Fertilizer fundamentals: Pick the single incorrect statement about common fertilizers and plant nutrition.

Introduction / Context:
Fertilizers supply essential nutrients—nitrogen (N), phosphorus (P), and potassium (K)—each with distinct physiological roles. This question checks common agronomic knowledge and clarifies misconceptions about nutrient functions and product terminology.

Given Data / Assumptions:

• Straight vs. complex fertilizers follow standard definitions.
• General horticultural guidance on nutrient roles is considered.
• Weed-suppressing behavior of calcium cyanamide is recognized in certain contexts.

Concept / Approach:
Phosphorus is primarily associated with root development, energy transfer (ATP), flowering, and early vigor. Stem strength and lodging resistance are far more strongly linked to potassium (K), which enhances turgor, cell wall strength, and overall plant robustness. Hence, stating that phosphorus is the primary nutrient for stem strengthening and branching is inaccurate.

Step-by-Step Solution:
Evaluate each statement against agronomic roles and definitions.(a) True: water solubility of nitrogenous fertilizers varies (e.g., urea vs. calcium cyanamide).(b) True: 'straight' means a single primary nutrient (e.g., urea for N).(c) True: calcium cyanamide can suppress weeds and act as a soil conditioner.(d) Incorrect: P is not primarily the 'stem-strength' nutrient; K is.

Verification / Alternative check:
Crop nutrition guides consistently attribute stem strength and lodging resistance to adequate potassium, while phosphorus is emphasized for roots and reproductive development.

Why Other Options Are Wrong:
They are factually correct within standard agronomy references.

Common Pitfalls:

• Overgeneralizing nutrient effects; multiple nutrients interact.
• Assuming all N fertilizers dissolve equally quickly in water.

Final Answer:
Phosphorus nutrient primarily strengthens stems and increases branching.