Praise be to Allah.

Firstly: 

In order for prayer to be valid, it is stipulated that the worshipper should avoid having any impurity on his clothing, because Allah, may He be exalted, says (interpretation of the meaning): “And your garments purify” [al-Muddaththir 74:4]. And the Prophet (blessings and peace of Allah be upon him) said, concerning menstrual blood that gets onto a woman’s garment: “She should scratch it, then rub it with water, then wash it, then pray in it.” Narrated by al-Bukhaari (227) and Muslim (291). 

Abu Dawood (650) narrated that Abu Sa‘eed al-Khudri (may Allah be pleased with him) said: Whilst the Messenger of Allah (blessings and peace of Allah be upon him) was leading his companions in prayer, he took off his shoes and placed them to his left. When the people saw that, they also took off their shoes. When the Messenger of Allah (blessings and peace of Allah be upon him) finished praying, he said: “What made you take off your shoes?” They said: We saw you take off your shoes, so we took off our shoes. The Messenger of Allah (blessings and peace of Allah be upon him) said: “Jibreel (peace be upon him) came to me and told me that there was some dirt – or filth – on them.” 

Secondly: 

There is no differentiation between dry impurity and wet impurity with regard to it being obligatory to remove it when wanting to pray. That is because of the general meaning of the texts referred to above, which indicate that it is stipulated that one’s clothing should be pure when praying. They do not differentiate between dry impurity and wet impurity. The words in the hadith quoted above: “She should scratch it, then rub it with water” indicate that dry impurity should be removed, because scratching is only done in the case of something that has dried out. 

Shaykh Saalih al-Fawzaan (may Allah have mercy on him) was asked:

If urine has got onto a garment then dried, is it still impure? Is it obligatory to wash the wet spot? If a person touches it, does he have to wash the area that touched it? 

He replied: 

Impurity cannot be removed from a garment except by washing it with clean water. It is not sufficient merely to let the impurity dry out. The Prophet (blessings and peace of Allah be upon him) said, concerning menstrual blood that got onto a woman’s garment: “She should scratch it, then rub it with water, then wash it, then pray in it.” Agreed upon. So it is essential to wash the impurity from the garment before praying in it. 

If a person touches a wet impurity, then he must wash off whatever part of his body touched it, because the impurity has been transferred to it. As for dry impurity, he does not have to wash whatever part of his body touched it, because the impurity is not transferred to him in that case. End quote. 

http://ar.islamway.com/fatwa/6511

And Allah knows best.