The correct answer is: a) Iron oxide.
Black soils, also known as ‘Regur’ soils, are rich in iron oxide. They are formed in semi-arid regions with a long dry season and a short wet season. The high iron content gives these soils their characteristic black color. Black soils are well-drained and have good water retention capacity. They are also rich in nutrients, making them ideal for growing a variety of crops.
Option b) Calcium carbonate is incorrect. Calcium carbonate is a common mineral found in many types of soil, but it is not a major component of black soils.
Option c) Potash is incorrect. Potash is a mineral that is essential for plant growth. It is found in many types of soil, but it is not a major component of black soils.
Option d) Organic matter is incorrect. Organic matter is a substance that is found in soil and is made up of the remains of plants and animals. It is important for soil health, as it helps to improve drainage, water retention, and nutrient availability. However, organic matter is not a major component of black soils.