicide • When applied, its ions hit both leaves and soil. • However, its ions are adsorbed onto solids in the surface of the soil and are inactivated as soon as they touch it. • This means it only kills the plants whose leaves it touches. • Ass 11 Controlling pests organically • Crop rotation • Prevents nutrient loss and prevents pests building up in the soil • Physical barriers • Netting, etc… • Encouraging predators • Ladybirds eat aphids • Weeding • Often needs to be done by hand so raises costs • Companion planting • E.g. marigolds reduce attacks of whitefly on tomatoes