I just saw the same thing and the answer is confluence.
Answer:
Latitude, attitude, pressure and winds
Explanation:
The Tropic of Cancer passes through the middle of the country from the Rann of Kuchchh in the west to Mizoram in the east
Im guessing volcanoes since Volcanic soil is quite fertile, namely because it the rocks dissolved contain a rich array of the necessary micro nutrients and trace elements essential for plant growth that most other soils lose over time.