Acknowledgements
The authors thank R. Gracia, M.J. Salvador, A. Bielsa and M.A.
Limón for theirfield and laboratory assistance and Dr. J. Machín for
his assistance with the characterization and classification of soils.
We are also grateful to all farmers for allowing access to theirfields.
This research was supported by the Comisión Interministerial de
Ciencia y Tecnología of Spain (Grants AGL2010-22050-CO3-02/AGR
and AGL2007-66320-C02-02/AGR) and the European Union (FEDER
funds). N. Blanco-Moure was awarded with a FPI fellowship by the
Spanish Ministry of Science and Innovation