Cotton grows best in black and alluvial soils. Major cotton producers, such as China, the USA, and India, have regions with these soil types, which support the high temperature and light rainfall cotton needs.

Primary Crop Grown in Black and Alluvial Soils: Cotton
Black and alluvial soils are known for their fertility and ability to support a variety of crops. Among these crops, cotton is primarily grown in these types of soils due to several reasons:

1. Soil Suitability:
- Black and alluvial soils have good water retention capacity, which is beneficial for cotton cultivation as cotton requires a consistent water supply.
- These soils also have good drainage properties, preventing waterlogging which can be harmful to cotton plants.

2. Nutrient Availability:
- Black and alluvial soils are rich in nutrients like nitrogen, phosphorus, and potassium, which are essential for the healthy growth of cotton plants.
- The high fertility of these soils reduces the need for excessive fertilizers, making them cost-effective for cotton cultivation.

3. Ideal Climate Conditions:
- Black and alluvial soils are often found in regions with warm climates, which are suitable for cotton cultivation. Cotton plants thrive in warm temperatures and ample sunlight.

4. Pest and Disease Resistance:
- Cotton plants grown in black and alluvial soils are less prone to certain pests and diseases that can affect their growth and yield. This reduces the need for pesticides and other chemical interventions.
In conclusion, cotton is the primary crop grown in black and alluvial soils due to their fertility, nutrient availability, ideal climate conditions, and resistance to pests and diseases. These factors make these soils well-suited for cotton cultivation, leading to higher yields and better quality cotton production.