In 1869, by which time total trade had reached $3,262,500, the second Rajah invited Chinese black pepper and gambier growers from Singapore to cultivate their crops in Sarawak. As a result, by the early 20th century, Sarawak became one of the world's major producers of pepper. The kingdom was a relative latecomer to the natural rubber boom due to the reluctance of the second Rajah to give over indigenous farmland to European companies. Only five large rubber estates were established during his reign. Oil reserves were discovered in his final years. From the 1930s, through the work of the Chinese businesses in the kingdom, it became a significant raw material supplier, with Singapore a major trading partner.