Comprehensive Study Notes on Agrochemicals: Fertilizers, Insecticides, Herbicides, and Fungicides

Definition and Scope of Agrochemicals

• Definition: Agrochemicals are chemical substances utilized for agricultural crop and animal protection to enhance production.

• Categories: The classification includes fertilizers, pesticides, herbicides, and fungicides.

• Origins: They can be synthetic, semi-synthetic, or of natural origin (natural products).

Fertilizers: Types and Applications

• Purpose: Chemical compounds applied to crops to improve production yields.

• Application Methods:     * Soil Application: For plant root uptake.     * Foliar Feeding: Absorption through the leaves.

• Main Categories:     * Organic Fertilizers: Prepared through natural processes such as composting (decayed plant matter) and using naturally occurring mineral deposits.     * Inorganic Fertilizers: Manufactured through industrial chemical processes. These often use naturally occurring deposits but chemically alter them into a form usable by plants.

Essential Plant Nutrients

• Major Nutrients (NPK):     * Nitrogen (N): Used by plants to produce leafy growth, stems, and branches. High-demand crops include leafy vegetables like cabbage and lettuce.     * Phosphorus (P): Crucial for seed germination and root development. Particularly needed by young plants forming root systems, as well as fruits, seed crops, and root vegetables like carrots.     * Potassium (K): Promotes flowering and fruit production, maintains growth, and helps plants resist diseases. It is involved in building starches and sugars. Required by vegetables and fruits like apples, tomatoes, and potatoes.

• Secondary Nutrients: Calcium ($Ca$), Sulphur ($S$), and Magnesium ($Mg$).

• Micronutrients (Trace Elements): Essential for overall healthy growth and wellbeing. Includes Boron, Chlorine, Manganese, Iron, Zinc, Copper, Molybdenum, and Selenium.

• Commercial Rating: NPK content is described numerically (e.g., $05-05-05$ indicates $5\%$ Nitrogen, $5\%$ Phosphorus, and $5\%$ Potassium).

Specific Fertilizer Compounds and Synthesis

• Urea:     * NPK Rating: $46-00-00$ ($46\%$ nitrogen).     * Highest nitrogen source among solid fertilizers; has surpassed ammonium nitrate in usage.     * Industrial Production: Synthesized from ammonia ($NH_3$) and carbon dioxide ($CO_2$) at $200^{\circ}C$ and $150\,atm$.     * Equation: $2NH_3 + CO_2 \rightarrow H_2NCONH_2 + H_2O$

• Ammonium Nitrate:     * NPK Rating: $34-00-00$ ($34\%$ nitrogen).     * More stable than urea; does not lose nitrogen to the atmosphere as easily.     * Urea must be applied in dry weather before rain to minimize loss, whereas ammonium nitrate is an acid/base product of nitric acid and gaseous ammonia.     * Equation: $NH_3(aq) + HNO_3(g) \rightarrow NH_4NO_3$

• Potassium Nitrate:     * NPK Rating: $13-00-44$.     * Production: Reaction of potassium chloride ($KCl$) with nitric acid ($HNO_3$) in the presence of oxygen, or reaction between sodium nitrate ($NaNO_3$) and potassium chloride.     * Equations:         * $2KCl + 2HNO_3 + [O] \rightarrow 2KNO_3 + Cl_2 + H_2O$         * $NaNO_3 + KCl \rightarrow NaCl + KNO_3$

• Ammonium Sulphate:     * NPK Rating: $21-00-00$.     * Used on alkaline soils because the sulphate ion helps lower the soil pH balance.     * Production: Reaction of ammonia with sulphuric acid.     * Equation: $2NH_3 + H_2SO_4 \rightarrow (NH_4)_2SO_4$

• Calcium Nitrate:     * NPK Rating: $15-00-00$.     * Production: Treating limestone ($CaCO_3$) with nitric acid followed by neutralization with ammonia, or from ammonium nitrate and calcium hydroxide.     * Equations:         * $CaCO_3 + 2HNO_3 \rightarrow Ca(NO_3)_2 + CO_2 + H_2O$         * $2NH_4NO_3 + Ca(OH)_2 \rightarrow Ca(NO_3)_2 + 2NH_4OH$

