GCE A/L Agricultural Science cheat sheet

Photosynthesis net equation: 6CO2 + 6H2O + light energy -> C6H12O6 + 6O2; light reactions occur in thylakoid membranes, the Calvin cycle in the stroma.

C3 plants (rice, wheat, soybean) fix CO2 via RuBisCO into 3-phosphoglycerate; photorespiration rises above about 25-30 C, cutting net assimilation.

C4 plants (maize, sugarcane, sorghum) use PEP carboxylase in mesophyll plus bundle-sheath decarboxylation, giving low photorespiration and high water-use efficiency; CAM plants (pineapple, sisal) fix CO2 at night.

Leaf area index (LAI) is leaf area per unit ground area; crop light interception rises with LAI until self-shading, so optimum LAI for many cereals is about 4-6.

Growth analysis: CGR = (W2 - W1)/(t2 - t1) per unit ground area; NAR = CGR/LAI; RGR = (ln W2 - ln W1)/(t2 - t1).

Crop rotation with a legume raises soil N through symbiosis (Rhizobium in root nodules); cereals following legumes typically yield more than continuous cereal.

Seed germination requires viable embryo, adequate moisture, suitable temperature and, for many species, light or scarification to break dormancy; orthodox seed stores dry, recalcitrant seed (mango, rubber) does not.

Photoperiodism: short-day crops (rice, soybean) flower when night exceeds a critical length; long-day crops (wheat, barley) flower when night is shorter; day-neutral crops (tomato) are insensitive.

Soil texture classes follow the percentage of sand, silt and clay; a soil with >40% clay behaves as clay, 40-60% sand as sandy loam, and loam is roughly balanced sand, silt and clay.

Soil pH governs nutrient availability: most crops grow best at pH 5.5-7.0; lime raises pH (corrects acidity), gypsum or elemental sulfur lowers it.

Cation exchange capacity (CEC) is the milliequivalents of exchangeable cations per 100 g soil; high-clay and high-organic-matter soils have higher CEC and buffer nutrients better.

Bulk density = oven-dry soil mass / total volume; porosity = 1 - (bulk density / particle density), with particle density about 2.65 g/cm3 for most mineral soils.

Field capacity is water held after free drainage; permanent wilting point is where plants can no longer extract water; available water = field capacity minus wilting point.

Soil erosion by water follows the sequence splash, sheet, rill and gully erosion; contour ploughing, terracing, mulching and cover crops reduce it.

Essential macronutrients (N, P, K, Ca, Mg, S) are needed in larger amounts than micronutrients (Fe, Mn, Zn, Cu, B, Mo, Cl, Ni); N deficiency shows as pale older leaves, P deficiency as purple-tinged leaves.

Organic matter improves structure, water-holding capacity and biological activity; the C:N ratio near 25:1 favours steady decomposition, while wide ratios (e.g. straw) temporarily tie up nitrogen.

Ruminants (cattle, sheep, goats) digest fibre in the rumen via microbial fermentation; monogastrics (pigs, poultry) rely on enzymatic digestion and need higher-quality protein.

Rumen microbes synthesize microbial protein from non-protein nitrogen such as urea, but urea must be introduced gradually and never fed to monogastrics.

Feed conversion ratio (FCR) = feed consumed / liveweight gain; lower FCR means more efficient production.

Digestible energy and crude protein are the main ration-balancing measures; a ration must meet maintenance plus production requirements and supply minerals and vitamins.

Oestrus cycle length: cattle about 21 days, sheep about 17 days, pigs about 21 days; gestation: cattle about 283 days, sheep about 147 days, pigs about 114 days.

Colostrum within the first hours of life supplies antibodies (passive immunity) because the newborn has no placental antibody transfer in ungulates.

Poultry incubation is about 21 days for chickens and 28 days for ducks; egg production is influenced by day length, with 14-16 hours of light stimulating lay.

Common livestock diseases: mastitis (udder infection), foot-and-mouth disease (viral, highly contagious), Newcastle disease (poultry), and internal parasites controlled by drenching/rotation.

Gross margin = gross output (yield x price) minus variable costs; fixed costs are excluded and deducted later to obtain net farm income.

Profit = total revenue minus total cost; break-even yield = total cost / (price per unit), and break-even price = total cost / output.

Law of diminishing returns: as one variable input rises with others fixed, marginal product eventually falls; apply inputs until marginal value product equals marginal factor cost.

Elasticity of demand = % change in quantity demanded / % change in price; inelastic demand (|E| < 1) for staple food means price swings cause large farm income swings.

Opportunity cost is the value of the next best alternative forgone; it belongs in economic (not accounting) cost comparisons.

Depreciation of machinery can be calculated by straight line: (cost - salvage value) / useful life in years.

Farm records (physical, financial, inventory) support enterprise budgeting, cash-flow planning and credit applications; a balance sheet shows assets, liabilities and net worth.

Supply of most farm products is inelastic in the short run because biological production cycles cannot be adjusted quickly, which amplifies price volatility.

Mouldboard plough inverts and buries soil for primary tillage; disc harrow and cultivator perform secondary tillage for seedbed preparation.

