Agriculture

\ce{SO2} and \ce{NO_x} dissolve in rain water to form \ce{H2SO4} and \ce{HNO3}, which lower the pH below 5.6. Carbon dioxide only makes normal rain slightly acidic (pH 5.6); methane and CFCs are linked to global warming and ozone loss, not acid rain.

Acid rain adds \ce{H+} ions to the soil, so the soil becomes more acidic and its pH falls. Limestone-rich soils resist the change for a time, but the general effect is a decrease in pH.

The \ce{H+} ions of acid rain replace nutrient cations such as \ce{Ca^2+}, \ce{Mg^2+} and \ce{K+} on soil particles, and these are washed away. Phosphorus is not leached but locked up as insoluble aluminium phosphate.

Soil microbes decompose organic matter and recycle nutrients. Acid rain harms these microbial communities, so nutrient cycling slows down. They are not unaffected, and most of them do not thrive in acidic soil.

Reduced agricultural biodiversity is listed in the book as a risk of GM crops, not a benefit. Higher yields, pest resistance and better nutrition (for example golden rice) are benefits.

A gene is inserted so that the plant makes the protein coded by that gene and so shows (expresses) the desired trait, such as insect resistance or drought tolerance. A higher yield (B) is often a result of the new trait, not the direct purpose of the insertion.

A gene gun fires tiny gold or tungsten particles coated with DNA into plant cells and is widely used for crops such as rice and maize. PCR only copies DNA, CRISPR-Cas9 edits genes already present, and microinjection is mainly used for animal cells because the plant cell wall gets in the way.

Golden rice makes beta-carotene, which the body converts into vitamin A. The yellow colour of beta-carotene gives the rice its golden colour.

Bt cotton carries a gene from the bacterium Bacillus thuringiensis that makes a protein toxic to bollworms and similar insects. Herbicide-tolerant soybeans resist a weedkiller, not insects.

By preventing damage by pests, pesticides give higher-quality fruits and vegetables and larger yields. The other three options are either the opposite of the truth (A, D) or a risk rather than a benefit (pesticides reduce biodiversity).

Pesticides do not make soil more fertile; persistent pesticides can instead harm earthworms and soil microbes. Resistance, loss of biodiversity and human health effects are all risks named in the book.

Rain washes pesticide residues from fields into streams, lakes and ground water. This contaminates water sources and harms fish and other aquatic organisms.

When pests become resistant, the usual dose no longer works, so farmers use larger doses or stronger, more toxic pesticides. This makes environmental and health problems worse.

Rain is the main source of water for crops, so changes in the amount and timing of rainfall (drought or floods) directly change the water available. Higher temperature and heat stress mainly increase water loss from plants.

Resilient varieties are bred or engineered for heat tolerance, drought tolerance and disease resistance, so they can withstand the stresses caused by climate change and still give a good yield.

Acid rain contains sulphuric acid and nitric acid formed from \ce{SO2} and \ce{NO2} in the air. When it falls on soil, its \ce{H+} ions react with soil minerals:

The soil loses its natural buffer (\ce{CaCO3}) and its nutrient cations (\ce{Ca^2+}, \ce{Mg^2+}, \ce{K+}), which are replaced by \ce{H+}. So the pH of the soil falls and poisonous \ce{Al^3+} ions appear in the soil water.

Phosphorus is a macronutrient absorbed by plants as phosphate ions (\ce{H2PO4-} and \ce{HPO4^2-}). It is needed at all stages of the plant's growth cycle.

• It reinforces (strengthens) roots and stems and promotes early root growth.
• It boosts flowering, seed production and fruit formation, and helps crops ripen.
• It is part of ATP, DNA, RNA and cell membranes (phospholipids), so it is essential for energy transfer in photosynthesis and respiration and for cell division.

A phosphorus-deficient plant has poor roots, stunted growth and dark green or purplish leaves. It is supplied by superphosphate, triple superphosphate or DAP.

Plants take up only part of the nitrogen applied. Extra nitrate is soluble, so it is easily washed away. The main impacts are:

• Eutrophication: nitrate runoff enriches lakes and rivers, causing algal blooms; when the algae die, their decay uses up dissolved oxygen and fish die.
• Ground water pollution: nitrate leaches into wells; high nitrate in drinking water is harmful to infants (blue baby syndrome).
• Soil acidification: ammonium fertilizers are oxidised by soil bacteria, releasing \ce{H+}: \ce{NH4+ + 2O2 -> NO3- + 2H+ + H2O}.
• Greenhouse gas emission: soil bacteria convert extra nitrate into nitrous oxide (\ce{N2O}), a strong greenhouse gas.

