Shafaqna (Shia International News Association) – The Grand Ayatollah Sistani answered some questions regarding Beliefs.According to Shafaqna the questions and answers are as follows:
Question 1: Why do Shias use Turbah (Sajdagah) when doing Sajda?
Answer: Sajdah should be performed on earth, and on those things which are not edible nor worn, and on things which grow from earth (e.g. wood and leaves of trees). It is not permissible to perform Sajdah on things which are used as food or dress (e.g. wheat, barley and cotton etc.), or on things which are not considered to be parts of the earth (e.g. gold, silver, etc.). And in the situation of helplessness, asphalt and tar will have preference over other non-allowable things. Sajdah should not be performed on the vine leaves, when they are delicate and hence edible. Otherwise, there is no objection. It is in order to perform Sajdah on things which grow from the earth, and serve as fodder for animals (e.g. grass, hay etc.). It is in order to perform Sajdah on flowers which are not edible, and also on medicinal herbs which grow from the earth. Performing Sajdah on a grass which is eaten in some parts of the world, but not in the rest, but it is classified as edible.
Question 2: Why do Shias combine Zuhr and Asr prayers? Is it permissible?
Answer: It is permissible to combine Zuhr and Asr prayers and to perform them separately as it is permissible in both manners. It is narrated that the Holy Prophet of Islam (pbuh) offered his prayers in both combined as well separate manners. He combined his prayers without being in an urgent situation like war or storm.
Source: Ayatollah Sistani