NOTE: the Directive 86/278/EEC of 12 June 1986 on the protection of the environment, and in particular of the soil, when sewage sludge is used in agriculture, indicated that nickel analysis must be performed on soil before spreading of sewage sludge. In this framework, the analysis must be carried out following strong acid digestion and the reference method of analysis must be that of atomic absorption spectrometry. Nevertheless, for other requirements, other type of analysis can be made using other reagents. Thus, the type of reagent used for the analysis must be given in the process parameter linked to the observable property. NOTE: Generally, the soil derived object will give the value of nickel content for a certain depth interval, layer or horizon that must be precised using the SoilDerivedObjectDescriptiveParameter.