“My lawn has brown spots. Do you know what is causing them?” This is one of the questions I get most often.

I ask, when presented with the spot question, is if the person has pets. Dog urine can cause small, dead-looking patches of grass in seemingly random places. The most obvious issue with pet urine spots is unsightliness, but they can potentially make your lawn susceptible to secondary pests or problems.

A common misconception is that the acidic pH of dog urine kills the grass and that you should feed your dog tomato juice or cider vinegar to adjust the pH of the urine. Not true.

Most common turf grasses prefer a slightly acidic pH but can tolerate a wide range — 5.5 to 7.5 and sometimes higher — and still do well. Dog urine has a pH range of between 6.0 and 8.0, depending on the dog’s diet and health, so pH is not the problem.

The real culprit is the concentration of nitrogen in the urine. Urine consists mainly of water and urea, a form of nitrogen, which results from the metabolism of protein that dogs consume in their food.

The spots form because when a dog urinates, the high-nitrogen urine is concentrated in a small area, which damages the grass. The same thing would happen if you spilled fertilizer in a small area. The center of the spot may die because of toxic levels of nitrogen, but each spot gets an outside ring of deeper green grass, because the nitrogen concentration around the perimeter is diluted enough to have a fertilizer effect.

You can minimize the damaging effects of dog urine by watering the spot where your dog “went” immediately after the event, but that’s generally impractical.

Instead, toss a handful of grass seed on any spots that do not clear up on their own. And if you are experiencing a big problem, train your dog to “go” in a specific area.

• UC Certified Master Gardeners are available to answer your gardening questions Tuesdays, Wednesdays, and Thursdays from 9 a.m. to noon at 953-6112. Questions for Heather can be submitted to ucmastergardener@gmail.com.