I want to expand my garden this year.Some of the lawn was used by the dogs.I cleaned up after them regularly.(I don't like the smell or the mess).I wonder is it safe to use this land now for gardening?
As long as the stool was picked up and removed fairly quickly after it was deposited any parasites or disease pathogens that might be in that stool would not have a chance to migrate out and into the soil. If that stool was allowed to accumulate for some time before cleanup you might have a problem with parasites or disease pathogens.
Considering all the animals that have left their deposits on the soil over the years I would say it is as safe as anywhere else.
Anyone concerned about safety should be washing their produce before eating it, particularly if the eaten part was in the soil or has soil splash on it.
Most of the potential problems from the soil are parasites in the soil left from animal feces. These parasites generally have to be ingested to be harmful.
I think mammal dung is a very interesting subject. Everyone has seen a dog pile sit and petrify in a lawn or field, but probably not many have watched it return naturally to the soil. I did some watching several years ago. I hesitate to call it experiments because all I did was observe the changes. All my dogs are both inherently averse to pooping within miles of where they eat (apparently a characteristic of chow mixed dogs) and are trained as much as practical to poop anywhere but the lawn. However when the dogs get older, sometimes they just go in the lawn. That happened several years ago with Tyler. He started pooping in my organic lawn. I tried a few things to see if I could speed up the decomposition process. Without dwelling on the myriad results, simply leaving them alone with plenty of moisture seemed to allow them to return to "dirt" in four days. One thing that probably affected my results was that it was winter and we had taken Tyler off of heartworm meds for his last few months. Heartworm meds persist through digestion and become insecticide in the dung. Thus parasites should be no problem in either case. Either the dung itself will kill the parasites or the dung processing critters will decompose the pile before the parasites have a chance to mature. Anyway I thought it was interesting that in four days, without heartworm meds, I could not find the dog piles at all (without my little flags).