The correct option for the question regarding the treatment of poor soil with lime is B. Calcium. Detailed Explanation:

1. Understanding Lime:
2. Lime, commonly referred to as agricultural lime, is primarily composed of calcium carbonate (CaCO₃). When lime is applied to soil, it undergoes a chemical reaction that can help to neutralize soil acidity.
3. Role of Calcium in Soil:
4. Calcium is an essential nutrient for plants. It plays a crucial role in various physiological processes, including cell wall structure, enzyme activity, and nutrient uptake. By adding lime to the soil, you are effectively increasing the availability of calcium, which can improve plant growth and health.
5. Soil pH and Nutrient Availability:
6. Poor soils are often acidic, which can hinder the availability of essential nutrients to plants. By raising the pH of the soil through the addition of lime, you not only provide calcium but also improve the overall nutrient availability. This is because many nutrients become more soluble and accessible to plants in a neutral to slightly alkaline pH range.
7. Chemical Reaction:
8. When lime is added to acidic soil, it reacts with hydrogen ions (H⁺) in the soil, which reduces acidity. The reaction can be simplified as follows: [ \text{CaCO}_3 + 2\text{H}^+ \rightarrow \text{Ca}^{2+} + \text{H}_2\text{CO}_3 ]
9. The calcium ions (Ca²⁺) released into the soil are what benefit the plants.

Why Other Options Are Incorrect:

• A. Sulphur:
• While sulphur is an important nutrient for plants, it is not supplied by lime. In fact, sulphur can sometimes be used to lower soil pH, which is the opposite effect of lime. Therefore, this option does not apply to the treatment of poor soil with lime.
• C. Phosphorus:
• Phosphorus is another essential nutrient for plants, particularly for root development and energy transfer. However, lime does not supply phosphorus. Instead, phosphorus availability can be affected by soil pH; in acidic soils, phosphorus can become less available. Thus, while lime can help improve phosphorus availability indirectly, it does not provide phosphorus directly.
• D. Sodium:
• Sodium is not a nutrient that is typically beneficial for most plants and is often associated with soil salinity issues. Lime does not supply sodium, and in fact, high sodium levels can lead to poor soil structure and reduced plant growth. Therefore, this option is also incorrect.

Summary of Key Points:

• Lime primarily supplies calcium to the soil, which is essential for plant health.
• It helps to neutralize soil acidity, improving nutrient availability.
• Other options (sulphur, phosphorus, sodium) do not relate to the benefits of lime treatment.
• Understanding the role of lime in soil management is crucial for effective agricultural practices.

This thorough understanding of lime's role in soil treatment will help you grasp the importance of calcium in agriculture and soil management.