Although the food crops used varies with location, the sequence of events in slash and burn agriculture are very similar around the world. Farmers begin to prepare a field by cutting down many of the trees and woody plants in an area. Trees that provide fruits, nuts, building materials or other useful products may be spared. The downed vegetation or "slash" is allowed to dry until right before the rainiest part of the year, at which time the slash is burned converting biomass into nutrient-rich ash. Burning also temporarily eliminates most pest and weed species. Seeds and cuttings are planted directly into the ash-covered soil. Farmers may add additional slashed vegetation from offsite as mulch which further enhances soil fertility while protecting the soil from erosion. Mulch made from the cuttings of plants which contain natural insecticides such as Piper nigrum may also be used to protect crops from pest species. Fields may be weeded one or more times, but inevitably, weed and pest species take their toll on productivity. Depending upon location, fields may be cultivated for three to five years, and again, depending upon location, be allowed to recover for as little as five to over twenty years before being cut and burned again.