Slash-and-burn is a technique that has been used for thousands of years. Usually at the start of the dry season, a plot of land is cleared of its vegetation. Very often, this is done manually, for example using machetes. This can take a long time, clearing a hectare can take up to 120 hours of work. If the land is to be used for crops that last more than two years, the roots need to be cleared as well. Usually this is not done for annual plants. The plants cleared are left to dry, often for weeks. When they are dry enough, they are burnt. The ash that is created makes the soil fertile. Weeds, and other pests have also been eliminated, so the field can be used for agriculture. After three to five years, the extra fertility is gone, and the farmer usually moves ot another field (or again uses slash-and-burn).