NCEA Level 3 Ag & Hort Science flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Cation exchange capacity (CEC)

The soil's capacity to hold positively charged nutrient ions on clay and organic matter surfaces, measured in cmolc/kg; higher CEC means greater nutrient retention and less leaching.

Soil texture triangle

A diagram using percentages of sand, silt and clay to classify soil into textural classes such as sandy loam or clay; texture strongly determines drainage, aeration and nutrient holding.

Liebig's law of the minimum

The principle that plant growth is limited by the single scarcest resource, not by total resources; correcting the most deficient factor gives the greatest yield response.

Photosynthesis equation

6CO2 + 6H2O + light energy → C6H12O6 + 6O2, occurring in chloroplasts; it converts light energy into chemical energy stored as carbohydrate in plants.

Limiting factors of photosynthesis

Light intensity, carbon dioxide concentration, temperature and water supply; whichever is in shortest supply restricts the rate of photosynthesis at that moment.

Macronutrients vs micronutrients

Macronutrients (N, P, K, S, Ca, Mg) are needed in large quantities; micronutrients (Fe, Mn, Zn, Cu, B, Mo, Cl) are needed in trace amounts but are equally essential.

Nitrification

The conversion of ammonium to nitrite by Nitrosomonas and then to nitrate by Nitrobacter; nitrate is readily taken up by plants but also easily leached.

Nitrogen fixation

Conversion of atmospheric nitrogen gas into plant-available forms by Rhizobium bacteria in legume root nodules, by free-living soil bacteria, or industrially by the Haber process.

Heritability

The proportion of variation in a trait due to genetic differences; high heritability means selection and breeding produce rapid, predictable improvement in that trait.

Soil pH and nutrient availability

Most macronutrients are most available at pH 5.8–6.5; below pH 5.5 aluminium toxicity and phosphorus fixation increase, while above pH 7 micronutrients become less available.

Transpiration

Loss of water vapour from plant leaves through stomata; it drives the upward movement of water and dissolved nutrients from roots to shoots.

Stocking rate

The number of animals carried per hectare over a defined period; overstocking reduces pasture regrowth because residual leaf area falls below the critical level for rapid recovery.

Grazing residual

The pasture dry matter left after grazing, ideally about 1200–1500 kg DM/ha for ryegrass; too low a residual slows regrowth and depletes root carbohydrate reserves.

Dry matter intake (DMI)

The amount of feed dry matter an animal consumes daily, typically 2–3% of body weight for ruminants; it determines the energy available for maintenance, growth and milk.

Feed conversion ratio (FCR)

Kilograms of feed dry matter required to produce one kilogram of product such as liveweight gain or milk solids; a lower FCR indicates more efficient feed use.

Integrated pest management (IPM)

A hierarchy of controls: cultural methods such as rotation and resistant cultivars first, then biological controls, and chemicals only when pest numbers exceed the economic threshold.

Economic threshold

The pest population density at which the value of crop loss equals the cost of control; below this level, applying pesticide is not economically justified.

Evapotranspiration (ET)

Combined water loss from soil evaporation and plant transpiration; irrigation scheduling replaces ETc (reference ET × crop coefficient) minus effective rainfall to avoid stress.

Controlled atmosphere storage

Post-harvest storage with reduced oxygen and elevated carbon dioxide, combined with cool temperature, to slow respiration and ripening and extend the shelf life of produce.

Biosecurity

Measures such as border controls, pest-free certification and traceability that prevent the entry and spread of pests and diseases, protecting production and export market access.

Riparian planting

Vegetation planted along waterways that filters nutrient and sediment runoff from farmland, stabilises banks and shades water, improving water quality and stream habitat.

Plant propagation from cuttings

Asexual propagation in which a stem or leaf segment is rooted to produce a genetically identical plant; used to preserve desirable cultivars that do not breed true from seed.

Crop rotation

Growing different crop species in sequence on the same land to break pest and disease cycles, balance nutrient use and maintain soil structure and organic matter.

Nutrient budget

An account comparing nutrient inputs (fertiliser, feed, fixation) with outputs (product, leaching, volatilisation); a positive nitrogen surplus signals leaching and greenhouse gas risk.

Nitrate leaching

Downward movement of soluble nitrate below the root zone into groundwater; risk increases with high rainfall, sandy low-CEC soils and excess nitrogen application.

Methane from ruminants

A greenhouse gas produced by enteric fermentation in ruminant animals, with a warming potential roughly 21–28 times that of carbon dioxide; reduced by improved feed quality.

Nitrous oxide emissions

A greenhouse gas released from nitrogen fertiliser and animal urine patches, with warming potential roughly 265–298 times carbon dioxide; nitrification inhibitors can reduce losses.

Eutrophication

Nutrient enrichment of water causing algal blooms; decomposition of the algae consumes oxygen, leading to hypoxia, fish kills and loss of aquatic biodiversity.

Gross margin

Gross revenue minus variable costs for an enterprise; it excludes fixed costs and allows comparison of profitability per hectare or per stock unit between enterprises.

Break-even analysis

Calculation of the output or price at which revenue equals costs, found as fixed costs divided by (price minus variable cost per unit); used to assess risk.

Water use efficiency

The amount of product (such as kilograms of dry matter or fruit) produced per cubic metre of water used; a key indicator of sustainable irrigated production.

Market access standards

Importing-country requirements such as maximum residue limits, food safety and animal welfare rules; failure to meet them causes rejection, recalls and loss of certification.