These species now constitute between 50% and 60% of all crops in cultivation worldwide. Pompeii was fortunate to have had fertile soil for crop cultivation. Some Caribbean islands have terrain that Europeans found suitable for cultivation for agriculture. The environment is damaged through deforestation caused by clearing fields for plant cultivation. Farm units included 160,000 hectares used for activities other than cultivation. The crop yields are very poor with "jhum" cultivation. In 1930 Nortier began conducting experiments with the commercial cultivation of the rooibos plant. In particular the area under rice and sugar cane cultivation increased. It is for this reason that many growers are reluctant to talk about their cultivation. There is evidence from seeds in Pharaoh tombs of watermelon cultivation in Ancient Egypt. Rice cultivation and tobacco were very labor-intensive. There are of land dedicated exclusively to corn cultivation in the United States. Erosion is much smaller with perennial cultivation. Forest clearances were undertaken to provide room for cereal cultivation and animal herds. Peat moss is a manufactured product for use in garden cultivation.