Draft of a tillage implement is the horizontal force required; it rises with soil resistance, working depth, working width and speed.

Tractor power take-off (PTO) transmits engine power to implements; PTO shaft speed is standardised at 540 or 1000 rpm depending on the system.

Wheel slip = (travelled distance with load - travelled distance without load) / travelled distance without load x 100; excessive slip wastes fuel and damages soil.

Field capacity (ha/h) = (implement width in m x forward speed in km/h) / 10, adjusted for field efficiency.

Irrigation: sprinkler and drip systems apply water under pressure; drip delivers water directly to the root zone and has the highest application efficiency, often 85-95%.

Pump power (kW) = (density x g x discharge x total head) / (1000 x efficiency), with density of water about 1000 kg/m3 and g about 9.81 m/s2.

Grain moisture content on a wet basis = (mass of water / total mass) x 100; safe storage moisture is roughly 12-14% for cereals depending on species and climate.

Mendel's law of segregation: two alleles of a gene separate during gamete formation; law of independent assortment applies to genes on different chromosomes.

Genotype is the genetic constitution, phenotype is the observable trait; homozygous carries identical alleles, heterozygous different alleles.

Monohybrid cross of two heterozygotes (Aa x Aa) gives a 3:1 phenotypic ratio; a test cross with a homozygous recessive reveals the unknown genotype.

DNA structure: double helix of nucleotides with adenine pairing thymine and guanine pairing cytosine; RNA substitutes uracil for thymine.

Mutation is a change in DNA sequence; it may be harmful, neutral or beneficial and is the ultimate source of genetic variation.

Plant breeding methods include selection, hybridisation, mutation breeding and tissue culture; hybrid vigour (heterosis) is the superior performance of an F1 cross.

Pest and disease control follows integrated pest management (IPM): cultural, biological, mechanical and chemical methods combined, with chemicals as a last resort.

Classification: binomial nomenclature gives genus and species, e.g. Zea mays (maize); legumes host nitrogen-fixing Rhizobium in root nodules.

Respiration rate determines perishability: climacteric fruits (banana, tomato, mango) ripen with an ethylene burst, while non-climacteric fruits (citrus, grape) do not.

Cooling after harvest is the most effective single treatment for perishable produce; each 10 C reduction typically slows respiration and deterioration substantially.

Controlled atmosphere storage lowers O2 (about 2-5%) and raises CO2 to slow respiration; excessively low O2 causes fermentation and off-flavours.

Drying grain to safe moisture (about 12-14% for cereals) prevents mould growth and mycotoxin formation; drying must be gradual to avoid cracking.

Post-harvest losses arise from mechanical damage, moisture loss, microbial decay, insects and rodents; careful handling, sorting and packaging reduce them.

Value addition includes cleaning, grading, milling, canning, drying, juicing and packaging, raising product value and shelf life.

Pasteurisation and canning use heat to destroy spoilage organisms; refrigeration slows but does not stop microbial growth.

Modified atmosphere packaging and edible coatings extend shelf life by reducing oxygen contact and moisture loss.

Sustainable agriculture balances productivity, profitability, environmental protection and social equity over the long term.

Conservation agriculture rests on three principles: minimum soil disturbance, permanent soil cover (mulch/cover crops) and crop rotation.

Agroforestry integrates trees with crops or livestock, improving soil fertility, microclimate and diversification of income.

Integrated nutrient management combines organic sources (manure, compost, green manure) with mineral fertilisers to match crop demand and reduce losses.

Eutrophication occurs when excess nitrogen and phosphorus run off into water bodies, causing algal blooms and oxygen depletion.

Greenhouse gases from agriculture include methane from ruminants and flooded rice, and nitrous oxide from fertiliser and manure; improved management reduces emissions.

Integrated pest management, biological control and crop rotation reduce reliance on synthetic pesticides and slow resistance development.

Soil and water conservation measures (terracing, contour bunds, mulching, water harvesting) reduce erosion and protect watersheds.

Photosynthesis net equation: 6CO2 + 6H2O + light -> C6H12O6 + 6O2; light reactions in thylakoids, Calvin cycle in stroma.

C4 crops (maize, sorghum, sugarcane) use PEP carboxylase and bundle-sheath cells, giving high water-use efficiency.

LAI is leaf area per ground area; light interception rises with LAI until self-shading limits it.

CGR = (W2 - W1)/(t2 - t1) per ground area; NAR = CGR/LAI; RGR = (ln W2 - ln W1)/(t2 - t1).

Legume rotation raises soil nitrogen through Rhizobium symbiosis, benefiting the following cereal.

Short-day crops (rice, soybean) flower when night exceeds a critical length; long-day crops (wheat) flower when night is shorter.

Seed germination needs viable embryo, moisture, suitable temperature and, for some species, light or scarification.

Orthodox seed stores dry and long; recalcitrant seed (mango, rubber) loses viability if dried.

Soil texture classes follow sand, silt and clay percentages; >40% clay behaves as clay, balanced proportions give loam.

Most crops grow best at pH 5.5-7.0; lime raises pH, gypsum or sulfur lowers it.

