Describe the criteria of essentiality of nutrients. Classify the essential nutrients for plants. Discuss the role of phosphorus and calcium in crops with special reference to pulses.

Criteria of Essentiality

A nutrient is considered essential if it meets the following criteria:

• Direct involvement in plant metabolism: It must be directly involved in a metabolic process or be a constituent of a vital plant structure.
• Deficiency causes specific deficiency symptom: The absence or deficiency of the nutrient must induce a specific, characteristic deficiency symptom.
• Deficiency can be corrected by supplying the nutrient: The deficiency symptom can be completely corrected by supplying the nutrient in a suitable concentration.
• The nutrient is not replaceable: No other nutrient can perform the same function.

Classification of Essential Nutrients

Essential nutrients are broadly classified into macronutrients and micronutrients based on their required quantities.

Macronutrients

• Primary Macronutrients: Nitrogen (N), Phosphorus (P), and Potassium (K) – required in large quantities.
• Secondary Macronutrients: Calcium (Ca), Magnesium (Mg), and Sulfur (S) – needed in lesser amounts than primary macronutrients.

Micronutrients

• Iron (Fe), Manganese (Mn), Zinc (Zn), Copper (Cu), Boron (B), Molybdenum (Mo), Chlorine (Cl), and Nickel (Ni) – required in trace amounts.

Role of Phosphorus and Calcium in Crops, with Special Reference to Pulses

Phosphorus (P) and Calcium (Ca) play crucial roles in plant growth and development, with significant implications for pulse crops.

Phosphorus (P)

Phosphorus is a vital component of ATP, nucleic acids (DNA and RNA), and phospholipids, involved in energy transfer and cell membrane structure. It also promotes root development, flowering, and seed formation.

• Role in Pulses: Phosphorus deficiency in pulses, like lentils and chickpeas, leads to stunted growth, delayed maturity, and reduced seed yield. Adequate P promotes nodulation in legumes, enhancing nitrogen fixation.
• Deficiency Symptoms: Dark green or purplish coloration of leaves, especially in younger leaves.
• Example: Studies in India have shown that phosphorus application significantly increases the yield of groundnut by 20-30% (ICAR, knowledge cutoff).

Calcium (Ca)

Calcium is a structural component of cell walls, essential for cell division and elongation. It also acts as a second messenger in various signaling pathways.

• Role in Pulses: Calcium influences pod development and seed quality in pulses. It also plays a role in maintaining cell membrane integrity.
• Deficiency Symptoms: Tip burn in leaves, distorted new growth, and poor pod development.
• Example: Calcium deficiency in soybean can lead to blossom drop and reduced pod set.

Nutrient	Function	Impact on Pulses	Deficiency Symptoms
Phosphorus	Energy transfer, root development, flowering	Promotes nodulation, increases yield	Dark green/purple leaves
Calcium	Cell wall structure, cell division	Influences pod development, seed quality	Tip burn, distorted growth

The interaction between phosphorus and calcium is also important. For example, high phosphorus levels can sometimes induce calcium deficiency.

In conclusion, essential nutrients are categorized based on their required quantities and play distinct roles in plant metabolism. Phosphorus and calcium are critical macronutrients, particularly for pulse crops, influencing various aspects from root development and nodulation to pod formation and seed quality. Addressing nutrient deficiencies through balanced fertilization is crucial for enhancing agricultural productivity and ensuring food security. Further research focusing on nutrient interactions and efficient utilization strategies is essential for sustainable agriculture.