This study is based on the work of Ricardo J. Melgar, Instituto Nacional de Tecnología Agropecuaria (INTA) Fertilizar.
The study benefited from the contributions of K. Isherwood, J. Poulisse and T. van den Bergen (FAO).
The background cover photograph is from the FAO Mediabase (FAO/8766/E. Kennedy) and the other photographs were provided by R.J. Melgar.
