South African President Pieter W. Botha took his program of cautious reforms a step further today when he announced that some blacks would be allowed for the first time to buy land in their segregated townships adjacent to white cities.

Botha also told Parliament in Cape Town that blacks would not necessarily lose their South African citizenship when tribal "homelands" demarcated by the white government are granted nominal independence. He did not explain what he had in mind.

Under the segregationist system called apartheid, blacks have been prohibited from owning property in the 87 percent of South Africa reserved for only the white minority.

Government policy has been that all blacks belong ultimately in the 13 percent of South Africa that has been demarcated into 10 tribal "homelands." Even there, under the traditionalist system that the white government encourages, a system of collective tribal ownership of land has meant that few blacks are property owners.

Seven years ago, in one of its earliest reforms, the Botha government allowed some urban blacks -- those who had worked there for 10 years for one employer, or for 15 uninterrupted years for several employers -- to buy houses in the segregated townships of "white" South Africa. But they were allowed only 99-year leases on state-owned land.

Nearly half of South Africa's 21 million blacks live in the cities, but only 31,000 have purchased houses under the 99-year leasehold plan.

Robin Lee, managing director of the business-funded Urban Foundation, which tries to upgrade living conditions in the black townships, said tonight that although Botha's new concession would make little practical difference, "symbolically it is decisively more important."

Permitting urban blacks to own land made a "symbolic difference to the government's recognition of their permanence," Lee said.