Why do shiaas prostrate to the soil?
Answer: Some people believe that prostrating to the soil or the torbat of the martyr means that you are somehow worshipping that and it means “shirk”.
For the answer, I should say that between these two meanings of (alsojood leallah) and (alsojood alal ardh) are differences, and the problem is that you don’t put differences between them. For sure the meaning of the first is that prostrating is for Allah, and the second is prostrate to the soil. And in other words we prostrate to the soil to worship Allah. All the Muslims worldwide prostrate to something, while their prostrating is only for Allah.
So till now we understand that prostrating to the soil, plants and… means worshipping Allah and doesn’t mean worshipping that certain thing.
Prophet Muhammad (PBUH): “The earth is the place to prostrate and it is a place to get clean for me.”(1)
 And also Allah tells us in the Quran:” Everyone in the skies and the Earth is prostrating me” (2)
.So prostrating to the soil isn’t something unusual and it actually shows the humility of mankind.
So now it’s good to point to one of the speeches of our imam Sadegh (AS) that: ”Hosham bin Hakam asked Imam Sadegh: what is the correct thing in which we have to prostrate to. He said: prostrating is only correct on the soil and what grows on it (except the things we eat and the things we wear) so I asked why? He said: prostrate is humility in front of Allah and so it is not good to prostrate to the things we eat and we wear because the people who worship the world (like the world a lot) are the slaves of food and clothes while the one who prostrates is only worshipping Allah so it is better to put our forehead on the soil and not on what the world worshippers love”.(3)
And prostrating to the soil is better because with the humility and lowliness towards Allah it is a better form.
So now there will be another question, why are shiaas bounded to prostrating to soil or to some plants and doesn’t prostrate to anything else?
As the answer we should say that as the base of any worship should be originated from Islamic law, the conditions of its parts as well should be clear by prophet’s Sonnat (tradition) because messenger of God as mentioned in Quran is model and sample for all pious human beings.

Now we will bring some narrates and evidences from tradition (Sonnat) of prophet PBUH our Prophet Muhammad PBUH. He always used to prostrate on soil or on some plants like palm’s Bast mat exactly in the same manner that shiaas believe.

The word “Ja’al” which here has the meaning of legislation clearly shows that this subject is a Divine Decree for followers of Islam. So legitimation of prostrate to soil and sand and other content of the ground will be proved.

Put your forehead on soil for Allah.(5)
 Two points get clear from the word “Tarreb”(put your forehead on soil), one is that we should prostrate on soil and put the forehead on soil while prostrating and the other is that this is an order that should be followed.

“I saw whenever prophet PBUH prostrate, put his forehead and tip of his nose over the ground”(6)
Anas-Ibn-Malek and Ibn-Abbas and some of prophet’s wives like Aisha, Um-Salama and a group of Narrators narrated that:
“prophet was prostrating on “Khumra” which was a kind of mat woven from palm’s Bast”(7)
Abu-Saeed who is one of prophet’s companions, narrates:
“I entered while prophet PBUH was prostrating on palm’s Bast mat”(8)
This will be another evidence for Shi’a’s theory that prostrating to whatever grows from the ground except of foods and clothes is permissible.

Jabber Bin Ansari says: we were praying Zuhur prayers with the Prophet (PBUH) I took the pebbles and kept it in my hand to get cool so that I could put them on my forehead.(9)
Narrator also says: if prostrating to the clothes that you were putting on was not a problem it was way easier than taking pebbles and keeping them in your hand!
Ibn-Saad in his book “Al-Tabaghat-Al-Kobra” says:
Masrough Bin Ajda used to take a brick while in a trip by ship so that he could prostrate to it while praying.(10)
It is necessary to say that Masrooq-Ibn-Ajdaa was one of the Tabe’een (Followers) of prophet and accompanying Ibn-Masood and is the author of “Al Tabaqat-Al Kobra” book. He has counted as first level of Kufa people after prophet and he was narrator of Abu Bakir, Omar, Othman and Ali AS and Ibn-Masood. This speech clearly proves that the word of those others which assume carrying a piece of soil is pagan, is baseless and followers also were doing the same.(11)
Nafe’e narrates that if Abdollah-ibn-Omar was praying and he had put on the scarf, he was removing that to put his forehead on the ground.(12)
Resin narrates that:
Ali-Ibn-Abdollah-Ibn-Abbas wrote me: send a piece of marble of Marva mountain to prostrate on that.(13)

THE Prophet (PBUH) saw a man that was prostrating over corner of his turban. He pointed to him to take away his turban and pointed to his forehead.(15)
Till now we understand that the Prophet (PBUH) prostrated to the soil and it obvious. And if anyone did something apart from this the Prophet used to tell him/her to correct.

Imam Sajjad (AS) says that: prostrating to the soil is order of Allah and prostrating to the mat (woven of palm’s Bast) is our Prophets tradition.(16)
And in other place says: prostration is not permitted but ground and what comes from the earth except of clothes and food items.
 
Conclusion:
Through all reasoning, it is obvious not only narrates of prophet’s family but also the tradition (sonnat) of prophet and manner of companions and followers are evidence of necessity of prostrating on the ground and whatever extracted from that (except of food and clothes items).
In addition, certainly prostration over mentioned things is permitted but there is doubt if we can prostrate over other things, so as per precaution-which is the way of Salvation-it deserves while prostrating just do it on those permitted.
At the end remind that this discussion is a jurisprudence subject and there are some variations between Islamic supreme but it should not make worries as similar differences are too many among four Sonni groups like touching the nose tip to the prostration area is suggested as per Maleki’s but Hanabele believe it void the prostration.
 
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