NCEA Level 3 Ag & Hort Science cheat sheet

Soil texture triangle: percentage sand, silt and clay determine class; clay particles (<0.002 mm) dominate cation exchange capacity (CEC), sand (>0.06 mm) drains fastest and holds few nutrients.

CEC is measured in cmolc/kg (or meq/100 g); higher CEC means greater nutrient retention, so sandy soils need smaller, more frequent fertiliser applications than clay loams.

Liebig's law of the minimum: growth is limited by the single most deficient resource, so adding a non-limiting nutrient gives no yield response until the deficient one is corrected.

Soil pH controls nutrient availability: most macronutrients are most available at pH 5.8–6.5; aluminium toxicity and phosphorus fixation increase below pH 5.5.

Photosynthesis summary: 6CO2 + 6H2O + light energy → C6H12O6 + 6O2, occurring in chloroplasts; net dry-matter accumulation = gross photosynthesis minus respiration.

Limiting factors of photosynthesis: light intensity, CO2 concentration, temperature and water supply; at low light, light intensity is limiting, at high light, CO2 or temperature usually becomes limiting.

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

Nitrogen cycle steps: fixation (Rhizobium, lightning, industrial Haber process), nitrification (Nitrosomonas, Nitrobacter), assimilation, ammonification, denitrification by anaerobic bacteria.

Genetic improvement via selection and cross-breeding relies on heritability: response to selection = selection differential × heritability, so traits with high heritability improve fastest.

Stocking rate is animals per hectare; stocking density is animals per unit area at a point in time. Overstocking reduces pasture regrowth because residual leaf area (grazing residual) falls below the critical leaf area index.

Pasture rotation rule: graze at the 2–3 leaf stage of ryegrass and leave a residual of about 1200–1500 kg DM/ha to maintain rapid regrowth; continuous hard grazing depletes root reserves.

Dry matter intake for ruminants is roughly 2–3% of body weight per day; feed conversion ratio = feed consumed ÷ product output (kg DM per kg liveweight or milk solids).

Integrated pest management (IPM) hierarchy: cultural controls (rotation, resistant cultivars) → biological controls (predators, parasitoids) → chemical controls only when economic thresholds are exceeded.

Economic threshold = pest density at which the value of crop loss equals the cost of control; below it, spraying is uneconomic.

Greenhouse climate control: ventilation and shading reduce temperature and humidity; CO2 enrichment to ~700–1000 ppm raises photosynthesis when light and temperature are non-limiting.

Irrigation scheduling uses evapotranspiration (ET) replacement: apply water equal to ETc (reference ET × crop coefficient) minus effective rainfall to avoid both waterlogging and deficit stress.

Post-harvest chain: rapid cooling, controlled atmosphere (low O2, elevated CO2) and cold storage slow respiration and ethylene-driven ripening, extending shelf life.

Quarantine and biosecurity: border controls, pest-free certification and traceability protect export market access; a single incursion can close a market for an entire product line.

Nutrient budgets compare inputs (fertiliser, feed, fixation) with outputs (product, leaching, volatilisation); a positive nitrogen surplus indicates risk of nitrate leaching and greenhouse gas loss.

Leaching risk rises with high rainfall, sandy soils and low CEC; riparian planting and reduced nitrogen application rates mitigate nutrient runoff into waterways.

Greenhouse gases from agriculture: methane (enteric fermentation, ~21–28× CO2 warming potential) and nitrous oxide (N fertiliser, ~265–298× CO2); mitigation includes nitrification inhibitors and stand-off pads.

Eutrophication sequence: nutrient enrichment → algal bloom → decomposition consumes oxygen → hypoxia → fish kills and loss of biodiversity.

Gross margin = gross revenue − variable costs; it excludes fixed costs and is used to compare enterprises per hectare or per stock unit.

Break-even yield or price = fixed costs ÷ (price − variable cost per unit); sensitivity analysis tests how profit changes with price, yield or cost shifts.

Sustainable practice indicators: soil organic matter, water use efficiency (kg product per m³), nutrient use efficiency, biodiversity and greenhouse gas emissions per unit of product.

Market access depends on meeting importing-country standards (residue limits, food safety, animal welfare); non-compliance results in rejection, recalls and loss of certification.

Water quality monitoring measures turbidity, nitrate-nitrogen, E. coli and dissolved oxygen; these link farm practices to measurable environmental outcomes.