CPAg cheat sheet

Growing Degree Days (GDD) = Σ[(Tmax + Tmin)/2 − Tbase]; use daily max capped at the crop's upper threshold and daily min floored at Tbase.

Crop water use: ETc = ET0 × Kc, where ET0 is reference evapotranspiration and Kc is the crop coefficient that changes by growth stage (initial, development, mid-season, late season).

Soil pH targets: most agronomic crops 6.0–7.0; alfalfa and sugar beet tolerate 6.5–7.5; blueberry and potato prefer acidic 4.5–5.5; lime raises pH, elemental sulfur lowers it.

Cation exchange capacity (CEC) in cmol(+)/kg approximates a soil's nutrient-holding capacity; higher CEC soils buffer pH and hold more K, Ca, Mg.

Liebig's Law of the Minimum: yield is limited by the most deficient growth factor, not by total resource supply.

Cool-season (C3) crops: wheat, barley, oat, canola; warm-season crops: corn, soybean, sorghum, cotton - match planting date to soil temperature thresholds (corn ≈ 10 °C, soybean ≈ 13 °C).

Legume N fixation requires rhizobia matched to the host species and pH near neutral; inoculant viability declines with heat, sunlight, and prolonged storage.

Crop rotation breaks pest and disease cycles and diversifies weed-control options; continuous monoculture increases pathogen inoculum and herbicide-resistant weed selection.

Soil texture triangle: sand 2.0–0.05 mm, silt 0.05–0.002 mm, clay <0.002 mm; texture class is read from the three percentage fractions.

Bulk density = oven-dry soil mass / total volume; root restriction commonly above 1.6 g/cm³ for clay-textured soils, higher for sands.

Available water capacity = field capacity − permanent wilting point; sand holds ~0.05–0.10, silt loam ~0.20, clay ~0.15–0.20 (volumetric fractions).

Nitrogen cycle steps: mineralization/ammonification, nitrification (NH4+ → NO2− → NO3−), immobilization, denitrification (anaerobic), volatilization (surface urea/ammonia), leaching of NO3−.

Phosphorus is most available at pH 6.0–7.0; it is fixed by Al/Fe at low pH and by Ca at high pH - band placement reduces fixation.

Potassium availability depends on exchangeable K and non-exchangeable (fixed) K in 2:1 clay minerals; soil test K correlates with crop response more weakly than P.

Fertilizer analysis N-P2O5-K2O: to convert P2O5 to elemental P multiply by 0.44, K2O to K by 0.83.

Soil organic matter holds roughly 58% carbon by mass; a 1% SOM increase in the top 15 cm represents large C and N reserves and improves aggregation and water retention.

Integrated Pest Management hierarchy: prevention, monitoring/economic threshold, biological control, mechanical/cultural control, chemical control as last resort.

Economic threshold = pest density at which the cost of control equals the value of yield loss avoided; action threshold is the operational trigger.

Herbicide mode-of-action groups (HRAC): Group 1 ACCase inhibitors, Group 2 ALS inhibitors, Group 4 synthetic auxins, Group 5 photosystem II, Group 9 EPSP synthase, Group 10 glutamine synthetase, Group 15 very-long-chain fatty acid inhibitors.

Resistance management: rotate herbicide sites of action, use multiple effective modes on the same target, and integrate non-chemical tactics; repeated Group 1 or Group 2 use selects resistance quickly.

Disease triangle requires susceptible host, virulent pathogen, and favorable environment; the disease pyramid adds human activity and vectors.

Fungicide FRAC groups: 3 DMI (sterol biosynthesis), 7 SDHI, 11 QoI (strobilurins, target-site G143A mutation risk), M multi-site protectants such as mancozeb and chlorothalonil.

Insecticide IRAC groups: 1A carbamates, 1B organophosphates, 4A neonicotinoids, 5 spinosyns, 28 diamides (ryanodine receptor), 22A voltage-gated sodium channel blockers.

Degree-day models predict insect development; each species has a developmental threshold and a biofix date that starts accumulation.

Seeding rate = (target plant population × seed weight) / (germination fraction × purity × field emergence); calibrate against thousand-kernel weight.

Plant population effect: corn yield is fairly stable across a range around the optimum; soybean is more plastic and compensates through branching and pod set.

Harvest maturity guides: corn black layer at ~30–35% grain moisture, soybean at 13–14% for storage, wheat at 13–14% safe storage moisture.

Storage: grain at 14% moisture or below keeps mold risk low at moderate temperature; each 1% moisture above safe level raises spoilage risk substantially.

Tillage systems: conventional (full inversion), reduced (fewer passes), no-till (slot planting, residue retained); no-till increases surface residue, organic matter near the surface, and stratification of P and K.

Cover crops reduce erosion and nitrate leaching; grass covers (cereal rye) scavenge N, legumes fix N, brassicas (radish) alleviate compaction.

Irrigation scheduling by soil moisture deficit: replace ETc minus effective rainfall; deficit irrigation maximizes water productivity where supply is limited.

Double cropping and relay intercropping increase land-use intensity but require a growing season long enough for both crops to reach maturity.

Fertilizer product needed = nutrient required / nutrient fraction in product; e.g., 60 kg N/ha from urea (46-0-0) = 60 / 0.46 ≈ 130 kg product/ha.

Nutrient removal = yield × nutrient concentration in harvested portion; balance against applied nutrients to track soil nutrient depletion.

Bulk blending check: sum of nutrient contributions must equal the target grade; verify with a weighted average of the component analyses.

Yield conversion: bu/ac × crop factor = kg/ha - corn ×62.77, wheat/soybean ×67.25, and metric tonnes = bu/ac × 0.0628 (corn).

Spray calibration: output (L/ha) = (nozzle flow L/min × 600) / (speed km/h × nozzle spacing m); verify with a measured catch test.

Tank mix concentration = (product rate per ha × area treated) / spray volume; account for product density when converting L to kg.

Soil test interpretation uses critical levels and buildup/maintenance: below critical, build; at or above, maintain removal-based rates.

Diagnostic sampling: plant tissue analysis compares to sufficiency ranges by growth stage; soil sampling depth consistency (0–15 or 0–20 cm) is essential for trend comparison.