NCEA L1 Ag & Hort cheat sheet

Production is limited by the factor in shortest supply (Liebig's law of the minimum): adding a non-limiting input such as nitrogen when water is limiting gives little or no yield gain.

Soil texture refers to the proportions of sand, silt and clay; soil structure refers to how those particles bind into aggregates. Texture is largely fixed, structure can be improved with organic matter.

Clay particles carry negative surface charge and hold cations such as Ca2+, K+ and Mg2+ (cation exchange capacity); sandy soils have low CEC and leach nutrients readily.

Soil pH controls nutrient availability: most macronutrients are most available between pH 5.8 and 6.8; phosphorus locks up in both strongly acid and strongly alkaline soils.

The three macronutrients supplied in largest amounts by fertiliser are nitrogen (leaf and stem growth), phosphorus (root development and energy transfer) and potassium (water regulation, fruit quality, disease resistance).

Photosynthesis equation: 6CO2 + 6H2O + light energy -> C6H12O6 + 6O2; light, CO2 concentration, water and temperature are the main limiting factors in a crop canopy.

Respiration releases energy from glucose and occurs day and night; net growth equals photosynthesis minus respiration, so high night temperatures reduce dry matter accumulation.

Plant transpiration pulls water and dissolved minerals up the xylem; stomata close under water stress, which also cuts CO2 uptake and therefore growth.

Integrated pest management ranks controls as biological, cultural, physical and chemical, using chemicals only when monitoring shows the economic threshold has been exceeded.

Ruminant digestion: microbes in the rumen ferment cellulose into volatile fatty acids; dietary protein is later digested in the abomasum and small intestine, so rumen function must be protected when changing feed.

Pasture management rule: graze before pasture reaches the three-to-four-leaf stage and leave a residual of about 1,500 kg DM/ha so regrowth reserves are not exhausted.

Animal production efficiency is measured as output per unit input, e.g. kg milk solids per hectare, kg lamb weaned per ewe, or feed conversion ratio (kg feed per kg gain).

Environmental management: nutrient budgets track inputs and outputs of N and P; leaching and runoff of nitrogen and phosphorus into waterways cause eutrophication, and riparian planting buffers this.

Greenhouse gas sources on farm: enteric methane from ruminants, nitrous oxide from urine patches and nitrogen fertiliser, and carbon dioxide from fuel and fertiliser manufacture.

Water management: irrigation scheduling is based on evapotranspiration replacement and soil moisture deficit, not on a fixed calendar, to avoid both drought stress and waterlogging.

Market and quality drivers: grade standards, food safety, animal welfare and traceability determine price received, so production decisions are matched to the target market specification.

Economic decision rule: a management change is worthwhile when marginal revenue from extra output exceeds marginal cost of the extra input.