I'm not sure what you mean by putting plastic under dirt but that seems like it would prevent downward movement of water increasing the likelyhood of soggy soil conditions. Likewise if the plastic were not very deep it would prevent deep rooting and thus make the plants more prone to drought conditions.
That said the problem with seeds could also be related to something added to the soil or if new soil or manure was brought in then is may be due to something in that soil or manure.
Thanks for the question. Please stop in again soon!
Q&A Library Searching Tips