The SA in OC SIman 312 SIf 9 says that it’s ossur for someone to go to the bathroom in a field lest them come to be ashaveh gomos. The question arised that perhaps it should be another problem since the human feces could be used as a fertilizer. The issue was investigated and indeed now a days it’s possible to use the feces for fertilizer (presumably with some ‘treatment’ having been done to it and in the right conditions.) However it also come out that this is some what of an old documented practice going back centuries. So the question remains then perhaps it should be ossur for that reason since the person is putting something onto the ground that can help in the growing process.
(Although perhaps one could say the SA (or the gemorah really) knew or understood the process of fertilization and therefore was never even a sofek as to why this should be ossur.)
Answer:
Both from Orach Chaim 312:9 and from Orach Chaim 336 (see Mishnah Berurah 28), we see that human excretion (both feces and urine) are considered as damaging to plants, and therefore there is no issue of zore’a (planting) with regards to using the bathroom in a field. See also Biur Halachah (336, s.v. oh), concerning the possible problem of charisha.
See Or Zaru’a Shabbos 54, where two opinions are mentioned with regard to urine; the Shulchan Aruch adopts the lenient opinion.
I don’t know about recent investigations in this matter, but in order for a problem to arise one would have to show that human waste is of direct benefit to the plants.
