Praxis Agriculture cheat sheet

Soil texture triangle: percent sand, silt, and clay determine class; loam is roughly 40% sand, 40% silt, 20% clay and balances water retention with drainage.

Cation exchange capacity (CEC) rises with clay content and organic matter; high-CEC soils hold more Ca2+, Mg2+, K+, and NH4+ and resist pH change better than sandy soils.

Soil pH near 6.0-6.8 maximizes availability of most macronutrients; phosphorus availability drops sharply below pH 5.5 (Al and Fe fixation) and above pH 7.5 (Ca fixation).

Liebig's law of the minimum: yield is limited by the scarcest essential factor, so adding a non-limiting nutrient does not raise yield.

Photosynthesis: 6CO2 + 6H2O + light yields C6H12O6 + 6O2; C3, C4, and CAM plants differ in carbon fixation pathway, with C4 crops (corn, sorghum) more water-use efficient in heat.

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

Nitrogen fixation by Rhizobium in legume root nodules converts atmospheric N2 to ammonia; rotating legumes reduces synthetic N requirements for the following crop.

Growing degree days (GDD) = sum of [(Tmax + Tmin)/2 minus base temperature] for each day, using a crop-specific base (often 50 degrees F for corn) to predict maturity.

Ruminant digestion: rumen microbes ferment cellulose into volatile fatty acids; rumen pH near 6.0-6.8 supports fiber digestion, and acidosis occurs when pH falls below about 5.5.

Monogastric animals (swine, poultry) cannot digest large amounts of fiber efficiently and require diets higher in starch and protein concentrates.

Crude protein = percent N x 6.25; ruminants can use non-protein nitrogen such as urea only when adequate fermentable energy is available.

Total digestible nutrients (TDN) and net energy systems estimate dietary energy; maintenance requirements scale with metabolic body weight (BW^0.75).

Beef cattle estrous cycle averages about 21 days; gestation averages about 283 days, so breeding-to-calving planning uses roughly 9 months.

Swine gestation averages about 114 days (3 months, 3 weeks, 3 days), and poultry incubation is about 21 days for chickens and 28 days for turkeys.

Colostrum must be consumed within the first hours of life because newborn livestock absorb maternal antibodies through the gut only briefly before gut closure.

Animal welfare and biosecurity: all-in/all-out housing, ventilation, and foot traffic control reduce disease transmission; record-keeping supports traceability.

Profit = total revenue minus total cost; a farm should continue producing in the short run if price exceeds average variable cost, even when below average total cost.

Break-even yield = total fixed and variable costs divided by price per unit; break-even price = total cost divided by expected yield.

Marginal analysis: apply an input until marginal value product equals marginal input cost (MVP = MIC) to maximize profit.

Partial budget format: added returns plus reduced costs versus added costs plus reduced returns; the net difference determines whether a change is profitable.

Depreciation methods include straight-line [(cost minus salvage)/useful life] and declining balance; depreciation is a non-cash expense but affects taxable income.

Enterprise budgets separate fixed costs (land, machinery ownership) from variable costs (seed, fertilizer, fuel) to compare enterprises fairly.

Risk management tools include crop insurance, forward contracts, futures and options hedging, and diversification across enterprises.

Farm financial statements: balance sheet shows assets, liabilities, and net worth at a point in time; income statement and cash flow statement cover a period.

Integrated pest management (IPM) sequence: prevention, monitoring and scouting, economic threshold, then biological, cultural, mechanical, and chemical controls in that order of preference.

Economic threshold is the pest density at which control measures should be applied to prevent pest populations from reaching the economic injury level.

Economic injury level is the lowest pest population that causes crop loss equal to the cost of control; it is always at or above the economic threshold.

Thresholds, not calendar sprays, drive insecticide timing; scouting with sweep nets, sticky traps, or pheromone traps provides the population data.

Herbicide resistance management: rotate sites of action (mode-of-action groups), use multiple effective modes, and include non-chemical tactics to slow selection.

Pesticide label is a legal document; the restricted-entry interval (REI) and pre-harvest interval (PHI) must be observed, and label directions cannot be exceeded.

Biological control agents include predators (lady beetles, lacewings), parasitoids (Trichogramma wasps), and pathogens (Bacillus thuringiensis for caterpillars).

Disease triangle: a disease requires a susceptible host, a virulent pathogen, and favorable environment; breaking any leg prevents epidemic development.

Tractor stability: the center of gravity rises and the wheelbase narrows stability; front-end loads and raised implements increase rollover risk, and ROPS with seat belts prevent fatalities.

Power take-off (PTO) operates at 540 or 1000 rpm; shields must remain in place, and loose clothing must never be worn near a rotating PTO shaft.

Electrical basics: volts = amps x ohms (Ohm's law); power in watts = volts x amps; grounding and GFCI protection reduce shock hazard in wet farm environments.

Hydraulic systems transmit force through incompressible fluid; pressure equals force divided by area, and larger cylinder area increases lifting force at the same pressure.

Precision agriculture uses GPS/GNSS guidance, variable-rate application, and yield monitors; RTK correction provides centimeter-level accuracy for auto-steer.

Grain storage safety: engulfment risk in flowing grain is severe; never enter a bin without lockout/tagout, harness, observer, and ventilation, and beware of toxic bin gases.

Anhydrous ammonia and other hazardous farm chemicals require appropriate PPE, and confined-space entry requires atmospheric testing before entry.

Internal combustion engine: four strokes are intake, compression, power, and exhaust; diesel relies on compression ignition while gasoline uses spark ignition.

Soil erosion by water follows the universal soil loss equation framework: rainfall erosivity, soil erodibility, slope length and steepness, cover, and support practice factors.

Conservation practices such as no-till, cover crops, contour strips, and grassed waterways reduce erosion and nutrient runoff.

The nitrogen cycle includes fixation, ammonification, nitrification, denitrification, and leaching; excess nitrate can contaminate groundwater and drive hypoxia.

The phosphorus cycle lacks a gaseous phase; P moves mainly by erosion and runoff, so buffer strips and soil-test-based application protect water quality.

4R nutrient stewardship: right source, right rate, right time, right place, to maximize nutrient use efficiency and minimize environmental loss.

Water quality: biological oxygen demand and nutrient loading cause eutrophication; riparian buffers filter sediment and nutrients before they reach streams.

Sustainability balances environmental, economic, and social dimensions; soil health indicators include organic matter, aggregate stability, and biological activity.

Carbon sequestration in soils increases with reduced tillage, perennial vegetation, and organic amendments; soil organic matter is roughly 58% carbon by weight.