The excrement used for fertilizer may come either from animals that are lawful to eat, or from animals that are unlawful or from humans. As for the animals whose meat is lawful to eat, it is lawful to use their dung in fertilizing the land, for the correct view is that their dung is pure as well. Also, there is nothing wrong in using the dung of animals whose meat is not lawful to eat and human excrement for fertilizer. If any such excrement has stuck onto a plant or fruit or vegetable growing in such soil, it is obligatory to wash it very well before eating it. The point is that the purity of the plants and their produce is not affected, so long as the manure used in fertilizing them have dissolved into the soil and have not stuck onto them.
Dr. Husam al-Din Ibn Musa `Afana, a professor of Principles of Islamic Jurisprudence at Al-Quds University, Palestine, issues this fatwa: ”The plants and fruits that are fertilized by impure materials are not impure in themselves; they become so if such impure materials get stuck onto them. If this happens, they are to be washed and cleansed and then they will be pure again. But the plants and fruits themselves do not become impure even if they have grown in an impure place or watered by impure water. Muslims have been accustomed to using manure in fertilizing lands. Though the majority of jurists are of the view that it is impure, they do not consider the crops grown in a soil fertilized by it as impure.”
Imam An-Nawawi (may Allah be pleased with him) said: ”According to the Shafi`i jurists, the plants that grow on a land fertilized by manure are not impure in themselves; yet they become impure, if an impure material has stuck onto them. The impurity of the plants in such case can be removed by washing the plants. If the part of the plant on which impurity has stuck is, for example, (unhusked) ears of corn or leaves of cucumber, or something like that, that is, a plant whose eaten part is covered in any way, the grains of corn and the cucumber will remain pure and thus will not need to be washed. This goes also for trees that have been watered by impure water, and hence their branches, leaves, and fruits remain pure.” (Al-Magmu`, vol. 2, p. 573.). According to the Kuwaiti Encyclopedia of Fiqh, it is lawful to use manure in fertilizing lands. The plants grown on such lands are not to be regarded impure, unless they are contaminated with filth.