SACE Stage 2 Agricultural Systems cheat sheet

A system is a set of interacting components bounded by a boundary; inputs (solar energy, water, nutrients, labour, capital) cross the boundary, outputs (product, waste, heat) leave it.

Feedback loops: negative feedback dampens change and promotes stability (e.g., predator–prey regulation, soil moisture recharge); positive feedback amplifies change and can cause collapse (e.g., erosion reducing infiltration, which increases runoff and further erosion).

Trophic levels transfer roughly 10% of energy to the next level (ecological efficiency); this limits the number of trophic levels and explains why meat production requires more land per unit of energy than plant production.

Biogeochemical cycles: carbon (photosynthesis, respiration, decomposition, combustion), nitrogen (fixation, nitrification, denitrification, leaching), phosphorus (weathering, uptake, runoff; no gaseous phase), water (evaporation, transpiration, precipitation, runoff, infiltration).

Soil texture triangle classifies soil by proportions of sand (2.0–0.05 mm), silt (0.05–0.002 mm) and clay (<0.002 mm); loam soils balance water retention, aeration and nutrient supply.

Soil pH affects nutrient availability: most macronutrients are most available at pH 6.0–7.0; aluminium toxicity and phosphorus fixation increase at low pH; micronutrient deficiencies (e.g., iron, zinc) increase at high pH.

Plant growth requires macronutrients N, P, K, Ca, Mg, S and micronutrients Fe, Mn, Zn, Cu, B, Mo, Cl, Ni; Liebig's law of the minimum states growth is limited by the scarcest resource.

Integrated pest management (IPM) combines biological, cultural, physical and chemical controls, using economic thresholds to decide when intervention is justified; this reduces selection pressure for pesticide resistance.

Crop rotation breaks disease, pest and weed cycles, balances nutrient use, and improves soil structure; legume phases fix atmospheric N2 via rhizobia symbiosis, reducing fertiliser N requirements for the following cereal.

Germination requires adequate moisture, temperature within a species-specific range, and oxygen; sowing depth must balance moisture availability against seedling emergence capacity.

Dry matter production is driven by intercepted photosynthetically active radiation (PAR) and water use efficiency (WUE = biomass produced per unit water transpired); closing canopy early increases seasonal light interception.

Livestock nutrition: feed intake is limited by rumen capacity and feed quality; metabolisable energy (ME) and crude protein (CP) must meet maintenance plus production requirements (growth, lactation, pregnancy).

Stocking rate is animals per unit area; carrying capacity is the maximum stocking rate sustainable without resource degradation; overstocking leads to pasture decline, erosion and reduced animal performance.

Animal health management includes vaccination schedules, quarantine of new stock, parasite control (drench rotation to slow resistance), and monitoring for lameness, mastitis and metabolic disorders.

Harvest timing affects quality and yield: grain moisture at harvest determines drying costs and storage risk; fruit maturity indices include sugar content, firmness, acidity and days from flowering.

Record keeping underpins management decisions: paddock history, input records, yield maps and financial records support benchmarking, traceability and continuous improvement.

Sustainability rests on three pillars - economic viability, environmental integrity and social acceptability; a practice is sustainable only if all three are maintained over time.

Soil erosion by water occurs when rainfall intensity exceeds infiltration capacity; erosion risk increases with slope, low ground cover and poor aggregation; contour banks, stubble retention and cover crops reduce risk.

Salinity: dryland salinity develops when rising water tables bring dissolved salts into the root zone after native vegetation is cleared; management includes deep-rooted perennials and drainage.

Eutrophication occurs when nutrient runoff (especially N and P) causes algal blooms, oxygen depletion and fish kills; riparian buffers and nutrient budgeting reduce off-site losses.

Greenhouse gases from agriculture: methane (CH4) from enteric fermentation and anaerobic rice paddies; nitrous oxide (N2O) from nitrogen fertiliser and manure; carbon dioxide from fuel and land clearing.

Biodiversity supports ecosystem services such as pollination, pest control, nutrient cycling and soil formation; field margins, remnant vegetation and habitat connectivity protect these services.

Water use efficiency and allocation: irrigation scheduling based on evapotranspiration (ET) and soil moisture monitoring reduces deep drainage and runoff; water trading allocates scarce water to highest-value uses.

Life cycle assessment (LCA) quantifies environmental impacts per functional unit (e.g., kg CO2-e per kg product) across production, processing, transport and waste stages, preventing burden shifting between stages.

Gross margin = gross income (yield × price) − variable costs; it compares enterprise profitability per hectare or per head and guides enterprise mix decisions.

Fixed costs (machinery depreciation, insurance, permanent labour) do not vary with output in the short run; variable costs (seed, fertiliser, fuel, casual labour) do.

Break-even yield = total costs ÷ (price per unit × area); break-even price = total costs ÷ total production; both are sensitive to yield and price variability.

Precision agriculture uses GPS guidance, variable-rate technology and yield mapping to match inputs to within-paddock variability, reducing over-application and improving efficiency.

Remote sensing (satellite NDVI, drones) and soil sensors provide data for monitoring crop vigour, moisture and nutrient status; data quality depends on calibration and timing.

Supply chain and traceability systems (e.g., National Livestock Identification System, grain quality assurance) underpin market access, food safety and provenance claims.

Risk management tools include enterprise diversification, forward contracts, futures hedging, crop insurance and maintaining financial reserves; each trades off certainty against potential returns.

Farm business planning integrates production, financial and environmental plans; key performance indicators (yield, cost of production, return on assets) allow benchmarking against industry standards.