Chennai, Aug 11 - Tamil Nadu's Leader of Opposition and former chief minister J. Jayalalithaa Tuesday demanded a central probe into the alleged diversion of potash meant for farmers to two chemical industrial units for manufacturing fireworks.

In a letter to Prime Minister Manmohan Singh, the AIADMK leader accused the two Karaikal-based companies - Vaigai Chemical Industries and Karaikal Chlorates - of diverting potash worth Rs.4,000 crore to make fireworks and safety matches.

'It is essential that the Central Bureau of Investigation (CBI) should probe the matter to unravel the truth,' Jayalalithaa said in the the letter, a copy of which was made available to the media.

Indian Potash Ltd imports 40,000 tonnes of the chemical per month from Russia and Israel at Rs.15,000 per tonne and distributes it among farmers at a concessional rate of Rs.5,000 per tonne.

The same product is also being used to make potassium chlorate, an essential commodity for making explosives and fireworks.

Jayalalithaa said both Vaigai Chemical and Karaikal Chlorates 'have been possessing stocks far beyond their production capacity. This has led to a suspicion vis-a-vis the source of supply of potash to these industries.'

'It has been brought to my notice that Vaigai Chemical is producing 40 tonnes of potassium chlorate per day, though its installed capacity is nine tonnes.