• Calcium Phosphate:     * NPK Rating: $00-44-00$ to $00-52-00$.     * Tricalcium phosphate ($Ca_3(PO_4)_2$) is the major component of phosphate rocks (phosphorite, apatite) but is insoluble in water.     * It is converted to water-soluble mono calcium phosphate (known as Super Phosphate) using sulphuric acid.     * Equation: $Ca_3(PO_4)_2 + 2H_2SO_4 \rightarrow 2CaSO_4 + Ca(H_2PO_4)_2$

Environmental Impact of Synthetic Fertilizers

• Runoff and Dead Zones: Excess fertilizer runs off into coastal regions, creating "dead zones" where the ocean is starved of oxygen.

• Eutrophication: A process where a water body becomes abundant in aquatic plants (due to nitrogen influx) but low in oxygen.

• Ammonia Toxicity: Toxic to aquatic life even at concentrations as low as $0.02\,ppm$.

• Human Health (Blue-Baby Syndrome):     * Nitrates in drinking water cause methemoglobinemia in infants.     * Nitrates are reduced to nitrites, which interfere with oxygen transport in the body, leading to a pale bluish skin color.

Inorganic Insecticides

• Historical Context: Most have been replaced by organic pesticides, though some remain in small-scale household use.

• Sulphur: Likely the oldest known effective insecticide, used as dusts and sprays in integrated pest management.

• Metals and Metalloids: Compounds of mercury, boron, thallium, arsenic, antimony, selenium, and fluoride have been used.

• Arsenicals:     * The only inorganic group used extensively.     * Forms: Arsenites (salts of arsenious acid) and Arsenates (salts of arsenic acid, $H_3AsO_4$).     * Examples: Paris Green ($Cu(CH_3COO)_2 \cdot 3Cu(AsO_2)_2$) was the first used water-soluble arsenite. Followed by Lead Arsenate ($PbAsO_4$) and Calcium Arsenate ($Ca(AsO_4)_2$), which is more toxic to insects and mammals.     * Mode of Action: They inhibit respiratory enzymes. The $As^{3+}$ ions specifically inhibit sulphydryl ($-SH$) containing enzymes (e.g., pyruvate oxidase) involved in cell respiration.

Organic Pesticides: Organochlorines

• General Characteristics: Chlorinated hydrocarbons used extensively from the 1940s to mid-1970s. Known for environmental persistence and fat solubility (bioaccumulation).

• Classification:     1. Dichlorodiphenylethanes: e.g., DDT, dicofol, methoxychlor.     2. Hexachlorocyclohexanes: e.g., Benzene hexachloride (BHC), chlordane, lindane, mirex, toxaphene.     3. Chlorinated Cyclodienes: e.g., aldrin, dieldrin, endrin, endosulfan, heptachlor.

• Specific Compounds:     * DDT (Dichlorodiphenyltrichloroethane): Synthesized by condensation of chlorobenzene with chloral in sulphuric acid. Highly persistent (half-life of $12$ years). Lethal by interfering with nerve function, causing $Na^+$ and $K^+$ ions to leak out of nerve cells, leading to paralysis.     * Lindane: Produced by adding chlorine to benzene under UV irradiation. Half-life is short ($18$ hours), and it is less stable than DDT. Isomers $\alpha$-hexachlorohexane and $\beta$-hexachlorohexane are more toxic to humans. Used as a second-line treatment for lice/scabies.     * Chlordane: Produced by chlorination of chlordene. Long residual activity (half-life in soil approx. $1$ year). Used as a wood preservative.     * Aldrin: Non-systemic insecticide for soil-dwelling insects. In soil/water, it undergoes epoxidation to Dieldrin. Half-life is $369$ days.     * Dieldrin: Produced by epoxidizing aldrin with peracetic acid. Highly persistent; half-life in temperate soil is about $5$ years. Used for public health pests and termites.     * DBCP (1,2-Dibromo-3-chloropropane): Former soil fumigant banned in 1985 due to causing male reproductive issues (decreased sperm count).