Single superphosphate is a mixture of calcium dihydrogen phosphate, \ce{Ca(H2PO4)2}, and calcium sulphate (gypsum), \ce{CaSO4}. It contains about 16 to 20% \ce{P2O5}. It is made by treating insoluble rock phosphate with sulphuric acid:

Triple superphosphate is almost pure \ce{Ca(H2PO4)2} (about 46% \ce{P2O5}), made with phosphoric acid instead.

Function: \ce{Ca(H2PO4)2} is water-soluble, so it supplies phosphorus that roots can absorb quickly. This strengthens roots and stems and improves flowering, seed and fruit formation. Its calcium and sulphur (from \ce{CaSO4}) are also plant nutrients, so it raises soil fertility.

Acid rain lowers soil pH, and this reduces the nutrients that roots can take up in three ways:

• Its \ce{H+} ions replace \ce{Ca^2+}, \ce{Mg^2+} and \ce{K+} on soil particles, and these nutrients are leached away from the root zone.
• It releases **toxic \ce{Al^3+}** ions, which damage root tips, so roots absorb less water and fewer nutrients. \ce{Al^3+} also locks phosphate as insoluble \ce{AlPO4}.
• It harms soil microbes and nitrogen-fixing bacteria, so less nitrogen is made available from organic matter.

Acid rain also damages the waxy layer of leaves, so the plant loses water and nutrients from its leaves as well.

Soil microbes (bacteria and fungi) decompose dead organic matter and recycle nutrients such as nitrogen, phosphorus and sulphur. Most of them work best near neutral pH.

Acid rain lowers soil pH and releases toxic \ce{Al^3+} ions, so the number and activity of these microbes fall. As a result, organic matter decomposes slowly, nitrogen fixation (for example by Rhizobium in soybean root nodules) and nitrification decrease, and fewer nutrients are released into the soil.

This further reduces nutrient availability and damages soil structure, which lowers crop yields.

Acid rain reduces crop yield both directly and through the soil:

• Leaf damage: it dissolves the waxy layer of leaves, increasing water loss and making plants more susceptible to disease.
• Nutrient loss: \ce{Ca^2+}, \ce{Mg^2+} and \ce{K+} are leached from the soil, so plants are poorly nourished.
• Aluminium toxicity: \ce{Al^3+} released in acidic soil damages roots and reduces water uptake.
• Microbial damage: nutrient cycling and nitrogen fixation fall.

Sensitive crops such as soybean, wheat and peanuts are most affected. For example, soybean yield falls because nitrogen fixation in its root nodules is reduced.

Genetic engineering (genetic modification) is a technique used to change an organism's genes to give it a desired trait. In agriculture it is carried out in four basic steps:

1. A desired trait is chosen, such as drought tolerance or pest resistance.
2. The gene responsible for this trait is identified and isolated from an organism (for example the Bt gene from Bacillus thuringiensis).
3. The gene is inserted into the target plant's genome, for example with Agrobacterium or a gene gun.
4. The GM plant is grown and tested to ensure it shows the trait and is safe to eat and for the environment.

Example: Bt cotton makes its own insect-killing protein.

Pests such as insects, weeds, fungi and rodents eat crops, compete with them for water and nutrients, and spread diseases. Pesticides kill or control these pests, so more of the crop survives to harvest:

• Insecticides stop insects eating leaves, stems and fruits.
• Herbicides remove weeds that compete for light, water and nutrients.
• Fungicides prevent fungal diseases such as rusts and blights.
• Rodenticides protect stored grain from rats and mice.

They also stop diseases spreading between plants and give higher-quality produce, so both yield and food security increase.

Nerve damage (neurotoxicity) is a major concern. Organophosphate and carbamate insecticides act by inhibiting the enzyme acetylcholinesterase. This enzyme is found in human nerves too, so the chemicals affect people in the same way as insects.

Farmers who spray without gloves or masks, and people who eat crops with pesticide residues, can suffer headache, sweating, muscle twitching, breathing difficulty and in severe cases death. Long exposure is linked to nerve disorders.

Some pesticides are also carcinogenic (can cause cancer). Protective clothing, correct doses and waiting periods before harvest reduce these risks.

