Soil Fertility and Plant Nutrition Fundamentals
Core concepts
Candidates must separate soil fertility (the capacity of soil to supply nutrients in available forms) from nutrient availability (the fraction actually plant-accessible at a given pH, moisture, and temperature). Know the 17 essential elements, the macronutrient/micronutrient split, and the distinction between mobile nutrients (N, K, Mg - deficiency shows on older leaves) and immobile nutrients (Ca, Fe, B - deficiency shows on new growth). Understand CEC and base saturation, how soil texture and organic matter drive CEC, and how pH governs availability (the classic availability chart: most nutrients peak near pH 6.0–6.8, with Fe/Mn/Zn declining as pH rises and P fixed by both Ca at high pH and Al/Fe at low pH).
Traps
Questions love to conflate total with available nutrient content, or to ask which nutrient is "most limiting" using Liebig's law of the minimum. Watch for the misconception that adding lime always improves availability - over-liming induces micronutrient deficiency. Also distinguish luxury consumption from toxicity and know that nitrate-N and ammonium-N behave very differently in soil.
Rule of thumb and worked example
Nitrogen is almost always the first limiting nutrient in non-legume systems. Worked example: a soil test reports 12 ppm P (Bray-1). To convert ppm to lb/acre in the top 6 inches, multiply by 2: 12 ppm × 2 = 24 lb/acre available P₂O₅-equivalent P. Compare that to a 40 bu/acre corn crop removing roughly 0.35 lb P₂O₅ per bushel (~14 lb) to judge sufficiency.