Agricultural Science Fundamentals: Soil, Crop Physiology and Plant Nutrition
This domain tests whether you can link soil properties, plant processes and nutrient supply into one working explanation. Questions are usually applied: read a soil test value, calculate a fertilizer dose, or explain why a symptom appeared in a field.
Soil fundamentals
- Texture (sand, silt, clay) governs water holding, aeration and workability; clay soils hold more water and nutrients but drain slowly.
- Soil pH controls nutrient availability; most field crops perform best between pH 6.0 and 7.5. Acid soils are corrected with agricultural lime, sodic/alkaline soils with gypsum.
- Cation exchange capacity (CEC) is the soil's capacity to hold exchangeable cations, expressed in cmol(p+)/kg; clay and organic matter raise CEC.
Crop physiology essentials
Photosynthesis converts light energy into carbohydrate; C3 crops (rice, wheat) photorespire more than C4 crops (maize, sugarcane), which is why C4 species produce more dry matter per unit of water used. Respiration, transpiration and translocation then decide how much of that carbohydrate reaches the grain.
Plant nutrition
Roughly 17 elements are essential: macronutrients (N, P, K, Ca, Mg, S) and micronutrients (Fe, Mn, Zn, Cu, B, Mo, Cl). Nitrogen drives vegetative growth, phosphorus root development, potassium quality and stress tolerance. Liebig's law of the minimum states that yield is limited by the scarcest nutrient, not by the most abundant one.
Worked example
The recommendation is 60 kg N/ha and the fertilizer is urea (46% N). Urea required = 60 / 0.46 = 130.4 kg/ha. Reasoning: divide the nutrient dose by the fraction of nutrient in the product; never multiply. For a 0.4 ha plot, 130.4 x 0.4 = 52 kg of urea.
Traps that cost marks
- Quoting product weight where the question asks for the nutrient dose, or the reverse.
- Treating pH as an arithmetic scale: pH 5 to pH 6 is a tenfold decrease in hydrogen ion concentration.
- Confusing CEC (a holding capacity) with base saturation (a percentage).
- Recommending lime for an already alkaline soil.