Correct Option: B. It can supply usable nutrients to plants Detailed Explanation: Soil fertility refers to the ability of soil to provide essential nutrients to plants in a form that they can absorb and utilize for growth. Here’s a step-by-step breakdown of why option B is the correct answer:

1. Definition of Fertility: Fertile soil is characterized by its capacity to supply adequate amounts of essential nutrients, such as nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur, among others. These nutrients are crucial for various physiological processes in plants, including photosynthesis, growth, and reproduction.
2. Nutrient Availability: For soil to be considered fertile, it must not only contain these nutrients but also have them in a form that is accessible to plants. This means that the nutrients should be soluble in water and present in the right proportions. Soil pH, texture, and organic matter content all influence nutrient availability.
3. Role of Soil Organisms: Fertile soils often have a rich community of microorganisms and earthworms that help decompose organic matter, releasing nutrients into the soil. This biological activity enhances soil structure and nutrient cycling, further contributing to soil fertility.
4. Water Retention and Aeration: Fertile soils typically have good water retention and aeration properties, which are essential for root development and nutrient uptake. The balance of sand, silt, and clay in the soil affects these properties.
5. Natural vs. Artificial Fertility: While artificial fertilizers (as mentioned in option D) can enhance soil fertility temporarily, they do not replace the need for a healthy soil ecosystem. Over-reliance on artificial fertilizers can lead to soil degradation over time.

Why Other Options Are Incorrect:

• Option A: It is black: The color of soil can be influenced by its organic matter content, but black soil does not necessarily mean it is fertile. Some black soils may lack essential nutrients, while other soils of different colors can be very fertile. Therefore, color alone is not a reliable indicator of fertility.
• Option C: The oxygen content is very high: While oxygen is important for root respiration and the activity of soil organisms, high oxygen content alone does not determine soil fertility. Soil can have adequate oxygen levels but still be low in nutrients. Fertility is more about nutrient availability than just oxygen levels.
• Option D: Artificial manure is added to it: Adding artificial manure (fertilizers) can temporarily increase nutrient levels in the soil, but it does not inherently make the soil fertile. Fertility is a more complex characteristic that involves the natural ability of the soil to sustain plant life over time, which includes the presence of organic matter, microbial activity, and nutrient cycling.

Revision Summary:

• Soil fertility is defined by its ability to supply usable nutrients to plants.
• Essential nutrients must be available in a soluble form for plant uptake.
• Soil structure, pH, and biological activity play critical roles in maintaining fertility.
• Color and oxygen levels are not reliable indicators of soil fertility; reliance on artificial fertilizers can lead to long-term soil health issues.