To get an answer to your question, it is necessary to take notice of the following points:
1. According to grand religious authorities “it is obligatory to pay Zakat on the following nine things: 1. Wheat, 2. Barley, 3. Dates, 4. Raisins, 5. Gold, 6. Silver, 7. Camel, 8. Cow, 9. Sheep (including goat).[1]
And if a person is the owner of any of these nine things he should, in accordance with the conditions which are mentioned in the Epistles of Islamic Laws, put their fixed quantity to one of the uses as prescribed.[2]
However, some jurisprudents have considered Zakat to be payable on the wealth in business, as a necessary precaution.[3] Some other scholars consider payment of Zakat on invested capital as recommended.[4]
2. Zakat is recommended on a number of things two of which are mentioned as under:
1-2. Female horse (mare) from which Zakat is paid in accordance with the following details:
The Zakat of every mare is two gold dinars per annum and if both of its parents are not original, the Zakat is one dinar.
There is a narration in hadith and jurisprudential books in this regard.[5] It is recommended to pay Zakat on a mare as stated in a narration from Imam Ali (a.s) who fixed two dinars as the tax for every mare.[6]
The fact that Imam Ali (a.s) levied Zakat on horses can be a governmental order that is particular to his time.[7] This is further endorsed by the fact that the Infallible Imams who came after him did not consider Zakat as mandatory on horses.
2-2. Some other things:
A) Things which grow from earth and which are sold by way of weight and measure such as rice, peas, lentils, mung and the likes, Zakat should be paid on them in the same way as it is paid on wheat, barely, dates and raisins but Zakat is not recommended on vegetables, melon, cucumber and the likes.
B) According to some jurists, it is recommended to pay Zakat on properties like orchards, caravansary, shops and the likes which are maintained or leased to someone[8] and also on wealth in business.[9]  There are also narrations according to which it is recommended to pay Zakat on everything that grows out of earth and which can be measured or weighed except vegetables and the likes.[10]
3. Gold and silver:
In old times, coins were minted from valuable metals such as copper, gold and silver. Those coins were used for every day transactions, evaluation units and calculation. In reality, these coins were used as “gold dinars”, “silver dirham” as currency.[11]
In hadith and jurisprudential books, the discussion revolves around coined gold and silver as “dinar”, “dirham” and “mithqal”[12] for fixing the taxable limit for Zakat.[13] As mentioned in the Manual of Islamic Laws (al-Resalah al-Amaliyah),[14] Zakat is payable on them, if certain conditions are met. One of those conditions is that gold and silver should be used as currency.
However, real cash like gold dinar and silver dirham which were used in the past in transactions were gradually replaced by pure credit cash (banknotes). Today credit money or what is known as “notes” is used as a replacement for gold and silver i.e. the very dinar and dirham that were used for every day transactions, evaluation units and calculation.
4. Zakat on Currency Notes
As for whether or not Zakat is payable on currency notes, we will cite some grand jurists’ fatwa in this regard:
Grand Jurists:
Imam Khomeini: “Zakat is not applicable to cash notes[15] (like currency notes[16]).”
Gulpaigani: “Zakat is not payable on currency notes.”[17]
Makarem Shirazi: “The recommended precaution is to give Zakat on current currencies such as currency notes, in the existence of other conditions.”[18], [19]

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