Plant and Soil Science: Physiology, Soil Fertility, and Integrated Pest Management
This domain covers plant anatomy and physiology, sexual and asexual propagation, soil texture and chemistry, nutrient deficiency diagnosis, fertilizer calculations, and integrated pest management (IPM). Expect calculation items on fertilizer grades and diagnosis items on symptom placement.
Plant physiology
- Photosynthesis: 6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂. Light reactions occur in thylakoid membranes; the Calvin cycle fixes carbon in the stroma.
- Xylem carries water and dissolved minerals upward in dead cells, driven by cohesion–tension as water evaporates through stomata. Phloem translocates sugars in living sieve elements.
- Anatomy terms: taproot versus fibrous root; node and internode; complete flower with sepals, petals, stamens, and pistil.
Soil and fertility
- The texture triangle runs sand–silt–clay; loam balances all three. Clay and organic matter supply cation exchange capacity (CEC) and water-holding capacity; sand drains quickly.
- Most crops perform best near pH 6.0–6.8. Lime raises pH; elemental sulfur lowers it.
- Macronutrients: N, P, K, Ca, Mg, S. Micronutrients: Fe, Mn, Zn, Cu, B, Mo, Cl. Nitrogen is mobile, so deficiency yellows older leaves first; iron is immobile, so new leaves yellow.
- Fertilizer grade N-P-K is percent N, P₂O₅, and K₂O by weight, not elemental P and K.
Worked example
A 50-lb bag of 10-20-10 holds 50 × 0.10 = 5 lb N, 50 × 0.20 = 10 lb P₂O₅, and 50 × 0.10 = 5 lb K₂O. To apply 40 lb N per acre: 40 ÷ 0.10 = 400 lb of product per acre. That same 400 lb also delivers 80 lb P₂O₅, so if the soil test calls for only 40 lb P₂O₅ you must blend a different grade rather than over-apply phosphorus.
IPM and propagation
- IPM order: prevention, scouting and monitoring, economic threshold, then biological, cultural, mechanical, and chemical controls, with resistant varieties where available.
- Asexual propagation (cuttings, grafting, layering, division, tissue culture) clones the parent; seed propagation recombines genes.
Traps that cost marks
- Treating percent P₂O₅ as elemental P (multiply by 0.44) or K₂O as K (multiply by 0.83).
- Saying lime lowers soil pH.
- Confusing transpiration (water loss) with translocation (sugar movement).
- Assigning nitrogen deficiency to new leaves, or iron deficiency to old leaves.
- Mixing per-acre and per-1,000-ft² fertilizer rates.