Good to Know — Rules of Thumb (Agricultural)
Field-tested approximations and sanity checks for agricultural engineers. Estimates for early-stage thinking and reasonableness checks — design follows ASABE standards, the equipment manufacturer's data, and the label (which is law for pesticides).
Soil-plant-water
- Available water: AW = field capacity − wilting point. Typical: sand ~0.7 in/ft, loam ~1.5–2.0, clay ~2.0–2.5. Coarse soils hold less but release it readily; clays hold more but grip it tighter.
- MAD: manage to ~50% depletion for most crops (turf 50%, sensitive vegetables 30–40%, drought-tolerant grains 60%). Irrigate to replace RAW = AW × root depth × MAD.
- Infiltration: sand ~1 in/h, loam ~0.5, clay ~0.2 — the ceiling on application rate. Exceed it and you're designing a runoff system.
- ET: peak-season ETo 0.25–0.35 in/day in temperate zones, 0.35–0.50+ in arid zones. Crop ET = ETo × Kc (Kc ~0.3 early, ~1.0–1.2 mid-season, ~0.5 late for many row crops).
- Water productivity: ~1 bushel of corn per ~4,000 gallons of ET — the scale argument for irrigation efficiency.
Irrigation hydraulics
- Hazen-Williams C: PVC 150, aluminum 140, new steel 120, old steel 100. Friction loss scales with Q^1.852 — doubling flow nearly quadruples loss (3.6×).
- Velocity: ≤5 ft/s in mains (water hammer and friction); drip laterals designed for flushing velocity ≥1 ft/s periodically.
- Elevation: 0.433 psi per foot — a 50-ft rise eats 21.7 psi, often the largest single term in the pressure budget.
- PR formula: PR(in/h) = 96.3 × GPM / (spacing_ft × spacing_ft). If you remember nothing else, remember 96.3.
- DU targets: ≥0.75 for turf rotors, ≥0.65 for sprays, EU ≥0.90 for drip. Below target, the schedule over-waters to compensate — efficiency dies in the uniformity, not the nozzle.
- Wire: 24 VAC valve circuits — keep voltage drop under ~10%; long runs need 14 AWG or heavier. A valve that "chatters" is usually a voltage-drop problem, not a bad solenoid.
Drainage
- Drainage coefficient: humid-region subsurface drainage ~0.5–1.0 in/day (the rate the system must remove). Spacing follows Hooghoudt/Ernst — but the coefficient sets the capacity.
- Tile depth: 3–4 ft typical in the US Midwest; grade 0.1–0.3% minimum to keep silt moving. Flatter than 0.1% and you're building a sediment trap.
- Surface drainage: 0.1–0.5% field grades; water should leave the field within 24–48 h of a storm to protect the crop.
Machinery and power
- Draft: tillage draft ~5–10 hp per foot of implement width (moldboard plow, medium soil) — size the tractor from the implement, not the brochure.
- PTO: 540 rpm standard, 1000 rpm for high-power; drawbar power ≈ 85% of PTO power roughly. A 150-hp tractor delivers ~125 drawbar hp on concrete, less on tilled soil (slip).
- Slip: 8–15% is the efficient range for traction; above 15% you're burning fuel to polish the soil.
- Fuel: diesel ~0.06 gal/hp-h at full load — the quick check on machinery cost estimates.
- ISO 4254 / OECD codes: machinery safety and ROPS testing are type-tested, not field-verified — specify certified structures.
Pesticide application
- GPA formula: GPA = (GPM × 5940)/(mph × spacing_in). Calibrate at field speed and pressure — the label rate is law.
- 1/128-acre method: 340.3-ft course for 20-in. spacing; ounces caught = GPA. The fastest field calibration ever devised.
- Droplet spectra (ASABE S572): fine drifts, coarse fails coverage — the label's "medium or coarser" is a legal requirement, not advice. Higher pressure = finer droplets = more drift.
- Inversions: calm, clear mornings trap spray — the #1 drift-weather trap. If there's fog in the low spots, don't spray.
- Buffers: label buffers are minimums; drift damage inside a complied-with buffer is still your civil liability.
Structures and storage
- Grain: dry to 13–15% moisture for storage (corn 15%, soybeans 13%, wheat 13.5%); aeration ~0.1 cfm/bu for maintenance, 1+ cfm/bu for drying. A 1% moisture mistake on 100,000 bu is a real-money spoilage event.
- Bin loads: Janssen's equation — wall loads saturate with depth (unlike fluids); the peak floor load is the design driver. Grain ~48 lb/ft³ (wheat 60, corn 56).
- Ventilation (animal housing): winter minimum ~10–20 cfm per finishing pig for moisture control; summer tunnel ventilation 300+ fpm airspeed for cooling. Moisture, not temperature, usually sizes winter ventilation.
Nutrient and environmental
- N rate: ~1.0–1.2 lb N per bushel of corn yield goal (minus soil credits) — the starting point, refined by soil tests and the 4R framework (right source, rate, time, place).
- Manure: test it — book values for manure nutrients are wrong often enough to cost real money; 1,000 gal of swine slurry ≈ 40–50 lb N, 25–35 lb P₂O₅, highly variable.
- Setbacks: wells, streams, and sinkholes get label- and regulation-driven setbacks — check the state, not memory.
- CAFO: NPDES thresholds (1,000+ animal units for large CAFOs, with species-specific head counts) — the regulatory cliff.
Economics
- Machinery cost: ownership (depreciation, interest, insurance, housing) typically equals or exceeds operating cost (fuel, repairs, labor) — the argument for custom hiring at small acreage.
- Irrigation payback: center pivots pencil out where yield gain × price > ~$150–250/acre-year all-in cost — highly water-price dependent.
Niche credentials worth knowing
- CAIS (Certified Agricultural Irrigation Specialist) — the IA's irrigation-management credential, beside the CID design credential.
- CCA / 4R NMS (Nutrient Management Specialist) — the agronomy-side nutrient credential; pairs with the engineer's system design.
- TSP (Technical Service Provider) — USDA's certification for conservation planners; the gateway to federal cost-share work.
Cross-discipline sanity checks
- Civil: farmstead drainage and earthworks — the rational method (Q = CiA) for small watersheds; culvert sizing per state DOT charts.
- Environmental: NPDES, SPCC (fuel storage 1,320 gal), and endangered-species bulletins (Bulletins Live! Two) increasingly gate pesticide use.
- Electrical: grain-dryer and irrigation-well loads — demand charges often dominate the electricity bill; power-factor correction pays.
- Aviation: Part 137 + Part 107 + 44807 for spray drones — three regulatory layers, all mandatory.
Original ETS reference writing, verified against public sources on 2026-09-30. Thin sourcing is flagged inline. Not legal advice — confirm current editions with the issuing body.