Organic Pesticides: Organophosphates (OPs)

• General Characteristics: Derived from phosphoric acid ($H_3PO_4$). Generally more toxic to vertebrates than organochlorines but chemically unstable and non-persistent (better substitutes for long-term environmental safety).

• Mode of Action: Inhibit cholinesterase ($ChE$) enzymes, leading to the accumulation of acetylcholine ($Ach$) and causing muscle twitching, paralysis, and death.

• Structural Requirements: Pentavalent phosphorus bonded to Oxygen or Sulfur, plus two alkoxy/alkyl/amino groups and one acyl/acidic residue.

• Specific Compounds:     * Malathion: Aliphatic OP with low mammalian toxicity ($LD_{50}: 1200\,mg/kg$ in rats). Used for crops and mosquito eradication.     * Parathion: Phenoxy derivative. Highly toxic ($LD_{50}: 6.4\,mg/kg$). Spectrum includes sucking and biting insects. Banned for food crops.     * Dichlorvos: Aliphatic OP used as a fumigant and agricultural pest control. Highly volatile and rapid decomposition (half-life in water $19$ to $79$ hours).     * Dimethoate: Systemic insecticide; also used as an animal spray.     * Chlorpyrifos: One of the most widely used insecticides globally. Acts as a neurotoxin. EPA revoked all food tolerances as of February 28, 2022. Linked to neurological effects in children.

Carbamate Insecticides

• Definition: Derivatives of carbamic acid ($NH_2COOH$).

• Mode of Action: Inhibition of cholinesterase ($ChE$).

• Evolution: Early $N,N$-dimethyl carbamates (e.g., Isolan, Dimetan) were less effective than later $N$-methyl carbamates.

• Key Compounds:     * Carbaryl (Sevin): Introduced in 1956. Most widely used carbamate. Low mammalian toxicity and broad spectrum.     * Systemics: Methomyl, Oxamyl, Aldicarb, and Carbofuran are taken up by roots/leaves. Aldicarb and Carbofuran are soil insecticides/nematicides.     * Propoxur (Baygon): Highly effective against cockroaches including those resistant to other classes.

• Synthesis Methods:     1. Reaction of heterocycles/enols with dimethylcarbamoyl chloride.     2. Reaction of $-OH$ compounds with methyl isocyanate.     3. Via chloroformates (e.g., Carbaryl produced from 1-Naphthol and phosgene, then methylamine).

Neonicotinoid and Phenylpyrazole Insecticides

• Neonicotinoids:     * Discovered in the late 1980s. Alternatives to OPs/carbamates where resistance occurred.     * Structural Basis: Related to nicotine (alkaloid); act as acetylcholine receptor agonists (mimics neurotransmitters).     * Examples: Imidacloprid (Confidor, Gaucho), Thiacloprid, Nitenpyram, Acetamiprid, Thiamethoxam, Clothianidin, Dinotefuran.     * Nature: Highly systemic (taken up by roots/leaves).

• Phenylpyrazoles:     * Structural Basis: Pyrazole ring.     * Mode of Action: Blocking glutamate-activated chloride channels in the central nervous system (absent in mammals).     * Examples: Fipronil (used worldwide) and Pyriprole (trialing in Europe).

Botanical Insecticides

• Rotenone:     * Extracted from the genus Derris (e.g., Derris elliptica in PNG).     * Action: Inhibits electron transfer, stopping ATP production.     * Usage: Stuns fish; kills mites on poultry. Half-life is short (one week or less).

• Pyrethrins:     * Extracts from Chrysanthemum (Chrysanthemum cinaeriofolium).     * Contains six insecticidal esters. Considered the safest insecticides for humans.     * Semi-synthetics: Chemists derived "pyrethroids" from these base molecules.

• Azadirachtin: Major component of Neem oil. Minor components include nimbolide and salannin.

Herbicides: Classification and Inorganic Types

• Primary Classification:     * Selective: Kills only weeds, not crops.     * Non-selective: Kills all vegetation.     * Contact: Kills only the part of the plant touched by the chemical.     * Translocated: Absorbed and circulated through the entire plant system.

