1. Abdu N, Yusuf AA, Abdulkadir A, Arunah UL, Chude VO, Pam SG (2007) Zinc soil test calibration based on 0.1 N HCl extractable zinc and cation exchange capacity from upland soils of Northern Nigeria. J Agron 6:179–182
2. Alloway BJ (2008) Zinc in soils and crop nutrition. International Zinc Association, 2nd edn. International Fertiliser Industry Association, Brussels
3. BOE (Spanish Official Bolletin) (1990) Real Decreto 1310/1990 29 de Octubre de 1990, que regula la utilizacion de los lodos de depuracion (Royal Decree 1310/1990 29th October 1990, that regulates the use of sewage sludge). Ministerio Agricultura, Pesca y Alimentacion, Madrid, Spain,
http://www.boe.es/boe/dias/1990/11/01/pdfs/A32339-32340.pdf
. Accessed 6 July 2017
4. Carlson DH, Sharrow SH, Emmingham WH, Lavender DP (1994) Plant-soil-water relations in forestry and silvopastoral systems in oregon. Agrofor Syst 25:1–12
5. CEM (1994) Microwave digestion applications manual. CED, Matthews