Pesticide resistance is the ability of a pest population to survive a pesticide dose that used to kill it.

It develops by natural selection: in a large pest population a few individuals carry genes that let them survive. When the same pesticide is used again and again, the others die and these few survive and breed, so the next generations are mostly resistant.

Concern: the pesticide becomes less effective, so farmers use larger doses or stronger, more toxic pesticides. This raises costs and increases harm to human health, beneficial insects and the environment. Crop losses may also rise.

First confirm the deficiency: look for symptoms (stunted growth, poor roots, dark green or purplish older leaves, delayed flowering) and get a soil test for available phosphorus and pH.

Then choose a fertilizer that supplies phosphorus in a soluble form:

• Single or triple superphosphate, \ce{Ca(H2PO4)2}, when only phosphorus is needed.
• DAP, \ce{(NH4)2HPO4}, or MAP, \ce{NH4H2PO4}, when nitrogen is also needed.
• In acidic soil, lime first, because \ce{Al^3+} and \ce{Fe^3+} lock phosphate.

Apply it near the roots at sowing, since phosphorus moves little in soil, at the dose the soil test recommends.

Insecticides and herbicides attack systems that are found only in their targets.

So insecticides paralyse and kill animals, while herbicides disturb the life processes of plants.

Excessive use of pesticides harms the environment in several ways:

• Water contamination: runoff carries pesticides into rivers, lakes and ground water, harming fish and other aquatic life.
• Harm to non-target organisms: beneficial insects, birds and pollinators such as bees are killed, reducing biodiversity and pollination.
• Soil contamination: residues harm earthworms and soil microbes.
• Biomagnification: persistent pesticides such as DDT build up along food chains and harm top predators.
• Resistance: pests become resistant, leading to even heavier use.

Runoff carries nitrate and phosphate into water and causes eutrophication. A practical plan follows the rule of right amount, right time, right place:

1. Soil testing so that only the needed amount of fertilizer is applied.
2. Split application of nitrogen in small doses when the crop needs it, not before heavy rain.
3. Placement of fertilizer near roots (banding) and use of slow-release or organic fertilizers.
4. Buffer strips of grass or trees along streams to absorb runoff.
5. Cover crops, contour ploughing and controlled irrigation to reduce erosion and water flow off the field.

Soil pH controls which nutrients dissolve and stay in the root zone. Acid rain lowers the pH, and:

• Loss of cations: \ce{H+} displaces \ce{Ca^2+}, \ce{Mg^2+}, \ce{K+} and \ce{Na+} from soil particles and they are leached away.
• Phosphate fixation: released \ce{Al^3+} and \ce{Fe^3+} combine with phosphate to form insoluble compounds such as \ce{AlPO4}, so phosphorus becomes unavailable.
• Toxic metals: aluminium and manganese become soluble and damage roots.
• Slower recycling: soil microbes are harmed, so less nitrogen and sulphur are released from organic matter.

If soil stays acidic for many years, the damage builds up:

• The soil's reserves of \ce{Ca^2+}, \ce{Mg^2+} and \ce{K+} are exhausted and phosphorus stays locked, so crops suffer chronic nutrient deficiency.
• Toxic \ce{Al^3+} and \ce{Mn^2+} accumulate and stunt roots, so plants suffer from drought even with normal rain.
• Soil microbes and earthworms decline, so organic matter, soil structure and nitrogen fixation all decrease.
• Sensitive crops such as wheat, soybean and peanuts can no longer be grown well.

The result is steadily falling yields and possible crop failure, unless the soil is limed regularly.

Genetic engineering can give crops their own protection against pests, so fewer chemical sprays are needed:

• Bt crops (Bt cotton, Bt corn) carry a gene from Bacillus thuringiensis that makes a protein toxic only to certain insects, such as bollworms. This reduces insecticide spraying.
• Disease-resistant crops, such as rainbow papaya resistant to papaya ringspot virus, need fewer fungicides and other chemicals.
• Herbicide-tolerant crops (for example glyphosate-tolerant soybean) allow one targeted weedkiller instead of several.

Less spraying protects farmers, pollinators and water sources.

