Wisdom behind making the Zakah rate of Rikaaz one-fifth

Why is the Zakaah of gold and silver one-fortieth whereas that which is due on Rikaaz extracted from the earth is one-fifth as stated by the Prophet ?

All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad , is His slave and messenger.

The basic rule that determines the rate of Zakah on Rikaaz (i.e. buried treasures) or that of gold and silver is in the texts that are authentically attributed to the Prophet , in this concern. The Prophet , stated that the Zakah of Rikaaz is one-fifth. Meanwhile, he used to take a fortieth as Zakah that is due on gold and silver.

It is narrated on the authority of Abu Hurayrah that the Prophet , said: "There is no compensation for one wounded or killed by an animal, working in a mine or falling into a well. However, one-fifth is due on Rikaaz." [At-Tirmithi and others]

Moreover, it is narrated on the authority of Ibn ‘Umar and ‘Aa’ishah that "The Prophet , used to take (as Zakah) half a dinar from every twenty dinars or more and one dinar from every forty dinars."  [Ibn Maajah] [Al-Albaani: Saheeh]

The reason behind the variance between the rate of Zakah of Rikaaz and other types of wealth is obvious. It is because of the abundant benefit that Rikaaz yields and being easily accessible in case it is found unlike other types. Al-Mubaarakfoori cited the opinions of some scholars in his commentary on Sunan At-Tirmithi as saying: "One-fifth is due on Rikaaz because of its abundant benefit and accessibility."

Allaah Knows best.