CEC is milliequivalents of exchangeable cations per 100 g soil; clay and organic matter raise CEC.

Bulk density = dry mass / total volume; porosity = 1 - (bulk density / particle density), particle density about 2.65 g/cm3.

Available water = field capacity minus permanent wilting point.

Water erosion sequence: splash, sheet, rill, gully; contour ploughing, terracing and mulching reduce it.

N deficiency pales older leaves; P deficiency gives purple-tinged leaves.

C:N ratio near 25:1 favours steady decomposition; wide ratios such as straw temporarily tie up nitrogen.

Ruminants ferment fibre in the rumen; monogastrics (pigs, poultry) need higher-quality protein and cannot use urea.

Rumen microbes make microbial protein from non-protein nitrogen such as urea; introduce gradually.

FCR = feed consumed / liveweight gain; a lower FCR means greater efficiency.

Oestrus cycle: cattle about 21 days, sheep about 17 days, pigs about 21 days.

Gestation: cattle about 283 days, sheep about 147 days, pigs about 114 days.

Colostrum supplies passive antibodies in the first hours of life; ungulates do not transfer antibodies across the placenta.

Chicken eggs hatch in about 21 days, duck eggs in about 28 days; 14-16 hours of light stimulates lay.

Mastitis is an udder infection; foot-and-mouth disease is a highly contagious viral disease; Newcastle disease affects poultry.

Gross margin = gross output (yield x price) minus variable costs; fixed costs are deducted later.

Break-even yield = total cost / price per unit; break-even price = total cost / output.

Law of diminishing returns: apply a variable input until marginal value product equals marginal factor cost.

Elasticity of demand = % change in quantity / % change in price; |E| < 1 is inelastic, typical of staple food.

Opportunity cost is the value of the next best alternative forgone and belongs in economic cost.

Straight-line depreciation = (cost - salvage value) / useful life in years.

A balance sheet lists assets, liabilities and net worth; farm records support budgeting and credit.

Farm supply is inelastic in the short run because biological cycles cannot be adjusted quickly, amplifying price swings.

Mouldboard plough performs primary tillage and inverts soil; harrows and cultivators prepare the seedbed.

Implement draft rises with soil resistance, working depth, working width and forward speed.

PTO transmits engine power to implements at standardised 540 or 1000 rpm.

Wheel slip % = (distance loaded - distance unloaded) / distance unloaded x 100.

Field capacity (ha/h) = (width in m x speed in km/h) / 10, adjusted for field efficiency.

Drip irrigation delivers water to the root zone with application efficiency often 85-95%.

Pump power (kW) = (density x g x discharge x head) / (1000 x efficiency); water density about 1000 kg/m3.

Wet-basis moisture % = (mass of water / total mass) x 100; cereals store safely near 12-14%.

Law of segregation: alleles separate during gamete formation; independent assortment applies to genes on different chromosomes.

Genotype is genetic constitution; phenotype is the observable trait; homozygote has identical alleles.

Aa x Aa gives a 3:1 phenotypic ratio; a test cross with homozygous recessive reveals an unknown genotype.

DNA is a double helix: adenine pairs thymine, guanine pairs cytosine; RNA uses uracil instead of thymine.

Mutation is a DNA sequence change and the ultimate source of genetic variation.

Breeding methods include selection, hybridisation, mutation breeding and tissue culture; heterosis is superior F1 performance.

IPM combines cultural, biological, mechanical and chemical control, with chemicals as a last resort.

Binomial nomenclature gives genus and species, e.g. Zea mays; legumes host nitrogen-fixing Rhizobium.

Climacteric fruits (banana, tomato, mango) ripen with an ethylene burst; non-climacteric fruits (citrus, grape) do not.

Cooling after harvest is the most effective single treatment for perishable produce; lower temperature slows respiration.

Controlled atmosphere storage uses about 2-5% O2 and raised CO2; too little O2 causes fermentation and off-flavours.

Cereals store safely at about 12-14% moisture; slow drying prevents cracking and mould.

Post-harvest losses come from mechanical damage, moisture loss, microbial decay, insects and rodents.

Value addition includes cleaning, grading, milling, drying, canning, juicing and packaging.

Pasteurisation and canning use heat to destroy spoilage organisms; refrigeration slows but does not stop microbial growth.

Modified atmosphere packaging and edible coatings reduce oxygen contact and moisture loss.

Sustainability balances productivity, profitability, environmental protection and social equity over the long term.

Conservation agriculture: minimum soil disturbance, permanent soil cover and crop rotation.

Agroforestry integrates trees with crops or livestock, improving soil fertility and microclimate.

Integrated nutrient management combines manure, compost and green manure with mineral fertiliser to match crop demand.

Eutrophication follows excess nitrogen and phosphorus runoff, causing algal blooms and oxygen depletion.

Agricultural greenhouse gases include methane from ruminants and flooded rice, and nitrous oxide from fertiliser and manure.

IPM and biological control reduce pesticide reliance and slow resistance development.

Terracing, contour bunds, mulching and water harvesting reduce erosion and protect watersheds.