Climate change brings droughts in some seasons and heavy rain in others, so irrigation must save water and match the crop's need:

• Use drip or sprinkler irrigation, which deliver water to roots with little waste, instead of flood irrigation.
• Schedule irrigation using weather forecasts and soil moisture, and irrigate in the early morning or evening to reduce evaporation.
• Harvest rainwater in ponds and tanks during heavy rain for use in dry periods.
• Use laser land levelling, mulching and good drainage so water spreads evenly and waterlogging is avoided.

Extreme weather events such as floods, storms, hail and heat waves cause crop failure. Then less produce is available for storage, processing and distribution.

• Food supply chains are disrupted, which leads to food shortages and price increases, especially where people depend on local production.
• Floods damage roads, warehouses and grain stores, so stored food rots or cannot be transported.
• Heat and humidity spoil stored grain and perishable food, which increases food waste.

Together these threaten food security. For example, the 2022 floods in Pakistan destroyed crops and stored food and raised food prices.

Each type attacks a process that is special to its target:

Soil health depends on living organisms and on clean, balanced chemistry. Long-term pesticide use harms both:

• Residues build up: persistent pesticides stay in soil for years and pass into crops and ground water.
• Soil organisms decline: earthworms, beneficial fungi and bacteria (including nitrogen-fixing bacteria) are killed or reduced.
• Nutrient cycling slows: less organic matter is decomposed, so fewer nutrients are released and soil organic carbon falls.
• Soil structure worsens: fewer earthworms and microbes means poorer aeration and water holding, and more erosion.
• Heavy metals from copper fungicides can accumulate.

Introduction

Biodiversity is the variety of living organisms in an area. Pesticides are chemicals used to kill pests such as insects, weeds, fungi and rodents. Most pesticides are not perfectly selective, so they also affect many organisms that are not pests. This reduces the number of species and upsets the balance of ecosystems.

Ways in which pesticides reduce biodiversity

• Killing non-target organisms: broad-spectrum insecticides kill beneficial insects such as ladybirds and spiders that eat pests, as well as butterflies and other harmless species. Birds and aquatic organisms are also affected.
• Harm to pollinators: neonicotinoids and other insecticides harm bees. Fewer pollinators means poor pollination of crops and wild plants, so plant diversity also falls.
• Water contamination: runoff carries pesticides into rivers and lakes, where they kill fish, frogs and aquatic insects.
• Biomagnification: persistent pesticides such as DDT are stored in fat and their concentration increases along the food chain. Birds of prey absorb so much that they lay thin-shelled eggs, which break, and their numbers fall.
• Loss of food and habitat: herbicides remove weeds and wild flowers that insects and birds use for food and shelter.
• Soil life: pesticides kill earthworms, fungi and bacteria that keep soil healthy.
• Resistance: pests become resistant, so stronger or more toxic pesticides are used, increasing all the effects above.

Result

The removal of natural enemies can cause secondary pest outbreaks, and the loss of pollinators and soil organisms upsets the ecosystem. The book notes that pesticides reduce beneficial insects, causing imbalances in the ecosystem and loss of biodiversity.

How the damage can be reduced

• Use selective pesticides only when needed, at the correct dose (integrated pest management).
• Avoid spraying during flowering, when bees are active.
• Keep buffer strips near water and use biological control and biopesticides.

Introduction

A genetically modified (GM) crop is a crop whose DNA has been changed by genetic engineering, usually by inserting a gene from another organism, so that it shows a new desired trait. Three important traits are:

1. Insect (pest) resistance

Example: Bt cotton and Bt corn. They carry a gene from the soil bacterium Bacillus thuringiensis. The gene makes a protein (Bt toxin) that is toxic to specific insects such as the cotton bollworm and corn borer, but harmless to humans. The crop protects itself, so the need for chemical insecticides is reduced. Bt cotton is widely grown in Pakistan.

2. Herbicide tolerance

Example: glyphosate-tolerant soybean, corn and cotton. These crops carry a gene that lets them survive the herbicide glyphosate. The farmer can spray the field to kill weeds without harming the crop, which makes weed control simple and cheap.

3. Disease (virus) resistance

Example: rainbow papaya. It contains a gene from the papaya ringspot virus which makes the plant resistant to that virus. This crop saved the papaya industry of Hawaii when the virus was destroying the plants.

Other traits

Food security

Food security means that all people always have access to enough safe and nutritious food. It is threatened by a growing population, pests and diseases, poor soils, malnutrition and climate change. Genetic engineering can help, but it is not a complete solution.

