JOHANNESBURG—South Africa’s government on Thursday criticized a tweet by President Donald Trump in which he said he asked Secretary of State Mike Pompeo to study efforts to overhaul land ownership and “large scale killing of farmers” in Africa’s most-developed economy. Mr. Trump’s overnight tweet referenced a report by Fox News host Tucker Carlson attacking the U.S. government’s stance on land reform in South Africa, which he suggested was too lax. South Africa’s rand fell against the dollar after the tweet, and was 1.3% lower in Thursday morning trade. The ruling African National Congress has said it plans to change the...