Obligation or desirability of the alms tax in jurisprudents` thought

Alms tax, zakat, is known to be one of the financial sources in Islamic government; in order to administrate society and omit economical poverty, Islamic government has regarded alms tax, beside other financial obligatory payment like tax and indulgence tax,  as another obligatory payment. Shiite jurisprudents have limited obligatory cases of alms tax to the well-known nine cases, while majority of Sunni jurisprudents have extended it to such a domain that it includes all cases known to be desirable by Shiite as obligatory. Among Shiite jurisprudent, ibn Babevayh and his son, Sheikh Saduq, have known it obligatory in trading finance. Ibn Junayd has known all grains as obligatory. One can conclude that relying on the measure for alms tax and with regard to social needs, the variety of the cases of alms tax is limited to the nine and the government can add to alms tax cases according to social needs in different ages.