How genetic engineering helps

• Higher yields: pest-resistant (Bt) and herbicide-tolerant crops lose less of their harvest to insects and weeds.
• Stress tolerance: crops engineered for drought, salinity and heat tolerance can grow on poor land and in changing climates.
• Disease resistance: rainbow papaya resists ringspot virus, protecting supply.
• Better nutrition: golden rice supplies beta-carotene (vitamin A) to people who depend on rice, fighting hidden hunger.
• Lower costs: fewer sprays reduce production costs, making food cheaper.
• Longer shelf life: less food is wasted during storage and transport.

Limitations and risks

• Little is known about the long-term effects of GM food on human health; some fear allergies and antibiotic resistance from marker genes.
• Loss of biodiversity: large areas of a few GM varieties replace many local varieties.
• Resistant pests and superweeds can develop, and genes may cross-pollinate into wild relatives.
• Economic issues: GM seeds are costly and patented by large companies; poor farmers may become dependent on them.
• Hunger is often caused by poverty, waste, war and poor distribution, which genetics cannot solve.

Conclusion

Genetic engineering is a powerful tool that can raise yields, improve nutrition and help crops survive climate change. It gives the best results when combined with good farming practices, fair seed policies, strict safety testing and better storage and distribution of food.

Definitions

Traditional breeding crosses two plants of the same or closely related species and selects the best offspring over many generations. Genetic engineering directly inserts a specific gene, which can come from any organism, into a plant's DNA.

Comparison

Benefits of GM crops over traditional breeding

• Pest resistance (Bt cotton) and disease resistance (rainbow papaya) reduce pesticide use.
• Higher yields and tolerance to drought, salinity and heat.
• Improved nutrition (golden rice) and food quality, lower production costs.

Risks of GM crops

• Long-term effects on human health are not fully known; possible allergies and antibiotic-resistance marker genes.
• Cross-pollination with wild relatives may spread genes with unforeseen consequences.
• Development of resistant pests and weeds; reduced agricultural biodiversity.

Similarities

Both aim to improve yield, quality and resistance, and both can reduce genetic diversity if only a few varieties are grown. Traditional breeding is slower but safer in public opinion; genetic engineering is faster and more powerful but needs careful testing. The best approach often combines both.

Role of micronutrients

Micronutrients (trace elements) are elements that plants need in very small amounts: zinc, manganese, copper, iron, boron and molybdenum (and chlorine, nickel). Although the amounts are small, a shortage of any one of them limits yield even when N, P and K are plentiful. They are parts of enzymes and chlorophyll formation, help photosynthesis, protein synthesis and nitrogen fixation, and improve grain and fruit quality. Micronutrient fertilizers are therefore important in modern high-yield farming, especially in the alkaline, calcareous soils of Pakistan where zinc and boron are often deficient.

Deficiency symptoms

Strategies to correct deficiencies

• Soil and leaf testing to find exactly which element is lacking.
• Soil application of salts such as zinc sulphate (\ce{ZnSO4}), borax, copper sulphate or iron chelates.
• Foliar sprays of dilute solutions, which act quickly and need small amounts.
• Seed treatment (coating seeds with Zn or Mo).
• Correcting soil pH by liming acidic soil or adding organic matter or sulphur to alkaline soil, which makes micronutrients more available.
• Organic manures and compost, which supply several micronutrients slowly.
• Growing biofortified or efficient varieties (for example zinc-rich wheat).

Environmental and economic evaluation

Economic: micronutrient fertilizers are needed in small quantities, so they are cheap compared with the yield gain; correcting zinc deficiency in rice and wheat can raise yields noticeably and improves the nutritional value of food. Environmental: overuse is harmful because the range between deficiency and toxicity is narrow. Excess copper, zinc or boron accumulates in soil, poisons plants and soil organisms and can reach water. Therefore they should be used only on the basis of soil tests, at the right dose.

Pesticides

Pesticides are chemical substances used in agriculture to control or kill pests, including insects, fungi, rodents and weeds. Each type has its own mode of action.

1. Insecticides

Chemicals used to control insects. Most attack the insect nervous system:

• Organophosphates and carbamates inhibit the enzyme acetylcholinesterase, which normally breaks down acetylcholine. Acetylcholine builds up, the nervous system is over-stimulated and the insect is paralysed.
• Pyrethroids disrupt sodium channels in insect neurons, producing repetitive nerve impulses and paralysis.
• Neonicotinoids affect the nicotinic acetylcholine receptors, disrupting nerve signals and causing paralysis.

