What is the Arnon and Stout (1939) criteria for plant nutrient essentiality? Give account of forms of each essential plant nutrient element absorbed by plants.?

The Arnon and Stout (1939) criteria for plant nutrient essentiality establish that a nutrient is considered essential if it meets three specific conditions:

1. Growth Requirement: The plant cannot complete its life cycle without the nutrient.

2. Specificity: The nutrient has a specific physiological function that cannot be substituted by another element.

3. Direct Role: The nutrient is directly involved in plant metabolism and biochemical processes.

Essential nutrients are categorized into macronutrients and micronutrients, each absorbed in distinct forms:

- Macronutrients:

- Nitrogen (N): Absorbed primarily as nitrate (NO₃⁻) or ammonium (NH₄⁺).

- Phosphorus (P): Taken up as phosphate ions (H₂PO₄⁻ or HPO₄²⁻).

- Potassium (K): Absorbed in the form of K⁺ ions.

- Micronutrients:

- Iron (Fe): Taken up as ferrous (Fe²⁺) or ferric (Fe³⁺) ions.

- Zinc (Zn): Absorbed as Zn²⁺ ions.

These criteria and forms are crucial for understanding plant nutrition and growth (Arnon & Stout, 1939).