• Inorganic Herbicides:     * Ammonium Salts: Ammonium sulfamate ($NH_4SO_3NH_2$), ammonium nitrate, and iron sulfate. Mechanism: Desiccation and plasmolysis.     * Borates: Sodium tetraborate and sodium metaborate. Absorbed by roots, persistent, and non-selective.     * Sodium Chlorate ($NaClO_3$): Soil sterilant used for $60$-$80$ years.     * Sulfuric Acid ($H_2SO_4$): Used as a foliar herbicide, though limited by its corrosiveness to equipment.

Organic Herbicides

• Petroleum Oils: Earliest organic herbicides (gasoline, kerosene, diesel). Disrupt plasma membranes. High atmospheric hydrocarbon release.

• Carbamate Herbicides:     * Esters of carbamic acid. Unlike insecticides, the aromatic ring is usually attached to the Nitrogen atom (e.g., Asulam).     * Action: Stop cell division and tissue growth.

• Thiocarbamates: Derived from thiocarbamic acid ($HSCONH_2$). e.g., Pebulate. Inhibit development of seedling shoots/roots.

• Nitrogen Heterocyclic Herbicides: Based on the triazine nucleus. Potent photosynthesis inhibitors.     * Examples: Atrazine, Metribuzin.     * Synthesis: Starting from cyanuric chloride ($2,4,6$-trichloro-$1,3,5$-triazine).

• Bipyridylium Herbicides:     * Diquat and Paraquat: Inhibit photosynthesis and the Hill Reaction (light-initiated water splitting to produce $O_2$).

• Glyphosate (Roundup):     * Organophosphate in chemical structure but not a cholinesterase inhibitor.     * Broad-spectrum, non-selective, systemic.     * Pathway: Inhibits the shikimic acid pathway (amino acid metabolism), which exists in plants and microbes but not animals.     * Synthesized from phosphorus trichloride, formaldehyde, and glycine.

Hormone Type Herbicides

• Mechanism: Mimic the plant growth hormone Auxin, causing uncontrolled growth that leads to death.

• Examples:     * Phenoxyethanoic and Phenoxypropanoic acids.     * 2,4-D ($2,4$-dichlorophenoxyethanoic acid) and 2,4,5-T ($2,4,5$-Trichlorophenoxyethanoic acid).     * Agent Orange: An equal ratio mixture of $2,4$-D and $2,4,5$-T used in the Vietnam War.

• Other Selective Grass Killers: Trichloroethanoic acid and $2,2$-dichloropropanoic acid (Dalapon), synthesized via the Hell-Volhard-Zelinski Reaction.

Fungicides

• Nature of Fungi: Parasitic plants living on hosts. They reproduce rapidly (up to $25$ generations in a $3$-month season).

• Disease Cycle: Infection $\rightarrow$ Colonization $\rightarrow$ Symptoms (e.g., Gray Leaf Spot lesions) $\rightarrow$ Spore Production.

• Types:     * Preventative (Protective): Barrier placed on plant before pathogen arrival.     * Curative: Stops pathogen growth within tissue ($24$ to $72$ hours post-infection).

• Inorganic Fungicides:     * Sulphur: Dust, colloidal, or wettable forms.     * Copper: Cupric sulphate, Copper hydroxide, Copper oxide.     * Mercury: Banned due to animal toxicity and environmental accumulation.

• Organic/Systemic Fungicides: Therapeutic and eradicant.     * Dithiocarbamates: e.g., Thiram. Synthesized from carbon disulfide and dimethylamine followed by oxidation.     * Triazoles: e.g., Hexaconazole (Anvil). Inhibits ergosterol biosynthesis. Used in PNG for Coffee Rust.

Alternatives and Limitations of Pesticides

• Limitations: Environmental persistence, biological magnification, non-selective toxicity, and the development of pest resistance.

• Alternatives:     * Biological Control: Introducing natural predators (e.g., fish for mosquito larvae).     * Pheromones: Chemical signals used to disrupt mating or trap insects.     * Sterilization: Releasing laboratory-raised males sterilized by gamma radiation.     * Hormones: Using synthetic hormones to stimulate premature metamorphism or prevent development.     * Agricultural Practices: Ploughing to expose dormant insects; changing planting times; breeding resistant crop cultivars.