2. Herbicides

Chemicals used to control unwanted plants called weeds:

• Glyphosate controls broadleaf weeds and grasses; it inhibits protein production and kills the plant.
• 2,4-D and dicamba mimic the plant hormone auxin, causing uncontrolled growth and eventual death.
• Paraquat inhibits photosynthesis and kills the plant.

3. Fungicides

Pesticides that kill or prevent the growth of fungi and their spores:

• Triazoles block the biosynthesis of ergosterol, a component of fungal cell membranes, so the membrane breaks and the cell dies.
• Strobilurins interrupt the electron transport chain in fungal mitochondria, stopping energy production.
• Copper-based fungicides interrupt various cellular processes in fungi.

4. Rodenticides

• Anticoagulant rodent poisons inhibit vitamin K, so blood cannot clot and the rodent dies of internal bleeding.
• Zinc phosphide reacts with stomach acid to form toxic phosphine gas: \ce{Zn3P2 + 6HCl -> 3ZnCl2 + 2PH3}.

5. Nematicides and biopesticides

Nematicides kill plant-parasitic nematodes (organophosphates disrupt their nervous system). Biopesticides come from natural materials such as plants, bacteria and minerals, for example canola oil and baking soda.

What is IPM

Integrated pest management (IPM) is an approach to pest control that combines biological, cultural, physical and chemical methods. Pests are kept below the level that causes economic loss, and chemical pesticides are used only when needed, in the smallest effective amount.

Main strategies

• Monitoring: fields are checked regularly and pests counted; spraying is done only when the pest crosses an economic threshold.
• Cultural control: crop rotation, resistant varieties, correct sowing dates, clean fields and destroying crop residues.
• Biological control: natural enemies such as ladybirds, lacewings and Trichogramma wasps, and biopesticides such as Bt.
• Physical and mechanical control: light traps, pheromone traps, sticky traps and hand removal.
• Chemical control as a last resort: selective pesticides at the correct dose and time, rotating chemicals to delay resistance.

How IPM reduces the harm of pesticides

Fewer and more targeted sprays mean less residue on food, less water and soil contamination, less harm to bees, birds and natural enemies, slower development of resistance and lower costs for farmers.

Examples of successful IPM

• Cotton in Pakistan: Farmer Field Schools taught scouting, pheromone traps and conservation of natural enemies, and farmers cut insecticide sprays while keeping yields.
• Rice in Indonesia: national IPM training greatly reduced insecticide use against brown planthopper, whose outbreaks had been caused by overspraying.
• Greenhouse vegetables: predatory mites and parasitic wasps control spider mites and whiteflies.

Evaluation and sustainability

IPM is sustainable in the long term because it protects biodiversity, soil and water and delays resistance. Its limitations are that it needs trained farmers, regular monitoring and more knowledge than routine spraying, and results may be slower. Government extension services, training and support are needed for it to succeed widely.

Formation of acid rain

Normal rain has pH 5.6 because of dissolved \ce{CO2}. Sulphur and nitrogen oxides from burning fuels form strong acids, giving rain of pH as low as 2.1:

Reactions in soil

Effects on crops

• Crop growth: acid rain damages the waxy surface of leaves, increasing water loss and disease. \ce{Al^3+} damages root tips, so growth is stunted.
• Nutrient uptake: \ce{Ca^2+}, \ce{Mg^2+} and \ce{K+} are leached, phosphate is fixed, soil microbes and nitrogen fixation decline, and damaged roots absorb less water and fewer nutrients.
• Crop yield: all these factors reduce yield. Sensitive crops such as soybean, wheat and peanuts are most affected; soybean yield falls because nitrogen fixation is reduced.

Mitigation strategies

• Liming the soil with powdered limestone: \ce{CaCO3 + 2H+ -> Ca^2+ + H2O + CO2}; dolomite also restores magnesium.
• Adding organic matter to buffer the soil and bind aluminium; balanced fertilizers to replace leached K, Mg and Ca.
• Growing acid-tolerant crop varieties.
• Reducing emissions: low-sulphur fuels, flue gas desulphurisation (\ce{CaCO3 + SO2 -> CaSO3 + CO2}), catalytic converters in vehicles and renewable energy.

