Botulinum toxin in water
I was told that if a bucket of water and dirt/organic matter sits around for a long time, anaerobic bacterial activity can result in the formation of botulinum toxin in the water and muck. I was warned to be very careful in cleaning and and reusing such a bucket due to the risk of poisoning. Also the same thing for vases that sit around for very long periods of time.
Is there any truth in this? Are there any risks in disposal of the bad-smelling contents of the bucket/vase?
Based on the information below, I do not believe that casual contact with water in buckets/vases where it was standing, it seems you would need to ingest the water for deleterious effects to occur. This information is from the Michigan Sea Grant Program, they developed it since botulism has become an issue with birds in the region: http://www.miseagrant.umich.edu/explore/coastal-communities/avian-botulism/faqs-botulism-in-the-great lakes/#canigetbotulism
"Where does botulism come fromBotulism spores (the resting stage of the bacteria) are abundant in anaerobic habitats, such as soils, and aquatic sediments of many lakes and can be readily found in the gills and digestive tracts of fish living in those lakes. The spores can remain in the ecosystem for extended periods of time, even years, and are quite resistant to temperature changes and drying. These spores, themselves, are harmless until the correct environmental factors and anaerobic conditions prompt them to germinate and begin vegetative growth of the toxin-producing bacterial cells.2]
The active bacteria that cause botulism only grow in a nutrient-rich substrate, such as areas with large amounts of decaying plant growth, which are free of oxygen (anaerobic). Fish that die for any reason and that contain the bacterial spores in their tissues are also suitable substrates for growth and toxin production by the bacteria. "
Can I get botulism?
Botulism in humans is usually caused by the consumption of improperly home-canned foods and is most often a result of the Type A or Type B botulinum toxin. A few cases of Type E botulism in humans have been reported in North America as the result of eating improperly smoked or cooked fish, but these cases are very rare. 
Thorough cooking is necessary to destroy the bacteria and bacterial toxins. Consult your local health agency for recommended cooking temperatures.
As a precaution, any fish or waterfowl that are sick or act abnormally should not be harvested or eaten because cooking may not destroy the botulism Type E toxin.  (More information on botulism from a human health and food safety standpoint can be obtained through the USDA Food Safety Research Information Office’s Clostridium botulinum resource list.)