I've heard recently that apple seeds contain arsenic. Is this true? Should apples not be eaten then?
So, if your soil doesn't contains arsenic in the first place, there is no way the apples you grow on it can contain arsenic. However, apples do contain cyanide which can be synthesized by the plant.
Looks like a simple google search yields my answer:
The seeds actually contain a cyanide compound, not arsenic. But, the amount is so trivial that there is no harm in eating the apples. And the seeds are encased in a indigestible material so they pass through our systems.
Arsenic in apple seeds is not so easy to discount:
Historically, the use of arsenic-based pesticides has led to considerable contamination of domestic and agricultural land, through their use as lawn herbicides, and insecticides for rice, orchards and cotton.
Thus, there's arsenic in many soils. Arsenic compounds are takn up by apple trees, and do show up in the fruit (ibid). I'd be surprised if arsenic compounds didn't appear in apple seeds when the material is taken up by the tree, but a quick search for actual measurements of Arsenic in apple seeds ran up against the paywall which hides so many academic publications.