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 two key conditions:

1. Growth Requirement: The plant cannot complete its life cycle without the nutrient, indicating its fundamental role in growth and development.

2. Specificity: The nutrient must have a specific physiological function that cannot be fulfilled by any other element, highlighting its unique contribution to plant health.

Essential nutrients are categorized into macronutrients and micronutrients, each absorbed in specific 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 as potassium ions (K⁺), crucial for various physiological processes.

- Micronutrients:

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

- Zinc (Zn): Absorbed as zinc ions (Zn²⁺).

- Copper (Cu): Taken up as cupric ions (Cu²⁺), essential for enzyme function.

These criteria and forms are vital for understanding plant nutrition and growth.