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 a high water-holding capacity. They are also rich in nutrients, making them ideal for growing crops such as cotton, sorghum, and millet.
Option b) Calcium carbonate is incorrect. Calcium carbonate is a white mineral that is found in limestone and chalk. It is not a major component of black soils.
Option c) Potash is incorrect. Potash is a mineral that is found in potassium salts. It is an important nutrient for plants, 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 living things. It is not a major component of black soils.