Fleas in a yard or field may bother cats and dogs and especially larger animals, such as horses. Borax is an inexpensive, easy way to kill outdoor fleas. Mixed with diatomaceous earth and salt, borax kills fleas by dehydrating them.
Check the weather. Make sure rain is not forecast for the next three days, and turn off your sprinkler system. It's essential that your lawn is completely dry for at least three days after you apply the borax mixture or it won't work.
Combine the ingredients for the borax mixture in your bucket, using a large mixing bowl as your measuring device. Follow this ratio of ingredients no matter how much you need to use to cover your yard: two parts borax, two parts diatomaceous earth and one part salt.
Sprinkle the mixture with a hand trowel all over the yard. You should apply the mixture liberally but not in clumps.
Once you have sprinkled the entire yard with the mixture, use a rake to comb the area and make sure the powder gets down through the grass or plant growth.
Keep your dogs or cats out of the yard as much as possible for the first three days after applying the mixture. If too much of the mixture gets on an animal's fur, the animal might accidentally ingest it while grooming and could become ill.
On the third or fourth day, liberally water your yard to wash away the borax mixture.