Ask Mr. Smarty Plants

Ammonia is a compound of one part nitrogen and three parts hydrogen. Household ammonia or "ammonium hydroxide" is combined with water, with usually only 5 to 10 per cent of the resulting product being ammonia. Since both nitrogen and hydrogen are already in the environment, and nitrogen, at least, is beneficial to the soil, it is unlikely that it is going to cause any damage. We wouldn't recommend spraying it directly onto the plant leaves, but it shouldn't harm the dirt or the roots.

If we may take this opportunity, we would like to remind you that the Lady Bird Johnson Wildflower Center is dedicated to the use, care and propagation of plants native not only to North America but to the area in which the plant is being grown. Nerium oleander is native to Africa, Asia and and the Mediterranean. We realize it is widely used in California, but would like to point out one other  characteristic of oleander that may be more important than whether ammonia will harm it. Nerium oleander is one of the most poisonous plants known, with all parts of the plant being toxic. Ingestion of just one leaf can cause heart attack and death. Some of the poisons it contains are cardiotonic glycoside (oldendrine), prussic acid and rutin. Skin contact with the plant can cause severe dermatitis. Burning it will release toxins that can cause intoxication. We would recommend that it be carefully removed, wearing gloves and protective clothing, bagged and disposed of properly, NOT burned nor consigned to the compost pile, where the toxins would continue to be dangerous. 

Now, back to the dirt in your garden and the cats. Rather than spraying ammonia or some other aromatic liquid on the dirt, try a good heavy mulching with shredded bark mulch. Cats don't like scratching in that, its fragrance will alleviate some of the urine smell which is attracting the cats back to the same place, and it will enrich and amend the soil.