Acid rain and soil pH

Acid rain (pH below 5.6) contains \ce{H2SO4} and \ce{HNO3}. Its \ce{H+} ions first react with the soil's calcium carbonate, which acts as a buffer. When this buffer is used up, the soil pH falls steadily.

Impact on soil chemistry and nutrient availability

• Cation exchange: \ce{H+} replaces \ce{Ca^2+}, \ce{Mg^2+}, \ce{K+} and \ce{Na+} held on clay and humus; these are leached out of the root zone.
• Aluminium and manganese become soluble and toxic: \ce{Al(OH)3 + 3H+ -> Al^3+ + 3H2O}.
• Phosphate is fixed as insoluble aluminium and iron phosphates, so it becomes unavailable.
• Microbial activity falls, so decomposition, nitrification and nitrogen fixation slow down.

Impact on plant physiology

• The waxy cuticle of leaves is damaged, so transpiration (water loss) rises and pathogens enter more easily; nutrients are washed out of leaves.
• \ce{Al^3+} damages root tips, so roots absorb less water and fewer nutrients.
• Shortage of \ce{Mg^2+} reduces chlorophyll and photosynthesis; shortage of \ce{Ca^2+} weakens cell walls.
• Physiological and biochemical processes of sensitive crops (soybean, wheat, peanut) change, and nitrogen fixation in legumes falls.

Long-term implications

• Soils become permanently infertile as nutrient reserves are exhausted and toxic metals accumulate.
• Soil organisms decline, organic matter and soil structure worsen, and erosion increases.
• Yields fall year after year, costs rise because of repeated liming and fertilizer, and sensitive crops can no longer be grown.
• Toxic metals leach into rivers and ground water, harming aquatic life and drinking water.

Therefore acid rain must be controlled at its source by cutting \ce{SO2} and \ce{NO_x} emissions, while farmers lime their soils to limit the damage.

Biotic and abiotic stresses

Biotic stresses are caused by living organisms: insects, weeds, fungi, bacteria and viruses. Abiotic stresses are caused by non-living factors: drought, salinity, heat, cold and floods. Genetic engineering adds genes that let crops withstand these stresses.

Examples of GM crops

Benefits

GM crops increase yields, reduce pesticide use and production costs, improve food composition and allow farming on stressed land, helping food security as the climate changes.

Challenges

• Little is known about long-term effects on human health; possible allergies and antibiotic-resistant marker genes.
• Pests can become resistant to Bt toxin and weeds to glyphosate.
• Cross-pollination with wild relatives may cause unforeseen consequences; agricultural biodiversity may fall.
• Abiotic stress tolerance is controlled by many genes, so it is harder to engineer than pest resistance.
• High seed cost, patents, strict regulation and public distrust.

Future prospects

New tools such as CRISPR-Cas9 gene editing can change a plant's own genes precisely without adding foreign DNA. Scientists are developing heat-, salt- and flood-tolerant crops, crops that fix nitrogen or use it more efficiently, and biofortified crops. Combined with good farming practices and careful safety testing, genetic engineering can make agriculture more sustainable.

Impact of climate change on agriculture

• Temperature: heat stress causes dehydration and reduces photosynthesis, respiration and water absorption, leading to slow growth, lower yields and crop failure; frost kills young plants.
• Precipitation: heavy rain leaches nutrients and causes floods; drought causes moisture stress and wilting.
• Extreme events: storms uproot crops and cause lodging, strong winds increase water loss, hail destroys leaves and fruit, and heat waves cause leaf burning and poor flowering.
• Food security: crop failures disrupt supply chains, cause shortages and price rises, and reduce stored food.

Regional differences

Adaptive measures being taken

• Resilient crop varieties with heat, drought and disease tolerance (by breeding or genetic modification).
• Changing sowing dates and crop choice; drip and sprinkler irrigation and laser land levelling.
• Rainwater harvesting, small dams and better drainage.
• Weather forecasting, flood warnings and crop insurance.

Evaluation

These measures reduce losses but are only partly effective. Improved varieties and efficient irrigation work well where farmers can afford them; poor small farmers often lack money, training and information, and very extreme events can still destroy crops. Adaptation must also be combined with cutting greenhouse gas emissions.

Additional strategies

• Climate-smart agriculture: conservation tillage, mulching and cover crops to keep soil moisture and carbon.
• Crop diversification and agroforestry to spread risk.
• Better storage and cold chains to reduce post-harvest losses.
• Research on gene-edited crops, and government support, credit and training for farmers.