GCE A/L Agricultural Science key terms

Loss of water vapour from plant leaves and stems through stomata. It pulls water and dissolved minerals upward from the roots and cools the plant, but excessive loss under dry, hot conditions can reduce yield.

Process in which green plants use light energy to convert carbon dioxide and water into sugars, releasing oxygen. It supplies the carbohydrates that drive crop growth, yield and stored grain or tuber mass.

Movement of sugars and other solutes through the phloem from sites of production or storage to growing or storage organs. It determines how much assimilate reaches grain, fruit or tubers.

Balance between plant parts that produce carbohydrates, such as mature leaves, and parts that consume or store them, such as grains, fruits or roots. A strong sink and healthy source raise harvestable yield.

Small pores, mainly on leaf surfaces, bounded by guard cells that open and close to regulate gas exchange and water loss. Their behaviour links photosynthesis, transpiration and drought response.

Response of plants to the relative length of day and night, controlling flowering and some developmental changes. Short-day and long-day crops flower only when the critical night length is met, which affects planting dates.

Promotion of flowering by exposure to a period of low temperature, especially in cereals such as wheat and barley. It explains why some varieties must be sown in cool seasons to produce grain.

Proportion of sand, silt and clay particles in a soil, determined by particle size. Texture strongly influences water holding, drainage, aeration, nutrient retention and workability of the soil.

Arrangement of soil particles into aggregates and the resulting pore space. Good structure improves aeration, root penetration and water movement, while poor structure encourages crusting, compaction and erosion.

Ability of a soil, mainly through clay and organic matter, to hold positively charged nutrient ions and release them to plants. Higher values generally mean greater nutrient retention and less leaching.

Measure of acidity or alkalinity of the soil solution on a scale from 0 to 14. It controls nutrient availability, microbial activity and the choice of crops, lime or gypsum applications.

Water content remaining in a soil after excess water has drained away by gravity and drainage has nearly stopped. It represents the upper limit of readily stored water available to most crops.

Soil water content below which plants can no longer extract water and remain wilted even when placed in a humid atmosphere. The gap between it and field capacity defines available water.

Downward movement of soluble materials, such as nitrate or salts, through the soil profile with drainage water. It can remove nutrients beyond the root zone and pollute groundwater.

Application of materials such as ground limestone or dolomite to raise soil pH and supply calcium or magnesium. It corrects acidity, improves nutrient availability and can reduce aluminium toxicity.

Detachment and removal of topsoil by water, wind or tillage. It depletes fertile surface layers, reduces rooting depth and causes sedimentation and pollution off site.

Herbivorous animal with a multi-chambered stomach and rumen microbes that ferment fibrous feed. Cattle, sheep and goats use forage efficiently, but their nutrition must supply protein, energy and minerals.

Amount of feed consumed per unit of animal product gained, such as kilograms of feed per kilogram of liveweight or milk. Lower values indicate more efficient feed use and better profitability.

Feed mixture combining forage, concentrates, minerals and additives in fixed proportions to meet animal requirements. It prevents selective feeding and helps stabilise rumen function and production.

First milk produced after birth, rich in antibodies and nutrients. Young animals need it within the first hours of life because it provides passive immunity before their own immune system develops.

Gradual or abrupt withdrawal of milk from young animals and transition to solid feed. Timing and method affect growth, stress and health, especially in piglets and calves.

Enterprise gross output minus variable costs, excluding fixed costs. It measures the return to fixed resources and helps compare which crops or livestock enterprises use limited land and labour best.

Cost that changes with the level of production, such as seed, fertiliser, fuel, casual labour and veterinary supplies. Variable costs are deducted from gross output to calculate gross margin.

Level of output or price at which total revenue equals total cost, so profit is zero. It indicates the minimum production or sales needed before an enterprise becomes profitable.

Value of the best alternative forgone when a resource is used for one purpose rather than another. It guides decisions on land, labour and capital allocation in farm planning.

Annual decline in the value of a farm asset such as machinery or buildings due to wear, age and obsolescence. It is a fixed cost used in enterprise budgets and machinery replacement decisions.

Shaft on a tractor that transmits engine power to implements such as mowers, sprayers and balers. Matching implement power demand to PTO output and speed prevents stalling and damage.

Hydraulic mounting system with two lower arms and one upper link that attaches and lifts implements behind a tractor. It transfers weight and draft, giving stability and depth control.

Force required to pull an implement through the soil, influenced by soil type, depth, speed and implement design. Higher draft demands more tractor power, fuel and traction.

Planning of when and how much water to apply, based on crop water need, soil moisture and weather. Good scheduling avoids stress and prevents waste, waterlogging and nutrient leaching.

Genetic cross involving one pair of contrasting traits, such as tall versus short plants. Its offspring ratios reveal dominance and segregation of alleles and form the basis of Mendelian genetics.

Genetic makeup of an organism, described by the alleles it carries at one or more loci. It interacts with the environment to produce the observable phenotype used in selection.

Observable characteristics of an organism, such as yield, colour or disease resistance, resulting from genotype and environment. Breeders select on phenotype but aim to improve underlying genotype.

Proportion of variation in a trait within a population that is due to genetic differences. High heritability means selection on phenotype is likely to produce rapid genetic progress.

Superior performance of crossbred offspring compared with their parents, often seen in growth, yield or vigour. It is exploited in hybrid seed production and crossbreeding programmes.

Speed at which harvested produce consumes oxygen and releases carbon dioxide and heat. Higher rates accelerate quality loss, so cooling and controlled atmosphere storage reduce respiration.

Unbroken series of refrigerated storage and transport steps from harvest to consumer. Maintaining the chain slows respiration, microbial growth and spoilage of perishable produce.

Storage with deliberately adjusted oxygen, carbon dioxide and sometimes ethylene levels, usually with refrigeration. Lower oxygen and raised carbon dioxide slow ripening and extend shelf life.

Planned sequence of different crops on the same land over successive seasons. It interrupts pest and disease cycles, improves soil fertility and reduces reliance on chemical inputs.

Crop grown mainly to protect and improve soil rather than for harvest, such as legumes or grasses. It reduces erosion, adds organic matter and can suppress weeds and pests.

Combined use of cultural, biological, physical and chemical methods to keep pests below economic thresholds. It reduces pesticide reliance, slows resistance and protects beneficial organisms and the environment.

Nutrient enrichment of water bodies, usually by nitrogen and phosphorus runoff, causing algal blooms and oxygen depletion. It damages aquatic life and is a key off-farm impact of fertiliser use.

Ploughing, planting and cultivating along lines of equal elevation on sloping land. The ridges slow runoff, trap soil and water, and reduce erosion compared with up-and-down slope cultivation.