Wudu is valid even with the dirt that normally collects under the nails. But if something is stuck on or under a nail that does not let water through, it must be removed before wudu. The ruling is given in this way in the Hanafi works ad-Durr al-Mukhtar and Fatawa Hindiyyah.
What blocks and what does not
Wudu and the obligatory bath are valid even if dust, earth, mud or the ordinary dirt of work is under the nails. The jurists said it is hard to avoid this altogether, whether one lives in a town or a village, so there is a concession.
Something like dried dough, on the other hand, forms a layer on the nail under which water does not pass. The books give the example of dough; scholars of our time include paint, glue and nail polish. Wudu or the bath is not valid while these are present; they must be removed first.
Why the difference
The Quran says that whoever intends to stand for prayer must wash the face and both arms to the elbows, wipe the head and wash both feet to the ankles. (Surah al-Maidah, verse 6) The nail is part of the hand. If any part of a limb that must be washed stays dry, wudu is not complete.
Keeping nails short is the sunnah
The Messenger of God (peace be upon him) counted cutting the nails among the acts of fitrah, the cleanliness natural to human beings. (Bukhari, hadith 5889) Anas (may God be pleased with him) said a time limit was set for them so that such things as cutting the nails and trimming the moustache were not left for more than forty nights. (Muslim, hadith 258)
With short nails there is less chance for dirt to collect and no doubt about water reaching the skin in wudu. The fingertips and the area round the nails should be washed well during wudu.
On the merit of wudu, Abu Hurayrah (may God be pleased with him) narrated that the Prophet said his community will be called on the Day of Resurrection with faces, hands and feet shining from the traces of wudu. (Bukhari, hadith 136)
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