1. Dep. of AgroecologyAarhus Univ.Blichers Allé 208830 TjeleDenmark
2. Univ. of São PauloCentre for Nuclear Energy in Agriculture (CENA/USP)Caixa Postal 96, 13416‐903Piracicaba (SP)Brazil
3. Dep. of Civil EngineeringAalborg Univ.Thomas Manns Vej 239220 AalborgDenmark