http://www.fao.org/docrep/007/y5608e/y5608e01.htm http://www.fao.org/docrep/007/y5608e/y5608e01.htm http://www.fao.org/docrep/007/y5608e/y5608e01.htm FAO, 1997 la definizione data dalla FAO http://www.fao.org/docrep/007/y5608e/y5608e01.htm http://www.fao.org/docrep/007/y5608e/y5608e01.htm