As per your query it is understood that you need information regarding buying the land from the individuals belonging to SC/ST community.
The Karnataka Scheduled Castes and Scheduled Tribes (Prohibition of Transfer of Certain Lands) Act, 1978 (Karnataka Act 2 of 1979) or PTCL is a statute of Karnataka.
This law which was introduced in 1978 is retrospective in nature and is considered an ex post facto law. Currently over 10000 cases are pending before the office of the Deputy Commissioners across the State Karnataka. This law was created as a social welfare Act aiming to protect and provide land for the poor members of the scheduled Tribes and castes. But due to a lack of clarity and understanding between the judiciary and the administration there exists this large number of cases still hanging in the balance.
You cannot purchase the land belongs to SC/ST as per PTCL (Prevention of Transfer of Certain Lands) Act 1978. The Most such lands granted under restriction of alienation are protected by PTCL (Prevention of Transfer of Certain Lands) Act 1978.
In some major court rulings, the Supreme court of India has declared that a land belonging to scheduled castes or tribes (SC/ST) cannot be purchased by anyone who does not belong to the same category, In case any person purchased the land then he will be liable to prosecute under Section 8 of PTCL ACT.
In case of land granted to SC/ST, even after the expiry of 15 years from the date of grant, permission should be obtained from the State Government before such land is transferred by the grantee under the provisions of Section 4 of the Karnataka Scheduled Caste and Scheduled Tribe (Prohibition of Transfer of Certain Lands) Act, 1978.
I would suggest you to not take any kind of risk and to ask for prior written permission from the competent authority before buying such land.
Hope it helps!