Probably stupid questions about Ricinus communis
 

The recent question about whether or not to compost rhubarb leaves got
me thinking about the Ricinus communis that I grow every year. As my
garden isn't used by children or pets, I let the plants set seed.
Last year some of them even germinated in the ground, although a little
too late to get more than 3 foot high before being cut down by frost.
I know that the seed coat is poisonous and that the substance is
water-soluble. Does anybody know if it can persist in the soil? Also,
is it okay to compost the leaves and stems? I grow some ornamental
food-plants in the same area, such as Swiss chard with coloured stems,
and Bulls Blood beet; is there any possibility of contamination from
the Ricinus seed in the ground? Thanks for any advice.